zotero/storage/TWTVEKE9/.zotero-ft-cache

Instruments of Darkness




Instruments of Darkness
The History of Electronic Warfare, 1939–1945
Dr Alfred Price
Frontline Books


Instruments of Darkness The History of Electronic Warfare, 1939–1945
A Greenhill Book
First published in 1967 by William Kimber & Co., London Expanded edition published in 1977 by Madonald and Jane’s Publishers, London
Revised hardback edition published in 2005 by Greenhill Books, Lionel Leventhal Limited www.greenhillbooks.com
This paperback edition published in 2017 by
Frontline Books an imprint of Pen & Sword Books Ltd, 47 Church Street, Barnsley, S. Yorkshire, S70 2AS
For more information on our books, please visit www.frontline-books.com, email info@frontline-books.com or write to us at the above address.
Copyright © Alfred Price, 1967, 1977, 2005, 2017
ISBN: 978-1-47389-564-5
All rights reserved. No part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of the publisher. Any person who does any unauthorized act in relation to this publication may be liable to criminal prosecution and civil claims for damages.
CIP data records for this title are available from the British Library
Printed and bound by CPI Group (UK) Ltd, Croydon, CR0 4YY


FOR JANE




Contents
List of Illustrations 9
List of Maps and Diagrams 10
Foreword 11
Author’s Acknowledgements 15
Prologue 19
Chapter 1 The Battle of the Beams 21
Chapter 2 The Instruments 51
Chapter 3 Discovery 62
Chapter 4 Towards the Offensive 97
Chapter 5 The Coming of the Yanks 109
Chapter 6 Doubts and Decisions 117
Chapter 7 The ‘Window’ Controversy 124
Chapter 8 The Pace Hots Up 135
Chapter 9 Operation ‘Gomorrah’, and After 155
Chapter 10 Approaching the Climax 179
Chapter 11 In Support of the Invasion 207
Chapter 12 The Final Months of the War in Europe 220
Chapter 13 Climax in the Pacific 240
Chapter 14 In Retrospect 251
Appendix A: Main Types of German Surface Radars 259
Appendix B: Main Types of Japanese Surface Radars 261
Appendix C: Air Forces, Equivalent Ranks 264
Glossary: Code-Names, Equipment Designations
and Unit Terms 265
Index 269




Illustrations
The airship LZ-130 Graf Zeppelin (Breuning) 81 Dr Ernst Breuning (Breuning) 81 Knickebein transmitter (Trenkle) 82 Heinkel 111 with Y-Gerät equipment (von Lossberg) 82 Wing Commander Edward Addison (Addison) 82 Heinkel 111 of Kampfgruppe 100 (Trenkle) 83 X-Gerät beam transmitter (Trenkle) 83 Dr R. V. Jones (Jones) 83 The Graf Spee and its Seetakt radar (IWM) 84 Reconnaissance photo of Auderville radar station (IWM) 84 Freya radars at Auderville (IWM) 84 Messerschmitt Bf 110 with Lichtenstein radar (Trenkle) 85 Wassermann early-warning radar at Bergen aan Zee (Chisholm) 85 Derek Jackson (Jackson) 85 Würzburg radar (Trenkle) 86 Generalmajor Josef Kammhuber (Studiengruppe der Luftwaffe) 86 Bruneval radar station (IWM) 86 Giant Würzburg radar on the island of Walcheren 86 (Crown Copyright)
German radar site with barbed-wire defences 87 (Crown Copyright) Himmelbett station (Heise) 87 H2S radar picture and map of Hamburg (Crown Copyright) 88 H2S indicator on a Lancaster bomber (Crown Copyright) 88 Aerial of a Korfu ground direction-finding station (Cockburn) 89 Generalfeldmarschall Erhard Milch (Milch) 89 Oberst Dietrich Schwenke (Schwenke) 89 Generalmajor Joseph Schmid (Studiengruppe der Luftwaffe) 90 Major Hajo Herrmann, with Hermann Göring (Herrmann) 90 Lancaster bomber over Berlin, 16 December 1944 (IWM) 90 B-17 Flying Fortress jamming escort aircraft (IWM) 91 Dr Robert Cockburn (Cockburn) 91 ‘Jostle’ communications jammer (IWM) 91 Junkers 88 night-fighter with SN-2 radar (Crown Copyright) 92


Upward-firing 20-mm cannon in a Junkers 88 (Chisholm) 92 Tail-mounted aerial for SN-2 radar in a Junkers 88 92 (Crown Copyright)
Jagdschloss fighter-control radar (Trenkle) 93 ‘Mandrel’ jamming on a Jagdschloss radar (Trenkle) 93 B-17 bombers being engaged by a flak battery (USAF) 94 The APQ-9 ‘Carpet III’ jamming equipment (USAF) 94 ‘Tuba’ jamming equipment (USAF) 95 B-29 bombers on one of the Marianas islands (USAF) 95 B-29 ‘Guardian Angel’ jamming escort aircraft (USAF) 96 ‘Little Boy’, the atom bomb dropped on Hiroshima (USAF) 96
Maps and Diagrams
The Lorenz Beam 23 Knickebein beam stations 38 X-Gerät beams during the attack on Coventry 44 The Seeburg Table 57 Himmelbett fighter control stations 60 The ‘Oboe’ system 120 Buck Ryan 128 ‘Serrate’ picture 145 Attack on Hamburg: 24/25 July 1943 159 Attack on Kassel: 3/4 October 1943 180 Attack on Nuremburg: 30/31 March 1944 201 The ‘Ghost Fleet’ 212 Deception operations: night of 5/6 June 1944 215 The Klein Heidelberg system 223 Attack on Bohlen: 20/21 March 1945 234 The Bernhard display 239
10 Illustrations


Foreword
by Sir Robert Cockburn, KBE, CB
(Written in 1967)
World War II was dominated by air power, which permeated every phase of the conflict. The aircraft became a major instrument of offence and defence in the air and a vital weapon in support of ground forces, in maritime warfare, in reconnaissance and in transport. This expansion of air power was stimulated by, and became critically dependent on, a series of remarkable developments in the fields of radar and radio communications. Both sides committed large resources to successive systems of early warning and detection, navigation, target identification and weapon guidance. Under the stimulus of war, technology advanced rapidly and each new system provided greater range, greater precision and greater capacity. Yet by modern standards they were still relatively naïve in concept and were soon found to be vulnerable to interference, deception and manipulation. It was a rude shock to designers to discover how quickly performance demonstrated in the laboratory was nullified in operation against a resourceful enemy; and as the war progressed scientists and engineers pitted their wits against one another to preserve their own systems, and to discover and exploit the weaknesses in those of their opponents. It is this story which Alfred Price describes in Instrument of Darkness.
Alfred Price is a serving officer in the Royal Air Force, at present with Bomber Command; and he has been able to recapture the excitement and drama of a struggle in which new techniques and tactics could have such immediate and catastrophic consequences. But he is also an electronics specialist well qualified to deal with the technical aspects of his subject and to appraise the relative importance of the various countermeasures of World War II. Rarely before or since has it been possible for the scientist in the laboratory to make such a direct impact on military operations. A few black boxes based on a new piece of intelligence, a revealing reconnaissance photograph or observations by a returning bomber


12 Foreword
crew could within a few weeks or months affect the fate of cities and the lives of hundreds of aircrew. Not all black boxes were equally effective; some were at best of psychological value, some were a temporary nuisance, and some were not only useless but positively dangerous. In the heat and fog of war any opportunity, however slight, must be exploited, but in retrospect it is clear that clever devices and adroit tactics were usually of limited value. Trivial weaknesses in a system were easy to exploit but equally easy to remedy, and over-sophisticated jamming and warning methods were incompatible with the nightly holocaust over German targets. Subtle countermeasures like ‘Moonshine’ were effective for special operations where surprise could be exploited, and they played an important part in the spoof invasions of the Pas de Calais. But it was straightforward noise jamming and the massive use of ‘Window’ which was most effective in sustained operations. Alfred Price has profited by the lapse of twenty years to put his story of World War II into perspective. He has gone to a lot of trouble to present at each stage both the British and the German story, and he shows how closely developments on one side were matched on the other. There was, for example, the extraordinary similarity in the evolution of British ‘Window’ and German Düppel. Nowadays it is accepted that major technical advances will occur almost simultaneously in a number of countries, even in highly classified military projects. In peacetime the time scale of development is long enough that a year either way in producing a new weapon may not seriously affect the issue, but in war six months can make the difference between victory and defeat; and it was by such narrow margins that the outcome of the Radio War was determined. Both sides entered the war believing that they possessed in radar a unique advantage over the other; and neither foresaw the profound effect that this scientific breakthrough would have on air operations. In 1940 our own radars were in many respects inferior to the Freya and the Würzburg, and we had no bombing and navigation systems to compare with the Knickebein beams and their successors. At the end of the war the Germans were introducing, well ahead of the Allies, a range of guided weapons, including the pilotless aircraft V-1 and the ballistic missile V-2. But the German High Command did not properly appreciate the pace of development or its inevitable


Foreword 13
impact on operations. For two critical years they failed to maintain the momentum of research in radar, and rapidly lost their initial advantage. By the end of the war British and American equipments were far superior both in performance and in range of application; and the German guided weapons came too late to redress the balance. In particular we were quicker to recognise and exploit the intrinsic vulnerability of radar and radio systems, and the initiative in the jamming war lay firmly in our hands. The importance of the electronic environment not only to aircraft and guided weapons but to the whole range of military operations is now well understood; and one of the most important criteria of any radar or radio system is the ability to discriminate against unwanted and irrelevant information. Vulnerability can be theoretically specified, and allowed for. Nevertheless, jamming and deception are always possible, with sufficient effort. Ideally an economic balance should be struck between complexity and vulnerability, so that the cost of nullifying a system is comparable to the cost of establishing it. But this condition can seldom be met in practice, and Alfred Price’s book is a salutary reminder that the impressive panoply of modern weapons is dependent ultimately on the survival, in war, of their guidance and control systems.




Author’s Acknowledgements
First Edition
In writing Instruments of Darkness I have been greatly helped and encouraged by the many busy men who have unhesitatingly given me their valuable time: to all of them I tender my grateful thanks. Space does not allow me to mention each one by name, but I am particularly indebted to Sir Robert Cockburn, Professor R. V. Jones, Professor D. A. Jackson, Air Marshal Sir Robert Saundby, Air ViceMarshal E. B. Addison, Air Commodore Chisholm, Dr B. G. Dickins and Mr J. B. Supper and, in Germany, the Studiengruppe der Luftwaffe, the Telefunken Company and Herr Hans Ring. I should like to thank Sir Donald MacDougall for allowing me access to Lord Cherwell’s papers and Mr. R. Bruce and Mr. C. Moore for allowing me to examine documents under their control. Also I am indebted to Mr. L. A. Jackets and the staff of the Air Historical Branch for much of the material I have used. I must stress, however, that I alone am responsible for the opinions expressed. My thanks go to Her Majesty’s Stationery Office for permission to quote from The Strategic Air Offensive Against Germany 1939–1945 by Sir Charles Webster and Noble Frankland. I should also like to thank Messrs. Cassell and Co. for allowing me to quote from Winston Churchill’s The Second World War; Messrs. Methuen and Co. for permission to quote from The First and the Last by Adolf Galland; Group Captain J. R. D. Braham for the use of the passage from his autobiography Scramble; and the proprietors of the Daily Mirror for permission to reproduce the Buck Ryan cartoon strip. I should like to record my debt to my wife, who gave valuable support when the going was tough, and my mother, who helped with the German translations.


16 Author’s Acknowledgements
This Edition
In the thirty-seven years since the initial appearance of this book, important additional material has become available. This completely revised edition takes the story up to the time of the Japanese surrender in August 1945, and details the huge US efforts in this area in both the European and the Pacific theatres of operations. I am grateful to the Association of Old Crows, Washington DC, for permission to use material from Volume 1 of The History of US Electronic Warfare, which I wrote to their commission. On a sad note, I must mention that almost all of the men and women I interviewed for this book are no longer alive. Let these pages serve as a memorial to their achievements.


‘The instruments of darkness tell us truths,
Win us with honest trifles, to betray’s
In deepest consequence.’
Shakespeare, Macbeth




Prologue
On the evening of 2 August 1939 the giant airship LZ130 Graf Zeppelin lifted off from her base at Frankfurt-am-Main and climbed slowly into the night sky. Leaving the German coast near Cuxhaven, she rumbled out over the North Sea heading for a designated search area off the coast of Great Britain. The LZ130, last of the long line of rigid airships built in Germany, had been designed to fly on the trans-Atlantic passenger service in conjunction with her sister ship, the LZ129 Hindenburg. Before the new airship was completed, however, Hindenburg came to a fiery end. With her died the notion of the airship as a passenger-carrying vehicle. For a time, Graf Zeppelin had no formally assigned role. Then, in the spring of 1939, she was modified for a quite different task. If war came, Luftwaffe intelligence officers would need to know the use potential enemies made of the radio spectrum for communications, navigation and even radar systems. Without such knowledge, and the method of operation and location of these systems, countermeasures would be impossible. Graf Zeppelin’s luxurious passenger compartments now carried a battery of radio receivers and a team of signals experts to operate them. The airship was the world’s first airborne electronic intelligence, or Elint, collector. From April 1939 the Graf Zeppelin flew a series of missions along Germany’s eastern and western frontiers, hunting for radio and other signals of interest emanating from neighbouring countries. On 12 July she ventured over the North Sea, and reached a point about a hundred miles off Middlesborough before turning for home. The mission on 2 August was a more determined attempt to examine the radio spectrum in the skies around Great Britain. For that mission the airship carried a crew of forty-eight, who included a twenty-five-strong team of radio specialists under the command of Dr Ernst Breuning. The airship crossed the North Sea and flew close to the east coast of Scotland, taking care to remain outside territorial waters. She then turned around and flew southeast down the coast to a point abeam Lowestoft, before returning to Germany.


While the Zeppelin was within their cover, Britain’s newly erected line of ‘Chain Home’ radars reported her every move. When she passed Aberdeen an RAF Magister training plane took off to get a closer look at the airship, and for a time flew in formation with her. Given the amount of British radar activity over the North Sea, it is surprising that the radio operators aboard the Zeppelin failed to identify British radar signals. Interviewed in 1969, Ernst Breuning told the writer that his team had concentrated their search on the radio spectrum above 100 MHz. That was where early German radars operated, and it seemed reasonable to expect that British radars might do the same. In that part of the spectrum the listeners found none of the expected pulsed radar signals. They did, however, hear transmissions from the new VHF radios being developed in Britain for the RAF. Breuning and his team did make a cursory search in the 20–50 MHz band – which was in fact that part of the spectrum where the early British radars operated. There the listeners found some pulsed signals, but these were discounted. The transmissions sounded just like those picked up during earlier flights, identified as coming from a station in Germany conducting experiments to measure the altitudes of the layers of ionised gas that surround the earth. Missing the British radar signals was an easy enough mistake, given that the German radio operators were learning the rudiments of Elint ‘on the job’ and their receivers had not been designed for this task. Graf Zeppelin’s marathon flight lasted 48 hours and covered a distance of 2,600 miles. It was the longest she would ever make. Less than a month after she returned from her sortie, on 1 September 1939, Germany invaded Poland. Within a couple of days, Great Britain and France entered the conflict in support of their eastern ally. With a major war on its hands, the Luftwaffe saw no further role for the big airship and she was scrapped. Graf Zeppelin’s unsuccessful missions off the coast of Great Britain are of historical interest only, for they achieved little of military value. Yet the notion of searching the radio spectrum for enemy or potential enemy signals, as a prelude to countering them, was an idea whose time had come. The airship’s flights marked the first tentative steps in a completely new form of warfare, one whose significance both sides would quickly learn.
20 Instruments of Darkness


Chapter 1
The Battle of the Beams
‘During the human struggle between the British and German air forces, between pilot and pilot, between AA batteries and aircraft, between ruthless bombing and the fortitude of the British people, another conflict was going on step by step, month by month. This was a secret war, whose battles were lost or won unknown to the public; and only with difficulty is it comprehended, even now, by those outside the small high scientific circles concerned.’
Winston Churchill, Their Finest Hour
The truth about military intelligence work is that much of its success depends on chance, and much upon tenacity. Little of it is glamorous in the way that readers of espionage thrillers would believe. In the case of a secret device to guide bombers to targets, for example, in time of war it is usually only a matter of time before an aircraft carrying it is shot down and falls in hostile territory. Then, a diligent examination of the wreckage should reveal its existence and survivors, perhaps still shaken after narrow escapes, may be induced to talk under interrogation. Aircrew cannot be expected to memorise detailed lists of radio frequencies, callsigns and the geographical positions of beacons. If that information is to be used in the stress of action, it has to be written down and taken on the sortie. Sooner or later, one of those briefing sheets is bound to be captured. If the system involves radio beams, those investigating it have another clear advantage: such beams cannot be concealed. One has only to look carefully enough and they will be found. Once the transmissions are found, they can be analysed and their purpose deduced. Thus, a handful of intelligence officers can have a bearing on the conflict that is out of all proportion to their numbers. This was why, on the night of 21 June 1940, Flight Lieutenant Harold Bufton came to be patrolling in the darkness over East Anglia in a twin-engined Anson aircraft. In the rear cabin his wireless


operator, Corporal Dennis Mackey, carefully searched the ether with his radio receiver. Suddenly Mackey found what he was looking for: a series of Morse dots, sixty to the minute, piercingly clear in his headphones. As the Anson continued on its heading, the dots merged into one steady note. A little later, the steady note broke up, not into ‘dots’ but into Morse ‘dashes’ at the same steady rate of sixty to the minute. Later in the flight, a second radio beam was located. After he landed at his base at Wyton, Bufton reported:
1. There is a narrow beam approximately 400–500 yards wide, passing through a position one mile south of Spalding, having dots to the south and dashes to the north, on a bearing of 104° to 284° True. 2. That the carrier frequency on the night of 21st–22nd June was 31.5 mc/s, modulated at 1,150 c/s and similar to Lorenz in characteristics. 3. That there is a second beam having similar characteristics but with dots to the north and dashes to the south synchronised with the southern beam, apparently passing through a point near Beeston on a bearing lying between 60° and less than 104°.
In terms of the effort involved, the flight of the Anson with the two-man crew was far removed from the Graf Zeppelin’s abortive Elint collection mission off the coast of Great Britain almost a year earlier. Yet in intelligence collection, success is often unrelated to effort involved. The Anson had located a couple of radio beams emanating from Germany, which intersected over the important Rolls-Royce aeroengine factory at Derby. It was a highly significant discovery.
***
To observe the background of that discovery, we need to look briefly at some scientific developments that had taken place in Germany in the early 1930s. There the Lorenz Company had developed a blindapproach system to help aircraft find airfields in bad weather. The so-called ‘Lorenz System’ used two adjacent radio beams to mark a path extending up to thirty miles from the airfield. In the lefthand beam Morse dots were transmitted, and in the right-hand beam Morse dashes. The signals interlocked, so that where the two beams overlapped a listener heard a steady note. Aircraft navigated
Instruments of Darkness
22


by flying down the steady-note zone until they came to the beams’ transmitter. By the mid-1930s the Lorenz system was in widespread use by civil airlines and some air forces. The Royal Air Force used it, as did the Luftwaffe. In Germany Dr Hans Plendl, a specialist in radiowave propagation, then adapted the Lorenz system to assist aircraft to attack accurately at night or in bad weather. This system became the X-Gerät (‘X-device’) which employed six Lorenz-type beams. Marking the approach to the target were three such beams, one coarse and two fine, all transmitted on different frequencies and all pointing straight at the target. The other three beams crossed the approach beams at three points leading up to the bomb-release point. The X-Gerät radio beams were transmitted on frequencies between 66 and 75 MHz (see map on p. 44).
A bomber using X-Gerät followed the approach beam to the target. When it was 50 km (30 miles) from the bomb-release point, the aircraft flew through the first crossbeam. That served as a warning that it was time to line up accurately in the approach beam. When it was 20 km (12 miles) from the bomb-release point, the aircraft flew through a second crossbeam. As it did so, the navigator pressed a button to start one hand of a special clock, similar to a stopwatch but with two hands that rotated independently. When the bomber was 5 km (3 miles) from the bomb-release point, it passed the third and final crossbeam. When he heard the steady-note signals from
The Battle of the Beams 23
The Lorenz Beam


that beam, the navigator pressed the button on his special clock a second time. The hand which had been moving stopped, and the other hand started rotating to catch it up. The distance from the second crossbeam to the third crossbeam was three times that from the third crossbeam to the bomb-release point (5 km or 3 miles), so the second hand on the clock travelled three times faster than the first. When the hands coincided, a pair of electrical contacts closed and the bombs were released automatically. All in all this was a sophisticated system, considering that it had been produced before World War II. The combination of the clock and the beams provided accurate data on the bomber’s speed over the ground, one of the most important facts to be known for accurate bombing once an aircraft was routed correctly over the target. The Luftwaffe established a special unit to operate with X-Gerät, No. 100 Air Signals Battalion (Luftnachrichten Abteilung 100) based at Köthen near Dessau and equipped with Junkers 52s and Heinkel 111s. Meanwhile Telefunken, a competitor of Lorenz, had produced another blind-bombing system for the Luftwaffe. Called Knickebein (‘Crooked Leg’) this system was much simpler than X-Gerät and it employed only two Lorenz beams. One beam marked the approach to the target, the other crossed the first beam at the bomb-release point. The system was less accurate than the X-Gerät, but it had two major advantages over it. Firstly, the device used the same frequencies – 30, 31.5 and 33.3 MHz – as the Lorenz airfieldapproach receiver fitted as standard in all German twin-engined bombers, so that receiver could pick up the Knickebein signals, and there was no need for the bomber to carry specialised equipment. Secondly, crews trained in the use of the Lorenz airfield-approach receiver could fly the Knickebein beams without further training. Thus Knickebein could be used by the entire Luftwaffe bomber force and not just part of it. The aerial array necessary at the Knickebein ground transmitter was a huge structure, more than 100 feet high and 315 feet wide. The whole thing rested on railway bogies running on a circular track, to allow the beam to be aligned accurately on the distant target. The system’s range depended on the altitude of the receiver aircraft: a bomber at 20,000 feet could receive the signals from a transmitter 270 miles away. The steady-note lane was one-third of
Instruments of Darkness
24


a degree wide, giving a theoretical accuracy of one mile at a distance of 180 miles. By the end of 1939, the Luftwaffe had erected three Knickebein transmitters to cover potential targets in Great Britain and western Europe. One was at Kleve close to the Dutch frontier, a second was at Stollberg in Schleswig-Holstein, and a third was at Lörrach in the southwest corner of Germany. Towards the end of 1939, No. 100 Air Signals Battalion was redesignated Kampfgruppe 100 (KGr 100) and now possessed twentyfive He 111s fitted with X-Gerät. During the campaigns in Norway and France, however, the unit did not use its night precision-attack capability and operated as a normal day-bombing unit. However, soon after the Allied evacuation from Dunkirk in June 1940, Luftwaffe signals personnel began erecting Knickebein and X-Gerät transmitters in Holland and northern France as part of the preparations for attacks on Great Britain.
***
Until the spring of 1940, the RAF had not considered it likely that German night bombing attacks would prove a serious threat. The general view was that the darkness that hid the bombers from the defences would also hide the targets from the bombers. Dr R. V. Jones, a scientist who a few months earlier had taken up a post at the Directorate of Intelligence at the Air Ministry, had a wide remit. His task was to determine which scientific developments taking place in Germany might affect the air war. He began receiving clues from various sources which suggested that the Luftwaffe possessed, or would soon possess, a radio system to guide bombers to their targets at night or in bad weather. In March 1940, an He 111 bomber crashed in England. In the wreckage, searchers found a scrap of paper which stated:
Navigational aid: radio beacons working on Beacon Plan A. Additionally from 0600 hours Beacon ‘Dunhen’. Light beacon after dark. Radio beacon ‘Knickebein’ from 0600 hours on 315°.
At about the same time, a prisoner admitted under interrogation that Knickebein was ‘something like the X-Gerät’, about which he assumed his captors already knew. He said that a beam was sent from Germany which was so narrow that it could reach London with
The Battle of the Beams 25


divergence of no more than one kilometre (the prisoner had exaggerated the fineness of the beam, though Jones had no way of knowing this). Two months later, the diary of a German airman was found in the wreckage of another He 111. Under 5 March it carried the significant entry:
Two-thirds of the Staffel on leave. In the afternoon we studied Knickebein, collapsible boats, etc.
From these snippets of information Jones deduced that Knickebein – and the X-Gerät which was ‘something like’ it – might be some sort of directional radio beam. The bearing of 315 degrees might point from the northwest coast of Germany to Scapa Flow, an area where Luftwaffe bombers had been active. What seemed unbelievable was the prisoner’s assertion that a radio beam from Germany to London – a minimum distance of 260 miles – could have a divergence of only one kilometre. In fact the prisoner had exaggerated: the beam’s divergence at that distance would have been nearly 11⁄2 miles. By now the Government Code and Cipher School at Bletchley Park was starting to produce a useful stream of decrypts of German radio signals transmitted in high-level Enigma ciphers. One such signal, picked up on 5 June and decrypted four days later, stated: ‘Knickebein at Kleve is confirmed [or established] at point 53° 21' N, 1° W.’ The signal had come from the Chief Signals Officer of Fliegerkorps IV and it seemed that the position, near Retford in Nottinghamshire, might be the location of an illicit radio beacon. A search of the area produced nothing but, significantly, the signal gave the location of a Knickebein. For, apart from being the home of the fourth wife of King Henry VIII, the town of Kleve lay at the part of Germany closest to Great Britain. The next logical step was to examine the radio equipments carried by the He 111 bomber, since this aircraft was linked with each intelligence report on Knickebein. Did it carry any device that could receive beam signals at long range? In October 1939, an He 111 had crash-landed near Edinburgh. At Farnborough, technicians had carefully dissected and analysed each item of the plane’s equipment. At the time they noted that the plane’s Lorenz blind-approach
Instruments of Darkness
26


receiver was far more sensitive than its British counterpart. Might this be the device that picked up the long-range beam signals? At first sight it might seem a simple matter to find out whether the Luftwaffe possessed a long-range radio-beam system. A few flights by aircraft carrying search receivers would settle the matter. But Jones was young and recently appointed and had no such aircraft under his control. Also, he knew he had to play his cards carefully. Jones saw his position as being analogous to that of a watchdog. He had to bark when he saw danger, but if he barked at the first whiff of trouble and none was subsequently revealed, people would learn to disregard his cries. On the other hand if he barked too late, the Luftwaffe could strike unhindered. There could be no mistaking the gravity of the situation, if the Luftwaffe really did possess an accurate method for attacking targets by night at a time when Britain’s air defences were ineffective. There was one man who could secure for Jones the influential backing he needed, and upon whom he could rely for a sympathetic hearing: his tutor at Oxford before the war, Professor Frederick Lindemann. Frederick Lindemann and Winston Churchill had been close friends since 1919, and when Churchill became prime minister in May 1940 the association continued. For all his superlative qualities as a war leader, Churchill had little grasp of scientific matters and he relied heavily on Lindemann to explain these to him. Clearly, if Jones could convince Lindemann of the possible danger of the Luftwaffe radio beams, his battle would be half won. On 12 June Lindemann sent for Jones to discuss another matter. At the end of the conversation, Jones steered the discussion round to Knickebein. Lindemann was unimpressed, however. He said he could not believe that a long-range beam on a frequency of around 30 MHz – the part of the spectrum covered by the Lorenz blindapproach receiver – would bend to follow the curvature of the earth. At that time such signals were thought to travel almost in a straight line, which limited their effective range to about 180 miles if the receiving aircraft was flying at 20,000 feet. That fell far short of the 260-mile range necessary to reach London from the nearest point in Germany. On the day after the unsuccessful encounter, Jones returned to Lindemann’s office carrying an unpublished paper he had discovered. Its author was Thomas Eckersley, scientific advisor to
The Battle of the Beams 27


the Marconi Company and a leading authority on the propagation of radio waves. The paper contained a series of graphs to illustrate the maximum ranges at which radio signals on various frequencies could be received. By taking the extreme end of one of the curves, it looked as if signals on 30 MHz might be picked up by an aircraft flying at 20,000 feet over much of England, provided the transmitter was situated on high ground in Germany. That satisfied Lindemann, who immediately wrote to the Prime Minister:
There seems to be some reason to suppose that the Luftwaffe have some type of radio device with which they hope to find their targets. Whether this is some form of RDF [radar] . . . or some other invention, it is vital to investigate and especially to seek to discover what the wavelength is. If we knew this, we could devise means to mislead them; if they use it to shadow our ships there are various possible answers . . . If they use a sharp beam this can be made ineffective. With your approval I will take this up with the Air Ministry and try to stimulate action.
Before passing the note to Sir Archibald Sinclair, his Secretary of State for Air, Mr Churchill jotted a brief comment at the bottom: ‘This seems most intriguing and I hope you will have it thoroughly examined.’ Now Jones had the big guns firing for him. Sinclair acted promptly and on the following day, 14 June, he placed Air Marshal Sir Philip Joubert in charge of the investigation. On that very day RAF interrogators were questioning another prisoner, who stated that Knickebein was a bombing device involving two intersecting radio beams which could be picked up by the aircraft’s Lorenz receiver. He added that bombers had to fly very high to pick up the beam signals at long ranges. For example, to receive the signals over Scapa Flow the aircraft had to be above 20,000 feet. Jones observed that from Scapa Flow to the nearest point in German-controlled territory – in western Norway – was 260 miles. That was exactly the same distance as from London to the nearest point in Germany. This intelligence was available in time for a meeting Air Marshal Joubert had called for 15 June, attended by Lindemann and Jones. Now there was sufficient evidence to justify bringing more people into the picture, and Joubert summoned a further meeting for the
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following afternoon. In addition to Jones the attendees included Air Chief Marshal Sir Hugh Dowding, C-in-C Fighter Command, and Air Commodore Charles Nutting, the RAF Director of Signals. Jones recounted the available evidence, and it was decided to fit special radio receivers to aircraft to hunt for the beams. Three Avro Anson general-purpose aircraft would be made available, and work began immediately to fit them with the necessary radio receivers. Squadron Leader Rowley Scott-Farnie, representing the RAF signals intelligence service at the meeting, opined that the beams would probably be found on a frequency of 30, 31.5 or 33.3 MHz. He said that every Lorenz receiver found in a wrecked Luftwaffe aircraft was tuned to one of those three frequencies. On the following Tuesday, 18 June, fresh evidence arrived in a miscellany of papers salvaged from a German aircraft shot down in France some weeks earlier. On one of them was written:
Long-range radio beacon = VHF 1. Knickebein (near Bredstedt, north-east of Husum) 54° 39' 8° 57' 2. Knickebein (near Kleve) 51° 47' 5" 6° 6'
That served to confirm some of the earlier pieces of information on Knickebein. If those were the locations of two Knickebein transmitters, that would make sense. Those positions were at points in Germany among the closest to England, but they were well separated to give the greatest possible ‘angle of cut’ between a pair of beams. Jones noted that Scapa Flow lay on a bearing of 315 degrees from Bredstedt, which explained the earlier reference. As if more proof were needed, a wrecked Heinkel provided a further clue. The wireless operator’s log had been recovered intact and it included a list of known radio beacons. At the head of the list was a jotted entry: ‘Knickebein Kleve 31.5’. Each of the other beacons was followed by a radio frequency, and the RAF listening service confirmed that the list was correct for the night in question. It was therefore reasonable to assume that the Knickebein at Kleve had on that night been radiating on 31.5 MHz. That fitted in with Scott-Farnie’s prediction.
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That evening, 19 June, an Anson took off on the first flight to search for beam transmissions; its receiver developed a fault, however, and the radio operator heard nothing. On the next night, the 20th, another Anson flew a patrol to look for beams but found nothing – the Luftwaffe had stayed at home. Even as that aircraft was airborne, however, an RAF intelligence officer was literally piecing together another clue. A Luftwaffe wireless operator had baled out of his crippled aircraft over England. Soon after landing the airman, more conscientious than many of his compatriots, realised he still had his notebook. He carefully tore it into more than a thousand pieces, but as he attempted to bury them he was captured and the pieces were recovered. The reward of much hard work was a table of data, which confirmed the positions of the Kleve and Bredstedt transmitters and the operating frequency of the former; it also gave the frequency of the latter as 30 MHz. Thus, by the morning of 21 June, R. V. Jones had established the positions and frequencies of two of the Knickebein transmitters. The findings were timely, for that very morning Churchill decided to summon a top-level meeting at No. 10 Downing Street to discuss the latest intelligence on the German radio beams. Among those present were Sir Archibald Sinclair (Air Minister), Lord Beaverbrook (Minister of Aircraft Production), Professor Lindemann, Sir Cyril Newall (Chief of Air Staff), Sir Hugh Dowding and Sir Henry Tizard (Scientific Advisor to the Air Staff). When Jones arrived, the meeting had already started. He arrived to find a discussion in progress on whether aircraft could in fact be guided by long-range radio beams. Both in rank and age, Jones was by far the most junior person present. He sat there waiting to be asked to speak, and after a few minutes the Prime Minister questioned him on a technical point. That was the excuse Jones had been waiting for and he asked ‘Would it help, sir, if I told you the story from the start?’ Churchill said it would, and Jones recounted various pieces of evidence supporting the theory that the Luftwaffe possessed a system of radio beams to direct bombers on to targets. That convinced the meeting that here was a matter worthy of further investigation. The axis of the discussion shifted from ‘Do the German radio beams exist?’ to ‘How can we find out more about them?’ That afternoon Air Commodore Nutting summoned Jones and Eckersley to his office, to discuss the technical details of the beam
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transmissions that might be used by Luftwaffe bombers over England. Then Eckersley dropped his bombshell: despite the series of graphs he had previously drawn, he said he could not agree with the widely held explanation of Knickebein. He said he did not believe that radio signals on 30 MHz would bend to follow the curvature of the earth. Jones pressed Eckersley to explain why he had produced the set of graphs upon which he, Jones, had relied so heavily in the Cabinet Room that morning. Eckersley renounced them, saying they applied to a different case when he had tried to stretch his theory. The feelings of Jones can be imagined more readily than described, since he had used Eckersley’s graphs to convince Lindemann that a long-range beam following the curvature of the earth was a possibility. Obviously someone was barking up the wrong tree; Jones could only hope it was not himself. (In fact partial bending to conform with the earth’s curvature does occur with transmissions on 30 MHz, but its extent was not realised in Britain at the time.) While Jones pondered on this unexpected development, the hunt for the beam signals continued. That very night, the patrol by Bufton and Mackey picked up the German beam transmissions over the Midlands, as described at the beginning of this chapter. Appropriately, given the seriousness of the new threat, the RAF code-named the beam system ‘Headache’. It fell to Air Commodore O. G. Lywood to initiate countermeasures. Wing Commander Edward Addison, one of Lywood’s staff officers, latter recalled:
One day he [Lywood] called me in and told me that a young fellow from scientific Intelligence called Jones had produced an extraordinary story of the Luftwaffe using a beam over this country to bomb London. It was known as Knickebein. He said: ‘I can’t tell you how he came by this information, but he has. It looks extremely dangerous. What do you think we ought to do?’
Addison suggested the formation of a specialised organisation to counter the German beams. Lywood agreed, and placed Addison in charge. Addison’s new unit, No. 80 Wing, was hastily established and set up its headquarters at Garston near Radlett in Hertfordshire. Thing were now moving ahead rapidly, but one further element of the defences was required. There needed to be an organisation
The Battle of the Beams 31


to build the specialised jamming devices to render the German beams unusable. That task fell to Dr Robert Cockburn, a young physicist who had recently joined the Telecommunications Research Establishment (TRE) at Swanage. He and a small team began work to design and build tailor-made jammers to counter Knickebein. The process of building specialised jammers would take time, however, and time was short; an all-out bombing offensive on Great Britain might begin any day. Accordingly, Addison requisitioned several diathermy sets (devices used in hospitals to cauterise wounds) and had these modified into crude spark transmitters to transmit a ‘mush’ of radio noise on the Knickebein frequencies. RAF personnel installed the diathermy jammers in selected police stations, where the duty constable had instructions to switch them on when instructed to do so from No. 80 Wing headquarters. Addison also secured some RAF Lorenz airfield beam-approach transmitters and modified these to radiate a beam similar to that put out by Knickebein. His idea was to produce a fake beam that could be laid across the German beam, in the hope that the German bombers might wander off course without the crews noticing it. Monitoring flights revealed that the deviation produced by the lowpowered device was negligible. Nevertheless, in the interests of getting countermeasures up and running as soon as possible, a few of these systems went into service with No. 80 Wing. An important component of the new wing was the flight of Anson aircraft commanded by Squadron Leader R. Blucke, which now flew patrols each night to determine the directions and crossing points of the German beams. Initially this unit operated under the cover name of the Blind Approach Training and Development Unit (BATDU for short). In September 1940 the unit was renamed the Wireless Intelligence Development Unit (WIDU), but its work continued exactly as before. Soon after the Anson aircraft had first picked up Knickebein signals, the RAF listening service discovered that suitably equipped ground stations could also receive these signals. Several outstations were now set up, which passed information on the beams’ frequencies and of their dot or dash transmissions to No. 80 Wing headquarters where they were plotted out on a special map. Those ground stations alone could not provide an exact picture of the German beam patterns. Yet they provided a useful cue for the Ansons
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to get airborne, and greatly narrowed the search area for their crews to hunt for the beams’ steady-note lanes. To underline the increasing potency of the new threat, during August RAF listening posts picked up signals that were traced to two new Knickebein transmitters erected on the north coast of France. One was at Greny near Dieppe, only 120 miles from the centre of London. The other was at Beaumont-Hague near Cherbourg, 150 miles from the capital. While Addison requisitioned equipment for his makeshift jamming organisation to begin operating, there was another commodity he urgently needed: capable personnel. With the highly technical war that was now in the offing, he had no use for other units’ misfits and throw-outs. He needed the best material available. Fortunately, interest in the wing’s well-being extended from the prime minister down, and Addison had free reign to choose people for his unit. A particularly useful source of recruits was the community of peacetime amateur radio enthusiasts.
***
At the same time as the wing began its improvised countermeasures to Knickebein, it took control of another jamming commitment that had been initiated some months earlier. Luftwaffe aircraft made considerable use of radio beacons set up in friendly territory, to assist navigation. Each German radio beacon transmitted its identification letters in Morse code, then a 50-second tone to allow aircraft radio operators to take bearings on the beacon. At that time in Britain there were fears that German agents might plant radio beacons near targets, to guide in their bombers. To counter the beacons, Post Office engineers had devised a clever device known as the Masking Beacon, or ‘Meacon’. The device comprised a receiver and a transmitter located about 12 miles apart. The receiver was linked to a directional aerial, aligned on the German beacon it was to counter. The receiver picked up the German beacon’s emissions, amplified them and fed them by landline to the ‘Meacon’ transmitter. The transmitter then radiated an exact replica of the German beacon signal, with the same Morse identification letters and 50-second tone, exactly in step with the German signals. But the ‘Meacon’ transmitter was, of course, in a quite different position.
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Professor Lindemann explained the operation of the Meacons in a paper he wrote for the Prime Minister early in August. The Luftwaffe, he said, had nearly eighty radio beacons in Germany, Norway and northern France, operating on the medium and long wave bands. Not more than twelve of these beacons were in use at any one time, the remainder being held in reserve. Different groups were used on different days. Lindemann continued:
There are two ways of dealing with such beacons. The first is to jam them, i.e. to make so much disturbance in the ether that their signals cannot be received. If one compares them with lighthouses, it is like turning on the sunlight so that they would become invisible. This method is difficult because they operate on so many different wavelengths that we must produce very strong signals in each band to cover the lot... Further, each lighthouse has its own colour (wavelength) which has to be outmatched, so that the general glare must be produced over the whole spectrum, ranging from 30 metres to 1,800 metres. In order to cover this range eight very powerful stations would be required, but this leads us to another difficulty. If we had eight such stations, the Luftwaffe would soon get to know where they were and could use them as lighthouses to guide them to their targets. It is much easier to fly towards a beacon than to navigate away from it on back bearings. In order to prevent this it would be essential to link our jamming stations in groups of three, making each group of three flash simultaneously. If this is done (though there is no exact optic analogy), the radio receiver cannot tell from whence the beam comes, so these could not be used as homing stations. On the other hand, this would imply the use of 3 x 8, i.e. 24 powerful stations, which would mean that all our home wireless had to be sacrificed for this purpose. By giving up the BBC and all other transmitters, this arrangement could possibly be made in four to six weeks. Even then we should have difficulty if the Luftwaffe chose to use, instead of their normal beacons, the super high-power stations normally used for wireless purposes in France and Holland. This brings us to the second method, called ‘Masking’. For this purpose, we require a number of small stations in England which
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pick up and repeat the German signals exactly in phase. If this is done, the wireless operator in the German machine cannot distinguish between the signals from his beacon and the echo signal from our station, and his direction finding is completely set to nought. Since these echo stations are in exact phase with the ground stations it is impossible to home on them, so that they cannot be used as a navigational aid by the enemy as a German station could. They are admittedly slightly more complicated to set up, but we have already six in action and a further nine will be operating within a week. Providing the Luftwaffe do not use more than twelve stations at a time we can mask them completely with these fifteen stations so this method of navigating will be nullified. All masking beacons are being provided as rapidly as possible and it is hoped in a few weeks to be able to cope with any possible German orchestra of beacons. Obviously, if we had eighty, we could deal with them if they turned on all their eighty beacons. On the other hand it is unlikely that they will use too many at a time as it would certainly confuse their own pilots very much. Thirty stations would probably suffice for anything the Luftwaffe are likely to do.
***
By 18 August 1940, No. 80 Wing had nine ‘Meacon’ stations in operation. Two days later, the strange assortment of hastily erected ‘Headache’ stations – modified diathermy sets and Lorenz transmitters – was ready to begin radiating on the Knickebein frequencies. It was a close-run thing for, on 28 August, a force of 160 bombers delivered the first heavy night attack on a British city, Liverpool. The bombers returned to the port in similar strength on each of the following three nights. The expected night onslaught had opened. On 7 September, the bombers shifted their night attacks to London. From then until 13 November an average of 160 aircraft raided the capital each night, except for one occasion when bad weather prevented operations. This assault coincided with the deployment of the first of the jammers which Robert Cockburn and his team had designed to counter Knickebein. The RAF code-name for the German beam system was ‘Headache’, so the code-name for the antidote system – the jammer – was ‘Aspirin’. The latter
The Battle of the Beams 35


transmitted Morse dashes on the beam frequencies. Those dashes were not synchronised with the beam signals, rather they were superimposed on them. The intention was that when a bomber pilot heard the Morse dashes, he would turn in the required direction. But when he reached what should have been the central steady-note lane he continued to hear dashes and so tended to overshoot. When in the ‘dot zone’, he heard a mixture of dots and dashes which did not resolve themselves into a clear note. The ‘Aspirins’ were prescribed for the more important sites, where they replaced the less efficient modified diathermy sets and Lorenz transmitters. Both of the older types of equipment were then moved to new sites, to increase the area where jamming cover was available. Throughout this period there was discussion as to whether it might be possible to design a countermeasures system to ‘bend’ the German beams, to push the bombers off course without their crews realising it was happening. Technically speaking, such a device was feasible. Such an elegant countermeasure would have taken time to design and build, however, and Addison had to meet a major threat to Britain’s cities; he did not have time to develop subtle approaches to the problem. In the event there was never any deliberate bending of the German beams, though it is widely believed that this was the case. Possibly the story began as a ‘plant’ by British intelligence, to weaken Luftwaffe confidence in Knickebein. As Addison told this writer:
Whenever anything unusual happened, people thought it was us. At the time we worked in such secrecy that when these funny ideas got around we had no means of correcting them – even had we wanted to. On one occasion a German aircraft unloaded its bombs in the castle grounds at Windsor. The next morning, the Comptroller of the King’s Household rang me; he was very cross, and wanted to know why we had bent the beams over Windsor – His Majesty might have been killed. It was the usual case of a lost German getting rid of his bombs – and we got the credit or the blame, depending on where they fell.
Dr Robert Cockburn voiced similar views during an interview with this writer. He commented:
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The myth has been established that we bent the beams. In fact we didn’t. I did rig up a system using a receiver at Worth Travers, near Swanage, and a transmitter at Beacon Hill, near Salisbury. I was going to pick up the modulation of the Knickebein, retransmit it, and thus push the beam over. In other words, my transmitter would have produced a beam similar to the German ground station but pointing to where I wanted it to. It was all very nice, but it didn’t happen. By the time the system was ready, the other jamming methods were in full swing and we could not spare the time or the effort to bring out a new system to supplement the old. So the deliberate bending of the German beams, which I had worked out, never happened.
When No. 80 Wing took over the Beacon Hill jamming station the unit used it to transmit unsynchronised Morse dashes, just like the other ‘Aspirins’. In October 1940, Edward Addison was promoted to group captain. No. 80 Wing now comprised twenty officers and 200 men and women, and operated fifteen ‘Aspirin’ sites to jam the Knickebein beams. How effective was this effort? The British ‘official line’, published in several books including the earlier editions of this one, was that the jamming was so effective in disrupting the Knickebein beams that the system fell soon out of use. The truth of the matter is rather different, though the outcome was the same. The Knickebein transmitter at Greny, 120 miles from London, often provided the main beam to the British capital. The BeaumontHague transmitter, 150 miles from the capital, was well placed to provide the necessary crossbeam. If those two transmitters trained their beams on London they could mark out a diamond-shaped patch of sky with sides measuring about 900 yards and 1,600 yards. The threat of over a hundred German night raiders delivering bombs to that degree of accuracy was a fearful prospect, but it never happened. Luftwaffe bomber crewmen who flew over Britain at that time have told the writer that usually they could easily hear the beam signals through the early jamming. Yet, even if it was not fully effective, the presence of the jamming was unsettling to the attackers because it indicated that the defenders were aware of the beams’
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existence and probably knew their location. One Luftwaffe bomber pilot told this writer:
At first we were very excited about Knickebein, a fine new method of navigation and a big help to find our targets. But after we had used it on operations once or twice, we realised that the British were interfering with it. Initially the jamming was weak and it hardly concealed the beam signals at all. But that fact that our enemy obviously knew that the beams existed and that they were pointing towards the target for the night, was very disconcerting. For all we knew, night-fighters might be concentrating all the way along the beam to the target. More and more crews used the Knickebein beams only for range, and kept out of them on the run up to the target.
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Knickebein Beam Stations
The eleven beam stations used during the attacks on Britain are shown. The map also shows attacks made by a Ju 88 pilot with III/KG 1, flying from Roye/Amy. A: raids on Cardiff on the nights of 1 and 3 March 1941, picking up the Beaumont-Hague beam at the coast and flying from there straight to the target. B: Plymouth 21, 28 and 29 April, again using Beaumont-Hague. C: Birmingham 16 May; this time he flew north to pick up the Kleve beam then northwest to find the Stollberg beam and the route to the target. During March and April the bombers often used the same route for repeat attacks. By May it was considered prudent to make a wide detour to avoid the defences in southeast England.


Other German aircrew echoed those sentiments, which became progressively stronger as the jamming became more powerful and the night defences became more effective. The primary effect of the jamming had therefore been its effect on the morale of the bomber crews, rather than the disruption it caused to the Knickebein signals. Yet the ‘bottom line’ of the makeshift efforts of No. 80 Wing and Dr Cockburn’s team was that they had successfully neutralised the German beam system. That gave a considerable boost to their prestige both in the RAF and in the corridors of power at Whitehall. It was also a triumph for Dr R. V. Jones and the cause of scientific intelligence; the next time the watchdog started to bark, people would listen. That would soon happen, for the Luftwaffe still had its other beam system.
***
On 13 August 1940 the Luftwaffe began its large-scale aerial bombardment of targets in Great Britain. On that day the first of the hard-fought daylight battles took place over southern England. After darkness fell, twenty-one He 111 bombers attacked the Nuffield factory at Castle Bromwich producing Spitfire fighters, and the nearby Dunlop tyre factory in Birmingham. The unusual feature of this attack was the inordinately high degree of concentration for a night raid – eleven bombs hit the sprawling collection of factory buildings at Castle Bromwich. The aircraft involved belonged to the special beam-flying unit Kampfgruppe 100, and carried the complex X-Gerät beam system. After its impressive start, KGr 100 delivered night attacks on several other small targets. The high degree of bombing accuracy achieved during the initial attack was not repeated often, however, and usually the attacks were not so effective. Halfway through August, the RAF monitoring service picked up unexplained signals on 74 MHz which appeared to originate from a point on the French coast. Jones code-named the new system ‘Ruffian’. By the end of the month the monitors had noted signals on nearby frequencies, and ground direction finders pinpointed their sources to the Calais and Cherbourg areas. The signals differed from those of Knickebein in their frequency and the keying rate, but they were sufficiently similar to identify them as an aid to navigation. During the following month Jones added further items
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to his file on ‘Ruffian’ and after his success with Knickebein, his new fears found ready ears. At the end of the third week in September his report, passed via Professor Lindemann, reached the prime minister’s desk:
It appears that the Luftwaffe are making great efforts to improve the accuracy of their night bombing. A number of new beams on a shorter wavelength than before have appeared. . . One Kampfgeschwader, KG 100 consisting of about forty machines [sic – in fact it was a Kampfgruppe, though the unit’s strength was given correctly], has been equipped with special new apparatus to exploit these beams with which apparently accuracies of the order of 20 yards are expected. With the technique they seem to be developing such a result does not seem impossible. We know the exact location of the sources of the beams in question. The parent beam is on the very tip of the Cherbourg peninsula; the crossbeams are in the Calais region. They will probably not reach much beyond London. Apart from attacks on the machines using the beams, our possible lines of defence would be: 1. to try to destroy the specially fitted KG 100 machines which are stationed at Vannes; home station Lüneburg and reserve station at Köthen; 2. to try to destroy the beam stations (a) by bombing, which would be very difficult since they are almost invisible targets (b) by a special [i.e. commando] operation; 3. to employ radio-countermeasures.
Early on in the investigation, RAF intelligence identified the distinctive footprint of the new series of attacks: great accuracy along a line running from Cherbourg, with a somewhat lower accuracy in range. The heaviest bombs used were 550-pounders. From the early summer of 1940 Luftwaffe signals personnel had worked hard to establish landline connections between each of the major bases and airfields in France, and that service’s main communications network in Germany. Starting in eastern France, they gradually worked their way westwards. As each airfield came on line it ceased using wireless for ground-to-ground communications, a move which deprived the cipher crackers at
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Bletchley Park of the opportunity to read their traffic. As luck would have it, however, KGr 100’s base at Vannes lay in the extreme west and so was one of the last to be connected to the network. Thus, from the autumn of 1940 until well into 1941, Bletchley Park was able to deliver to Jones’s office a steady stream of decrypted signals on KGr 100’s activities. On occasions the intercepted signals listed the beam frequencies and alignments for a night’s attack. Yet, at that stage of the war, the painstaking task of decryption often took several days. Although the raids were therefore over long before Jones read the signals, the information they contained was invaluable to build up a general picture of how the system worked. He noted that the beams were aligned to within five seconds of arc, which implied a maximum accuracy of the order of twelve feet at a distance of 100 miles from the beam transmitter. From this, Jones estimated accuracy of the new system to be ‘of the order of twenty yards’. In fact his report exaggerated the accuracy of the system by a factor of about six, but even so it represented a potent threat. To counter the X-beams Dr Cockburn and his team hastily modified the transmitter from an Army gunlaying radar to jam the beam frequencies, and code-named the device ‘Bromide’. When the prototype appeared to work satisfactorily, Cockburn’s section began a crash programme to build sufficient ‘Bromides’ to provide cover for potential targets. The immediate plan was to install these jammers at ground stations between Cherbourg – the source of the approach beams – and the Midland towns, Manchester and London. By the end of September KGr 100 had taken part in more than thirty attacks, more than half of which were on London. For the rest of the attacks the unit usually visited targets alone, attempting precision attacks using the beams. Early in October, RAF intelligence noted that KGr 100 had started dropping 1-kg stick-shaped incendiary bombs during some of its attacks. On the face of it that was a strange development; these small weapons scattered over a large area and could not be aimed accurately, which seemed to nullify the major advantage of the X-beam system. There seemed only one reasonable explanation for the change: KGr 100 was practising to lead the rest of the Luftwaffe bomber force to its targets. Apprised of the new information, Lindemann advised Mr Churchill on 24 October:
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There is some reason to believe that the method adopted is to send a few KG 100 aircraft fitted with special devices to assist in blind bombing on these expeditions, in order to start fires on the target which any subsequent machines without special apparatus can use.
The note accurately predicted the course the Luftwaffe would adopt a few weeks later. In the meantime, fate played into the hands of British intelligence in one of the more inept episodes of the secret war. Early on the morning of 6 November, a raiding Heinkel bomber suffered a compass failure over England. After tuning their radio compass to the beacon at Saint-Malo in Brittany, the crew turned for home. When the radio compass indicated that the aircraft had passed over the beacon, the aircraft descended but as it broke the cloud the pilot saw he was still over the sea. This had to be the Bay of Biscay, so he reversed his course and headed back to the beacon. By now his fuel was almost exhausted and, when a coastline came into view, he decided to set the bomber down on the beach. The pilot misjudged his approach, however, and in the resultant crash one crewman was killed and two were injured. The survivors scrambled up the shingle beach where, to their great surprise, they were immediately surrounded by soldiers in khaki uniforms. The beacon in which the airmen had misplaced their trust had been covered by a No. 80 Wing ‘Meacon’ transmitter at Templecombe in Somerset. What the crew had thought to be the southwest coast of Brittany, was in fact the beach at West Bay near Bridport. Some soldiers waded out to the wreckage and secured a rope round it, and all would have been well had a Royal Navy vessel not arrived. The ship’s captain pointed out that, since the bomber was in the sea, its salvage was technically a Navy matter. After some wrangling, the Army grudgingly agreed. The sailors took the line aboard their ship and solemnly towed the aircraft out into deeper water, preparatory to lifting it out. Unfortunately, in the course of that operation, the rope snapped and the Heinkel sank to the bottom. Dawn broke to reveal the upper part of the wrecked bomber protruding from the waves, looking rather like a stranded whale. As the tide receded, the plane’s markings gradually came into view; painted on the rear fuselage in characters nearly four feet high were
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the identfication markings 6N+BH. And 6N was the unit code for KGr 100. Professor Lindemann was understandably bitter when he learned what had happened. A week later, he wrote to the prime minister:
The KG 100 squadron is the only one known to be fitted with the special apparatus with which the Luftwaffe hope to do accurate night bombing using their very fine beams. As it is important to discover as much as possible about this apparatus and its mode of working, it is a very great pity that inter-Service squabbles resulted in the loss of this machine, which is the first of its kind to come within our grasp.
All was not lost, however. RAF technicians removed the invaluable X-beam receivers from the waterlogged hulk and, looking somewhat the worse for their immersion, they went to Farnborough for examination. Any last doubts regarding the advanced nature of German radio-beam technique were dispelled by the dates on some of the receivers’ inspection-stamps. They went back to 1938. In the second week of November, the Luftwaffe shifted the burden of its attack from London to the Midlands cities. In the first of these attacks KGr 100 was to lead the bomber force to its target, Coventry. On the afternoon of 14 November the Boulogne headquarters of 6th Air Signals Company – the unit which operated the X-beam transmitters – received its orders on beam alignments from KGr 100’s headquarters at Vannes. It relayed these instructions to transmitters Elbe, Oder and Rhein nearby, and to Weser, the ‘approach-beam’ transmitter, on the Cherbourg peninsula. The approach-beam crossed the English coast near Christchurch, ran to the east of Salisbury and Swindon, and passed over Leamington and Coventry. Shortly before the target city the three ‘crossbeams’ intersected it. That night KGr 100 sent thirteen He 111s to mark Coventry. Soon after 1900 hours, the leading aircraft crossed the Thames. Six minutes later it flew through the first of the crossbeams, and moved into the centre of the main approach beam aligned on the city. On that night, only four ‘Bromide’ transmitters were available to counter the X-beams. One of these, at Kenilworth, lay almost directly beneath the bombers’ run in. Yet the jammers caused the raiders little trouble; the system had been conceived in haste at a
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time when too little had been known about X-Gerät. Consequently, although the jammers probably emitted on the correct frequency, the radiated note was modulated at 1,500 cycles instead of 2,000 cycles. The difference – between a whistle and a shriek – is just perceptible to the human ear; but the filter circuits in the German receivers were sensitive enough to pick the beam signals out of the jamming with ease.
The leading Heinkel continued on its northerly course undisturbed and at 1906 hours, three miles south of Leamington, it flew through the second crossbeam. The observer started the automatic bomb-release clock and two and a half minutes later, about a mile east of Bagington, the aircraft flew through the third and final crossbeam. The second pointer on the clock began moving rapidly to catch up with the first. Fifty seconds later the two pointers
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Layout of the X-Beams over Coventry, Night of 14 November 1940


overlapped, the pair of electrical contacts closed and the bombs were released. The time was 1920 hours. During the next 45 minutes, the twelve remaining Heinkels from KGr 100 dropped their bombs on the city, starting some fires. By then the first of the main force of bombers had arrived, and in the hours to follow their loads reinforced the destruction already caused. Yet, even without the assistance of KGr 100, it is likely that the main force of bombers would have found the target with little difficulty. It was a bright moonlight night, with clear skies. Feldwebel Günther Unger of Kampfgeschwader 76 (KG 76), piloting a Dornier 17, attacked later that night. He recalled:
While we were still over the Channel on the way in we caught sight of a small pinpoint of white light in front of us, looking rather like a hand torch seen from two hundred yards. My crew and I speculated as to what it might be – some form of beacon to guide British night-fighters, perhaps. As we drew closer to our target the light gradually became larger until suddenly it dawned on us: we were looking at the burning city of Coventry.
One stream of bombers came in over the Wash, another over the Isle of Wight and a third over Brighton. Altogether 449 bombers hit Coventry during the ten hours of the attack. Between them they dropped 56 tons of incendiaries, 394 tons of high-explosive bombs and 127 parachute mines. The city was hit heavily and several factories were forced to cease production, albeit temporarily. Nearly 400 people were killed and a further 800 were seriously injured. The attackers had the rare benefit of a combination of perfectly clear skies, a full moon, a combustible target (there were many old timbered buildings), and weak anti-aircraft defences. Although the X-Gerät beams played a part in the raid’s success, this should not be exaggerated. Crews in the follow-up bomber units, including Günther Unger, have said the night was so bright that they would have found the city even without the assistance of marking by KGr 100. Meanwhile, examination of the captured X-Gerät receiver had revealed the weakness of the ‘Bromide’ jammer, and within a few days of the attack they had all been modified to transmit the correct jamming note. With modified ‘Bromides’ emerging from Cockburn’s workshop at an encouraging rate, it seemed that a much
The Battle of the Beams 45


more effective No. 80 Wing was ready to counter KGr 100’s next major pathfinder marking effort. Birmingham came under attack on the night of 19 November (by thirteen bombers from KGr 100 and 344 aircraft in the followup force), on the 20th (eleven and 105) and the 22nd (nine and 195). Yet that city suffered nothing like the concentrated damage inflicted on its neighbour. Nor, during the months to follow, would any other British city. At the time British intelligence attributed the sudden deterioration in the effectiveness of KGr 100-led attacks to the improvements made to the ‘Bromide’ jammer. With the benefit of hindsight, however, it is possible to offer a quite different explanation for the failure to repeat the destruction inflicted on Coventry. The steady improvement in Britain’s night air defences during the winter of 1940 forced the Luftwaffe to cease large-scale attacks when there was a full moon. Also, during that period, the raiders had to contend with frequent spells of bad weather. So, when Kampfgruppe 100 led attacks on nights when there was little or no moon and poor weather, against targets that were better defended than Coventry and less combustible, it is not surprising that the results were less impressive. Although other cities suffered damage comparable with that suffered by Coventry in relation to their size – notably Liverpool and Plymouth – in their case it was the cumulative effect of several attacks and not just one. Even when X-Gerät worked perfectly, the weak system of target marking usually mitigated against an effective attack. The Luftwaffe was evolving its technique from scratch, and it still had a lot to learn. For one thing, it employed far too few pathfinder aircraft. Sending ten or a dozen bombers mark the target, for a follow-up attack that might last six hours or more, was not enough. Moreover, the 1-kg ‘stick’ incendiary bomb dropped by KGr 100 to start fires had poor ballistics and was a relatively inaccurate weapon. Even if the initial fires were accurately laid, the pathfinder marking rapidly became diluted as bombers in the follow-up force dropped much greater numbers of incendiary bombs with varying degrees of accuracy. Since the bombers all carried more or less the same types of high explosive and incendiary bombs, it was impossible to tell which fires had been started by pathfinders and which by the follow-up bombers. If there was cloud cover, the follow-up bombers often
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failed to find their target even if the initial marking had been accurate. In a later chapter we shall observe how the RAF applied the lessons it learned from the Luftwaffe attacks in 1940 and 1941. In the autumn of 1940, No. 80 Wing took control of a special unit to counter the Luftwaffe pathfinder tactics. If the follow-up raiders aimed their bombs at fires on the ground, why not light fires in the countryside for them to bomb? The job of establishing the necessary decoy fires – known as ‘Starfish’ – fell to a section headed by Colonel J. Turner, one-time head of the RAF Works Department. Since the decoys needed to look plausibly like cities under attack, timing was critical. The fires had to be well alight in time to catch the follow-up attackers, and ideally the bombers should need to fly over a decoy to reach the real target. That called for careful control and good communications. The ‘Starfish’ operation was an integral part of No. 80 Wing’s activities, directed from its headquarters. By the end of November twenty-seven decoy sites were ready for action. The first two ‘Starfish’ were ignited on the night of 2 December 1940, just over two weeks after the Coventry attack, during a raid on Bristol. In the course of that action the sites collected sixty-six high-explosive bombs. From then on, the ‘Starfish’ were a regular feature of Britain’s passive defences. If the local fire and civil defence services were able to extinguish the pathfinders’ fires promptly, decoy fires often drew away a large proportion of the bombs intended for city targets. By February 1941, No. 80 Wing had sufficient ‘Bromide’ transmitters to jam all of the approach and crossbeam frequencies. That brought about a significant improvement in jamming effectiveness, and a resultant deterioration in the performance of X-Gerät. Now, when KGr 100 delivered an attack on its own, the unit’s ‘bomb signature’ often showed that a large proportion of its bombs had fallen outside the target area.
***
In November 1940 the RAF monitoring service noticed more unusual signals on frequencies in the 42–48 MHz band, and R. V. Jones allocated these the code-name ‘Benito’. The new system, called Y-Gerät by the Germans, was another brainchild of Dr Hans Plendl. This employed a single beam made up of 180 directional signals per minute, which was aligned on the target. That was too
The Battle of the Beams 47


fast for human interpretation, and the aircraft carried an electronic analyser to determine its position relative to the beam. To measure the aircraft’s range from the ground station, the ground station transmitted additional signals which the aircraft picked up and re-radiated on a different frequency. The ground station then measured the range using normal radar methods. When the aircraft arrived at the pre-computed bomb-release point, the ground station transmitted the bomb-release signal. Since it used only one ground station, Y-Gerät was more flexible than either of the predecessor systems. It was more accurate than X-Gerät, though it was also a great deal more complex. General Wolfgang Martini, head of the Luftwaffe signals service, later recounted how he tried to explain the operation of Y-Gerät to Hermann Göring. The Reichsmarschall listened for about two hours, then asked a few questions which showed he was none the wiser. Göring, a World War I fighter ace, had little time for such things. He thought wars should be fought by brave men with guns, not like this. He is said to have commented on another occasion, ‘Radio aids contain boxes with coils, and I don’t like boxes with coils.’ It is hard not to feel for him. After a false start in the summer of 1940, Plendl’s new system resumed operations at the end of the year. The Y-Gerät was fitted to the He 111 bombers of III Gruppe of KG 26 (III/KG 26) based at Poix near Amiens, and beam transmitters were situated at Poix, Cherbourg and Cassel in France. When he examined the Y-Gerät signals, Dr Cockburn noted that there were separate transmissions to fix the aircraft’s bearing and range from the ground beacon. To counter the system, therefore, he worked out a separate jamming method for each. At the time it was discovered the new beam system was still at the working-up stage, so Cockburn was under no great pressure to rush his jammers into operation. He even had time to introduce subtlety into his countermeasures. The prototype of Dr Cockburn’s new jammer for Y-Gerät code-named ‘Domino’ – employed a receiver at Highgate and the BBC’s dormant television transmitter at Alexandra Palace north of London. The receiver picked up the ranging signal from the bomber’s transmitter and passed it to Alexandra Palace, where the powerful equipment there returned the ‘echo’ to the German
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ground station to produce an erroneous range indication. The first ‘Domino’ went on the air in February 1941, soon followed by another at Beacon Hill near Salisbury. It soon became obvious that the ‘Domino’ transmitters were causing embarrassment to bomber crews relying on the Y-Gerät. On 9 March, the Y-beam signals changed frequency in the middle of an operation in an attempt – unsuccessful – to shrug off the jamming. Two nights later a small force of bombers attacked the Beacon Hill jammer, and one scored a near miss which put the station out of action for a few days. On the following night that jammer was still off the air, when III/KG 26 operated once more. The Alexandra Palace jammer covered the transmitter at Cassel, and none of those crews received the bomb release signal. There was no ‘Domino’ cover for the Beaumont-Hague transmitter, however, and crews using that ground station made accurate attacks. On the next night the Beacon Hill station was back on the air, and full jamming cover returned. Of eighty-nine Y-Gerät sorties flown over England during the first two weeks of March 1941, only eighteen aircraft received the bombrelease instruction. On the night of 3 May 1941, during an attack on Liverpool and Birkenhead, the Y-beam unit lost three Heinkels shot down. In each case the Y-Gerät equipment was removed from the wreckage and sent to Farnborough for examination. This revealed that between each pair of direction signals there was a short gap, which locked the electronic bearing-analyser to the beam. Later Dr Cockburn commented:
Unlocking the Y-beam was a piece of cake: they had fallen into the trap of making things automatic, and when you make things automatic they are more vulnerable. All one had to do was radiate a continuous note on the beam’s frequency. This filled in the gap between the signals, unlocked the beam analyser and sent the whole thing haywire.
Dr Cockburn’s new jammer, code-named ‘Benjamin’, first went on the air on 27 May 1941. By then, however, a major development in the German war strategy had brought about a profound change in the air war over Great Britain. During May 1941 the bulk of the Luftwaffe bomber force moved to bases in eastern Europe, in preparation for the long
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planned attack on the Soviet Union. On 21 June the offensive began, and the intensity of the night Blitz suddenly gave way to the boredom of waiting for an enemy who rarely came. An air of almost unreal calm descended on Great Britain, but Group Captain Addison could not rely on the respite lasting for long. If German forces achieved a rapid victory in the east, the Luftwaffe would resume its onslaught on Great Britain. During the summer and autumn of 1941 No. 80 Wing continued its build-up, and replaced the last of the makeshift jammers with purpose-built systems. By September the unit had a strength of some 2,000 men and women of all ranks. It operated eighty-five ground stations scattered throughout the British Isles, and controlled more than 150 ‘Starfish’ decoys.
***
So ended the first-ever electronic warfare action. Knickebein had surrendered almost without a fight. X-Gerät proved more difficult to jam, but in the time available its capabilities were not exploited sufficiently to provide effective target-marking for Luftwaffe bombers. Y-Gerät played only a minor role, its effectiveness much reduced by jamming, before the bombing campaign ended. We now know that the jamming of Knickebein and X-Gerät was less disruptive than had been thought at the time. Yet, although the initial jamming of Knickebein was insufficiently powerful to conceal the beam signals, it dissuaded Luftwaffe crews from using their beams when flying over England. Although the Luftwaffe reaction to the various countermeasures had been slow in coming, it would unwise to conclude that Luftwaffe technicians could not have modified their radio aids to operate more effectively in the face of the jamming. In fact, before they could do so, the night bombing offensive against Britain had come to a halt. As we shall see, the Luftwaffe could do much better than this.
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Chapter 2
The Instruments
‘I have done my best during the past few years to make our air force the largest and most powerful in the world. The creation of the Greater German Reich has been made possible largely by the strength and constant readiness of the air force. Born of the spirit of the German airmen in the First World War, inspired by its faith in our Führer and Commander-in-Chief – thus stands the German Luftwaffe today, ready to carry out every command of the Führer with lightning speed and undreamed-of might.’
Order of the Day from Hermann Göring to the Luftwaffe, August 1939
Radar, like many major inventions in the twentieth century, did not result from a sudden and inspired line of thought pushed to the point of fulfilment by a single inventor. As with many innovations, the basic idea preceded the invention by several decades. Only when each of the major components had been developed, could its realisation become practicable. Again, as with many other inventions, once the background work was complete, development of the device proceeded independently in several nations simultaneously. In the case of radar, by the early 1930s the major components necessary to assemble such a system already existed. These were: a high powered pulsed transmitter; a very sensitive receiver; a device for measuring small time differences very accurately (the cathoderay tube); and a highly directional aerial system. During that decade scientists working independently in Great Britain, the USA, France, Germany, the Netherlands, Japan and the Soviet Union all produced working radars. Each nation claimed the device as its own, and in each the fighting services believed that it offered them a unique advantage. Since they would be the target systems in the next phase of the electronic warfare battle, however, the account that follows will concentrate on radar developments in Germany.
***


When World War II began in September 1939 the German armed forces had two separate types of radar in service and a third was at the advanced testing stage. For early warning against air attack the Gema Company had produced the Freya radar operating on frequencies around 120 MHz, initially with a maximum range of about seventy-five miles. Gema also produced the Seetakt radar operating on 370 MHz, for installation in warships and at shore batteries. This radar provided surface-search and also accurate range information to assist naval gunnery. Late in the 1930s the rival Telefunken Company also entered the field of radar, and its Würzburg equipment was the impressive result. When the war began this equipment was still in the trials stage. The small, highly mobile set operated on what was then the extremely high frequency of 560 MHz, and could plot the position of aircraft to within fine limits at ranges up to twenty-five miles. The Würzburg was the first radar in the world with the precision to allow anti-aircraft gunners to engage targets accurately at night or through cloud. Also at this time, Telefunken had commenced testing a small airborne radar. How well did the German radars compare with their British equivalents at the outbreak of World War II? The Freya, the only German early-warning equipment, had a maximum range of seventyfive miles, its rotating aerial array gave it full 360-degree cover and its wheeled carriage allowed a high degree of mobility. Yet, it could not measure the altitude of approaching aircraft. Its nearest British equivalent, the ‘Chain Home’ operating on the far lower frequencies between 20 and 52 MHz, had a maximum range of 120 miles and could determine the altitude of approaching planes. But the ‘Chain Home’ stations gazed out to sea with a fixed 120-degree angle of look, and for technical reasons the radar could not give reliable plots on aircraft flying over land. Moreover, the four 300-foot high towers – each twice the height of Nelson’s column – supporting each station’s transmitter aerials precluded mobility. It was not in its radar hardware that the RAF had established a lead, but rather in the way in which the information was exploited. Only in Fighter Command could reliable and up-to-date information based on radar plots be passed to fighter pilots by radio. By September 1939 the RAF had nineteen ‘Chain Home’ radars operational, covering the approaches to the east coasts of England
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and Scotland. Its system of fighter control had been developed and tested during scores of exercises. The Luftwaffe had made no attempt to perfect such a system of its own before the war. Since it had little to fear from hostile bombers, that service had logically concentrated its energies on offensive developments – like the radio beams. The Seetakt and Würzburg precision radars were the most advanced equipments in the world in their respective categories. When war broke out, the Royal Navy had no equivalent of the Seetakt for its ships nor would it have one for another two years, and the Würzburg was considerably in advance of its nearest British equivalent in both range and plotting accuracy. But Britain had established a lead in the development of radar sets small enough to be carried in aircraft; there were two types on the point of entering service, one for coastal patrol aircraft and one for night-fighters.
***
In the autumn of 1939 the Luftwaffe had eight Freya stations operating on the chain of islands along the northwest coast of Germany: two on Heligoland, two on Sylt, two on Wangerooge, one on Borkum and one on Norderney. At that time the RAF was prohibited from attacking targets on the German mainland, because this would endanger civilian lives. It therefore sent bombers in probing attacks, to feel out the defences with attacks on German warships in the Heligoland Bight. The first three daylight bombing attacks were inconclusive. Then, on 18 December 1939, twenty-four Wellingtons of Nos. 9, 37 and 149 Squadrons set out from their bases in East Anglia to patrol the Schilling Roads, Wilhelmshaven and the Jade Roads. Two of the Wellingtons turned back early with technical problems, the rest continued with the mission. Just after midday, a Freya radar on the pre-war holiday island of Wangerooge picked up the approaching Wellingtons at a range of seventy miles. The operator reported the formation to the nearby fighter station at Jever and, after a delay, the defending fighters, sixteen Messerschmitt Bf 110s and thirty-four Messerschmitt Bf 109s, took off to engage. By then, having found no warships at sea, the bombers had turned around and were heading for home. It was a clear winter’s day and the fighter pilots could see the RAF formation from several miles away. They quickly caught up with the
The Instruments 53


raiders and, in the heated engagement that followed, bomber after bomber went down. Of the twenty-two bombers involved in the action, only ten regained friendly territory. So the RAF had learned the hard way, as the Luftwaffe was to learn during the Battle of Britain and the US Army Air Force would learn in 1943, that formations of bombers operating over enemy territory by day without fighter escort risked heavy losses. The lesson was clear, and throughout most of the conflict that followed RAF Bomber Command sought to avoid the German defences by delivering attacks under cover of darkness. On 14 May 1940, following the destructive Luftwaffe attack on the city of Rotterdam, Prime Minister Winston Churchill lifted the ban on air attacks on the German mainland. By 4 June, RAF bombers had flown some 1,700 night sorties over Germany at a cost of thirtynine aircraft, most of them lost in accidents. Compared with what Bomber Command would achieve later in the war, those early operations were no more than gestures of defiance. Yet they caused a degree of consternation in Germany. Had not Hermann Göring declared that the Ruhr industrial area would not be exposed to a single bomb from an enemy aircraft? After inspecting the AA gun defences in the Essen area in August 1939, Göring’s imagination had been captivated by the Würzburg radar. Here was a device to enable the gunners to engage enemy planes even through the thickest cloud or at night. It was the success of the early Würzburg trials that had inspired him to make his muchquoted declaration regarding the invulnerability of the Ruhr. Yet the introduction of Würzburg into service took somewhat longer than anticipated, and the first sets did not become operational until the summer of 1940. Lacking radar, the gunners sought out the night raiders using searchlights and largely ineffective sound locators, with poor results. Göring was not at all satisfied by this – while his reputation had suffered, the raiders had not. Since the AA guns alone could not inflict losses sufficient to deter the night raiders, he decided to form a specialised night-fighter force. Up to that time a few intrepid pilots had flown night patrols in single-seat Bf 109 fighters, and on occasions they had shot down raiding bombers. Yet, lacking any formal system of ground control, an interception at night remained a matter of chance.
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In July 1940 Göring summoned Oberst Josef Kammhuber to his headquarters, and ordered him to set up a specialist night-fighter unit and a ground-control system to support it. Kammhuber was forty-three when he took on his new appointment. In his previous posts he had demonstrated that he was a methodical worker, displaying great drive tempered by sound judgement. He was to use these qualities to the full in his new post. Kammhuber worked fast. By mid-August 1940 the first specialised night-fighter unit, Nachtjagdgeschwader 1 (NJG 1), had a strength of seventy Bf 110s, seventeen Ju 88s and ten Do 17s. None of these planes carried radar or other specific equipment for their new role, however. Backing these fighters, on the ground, was a regiment of searchlights and a few Freya early-warning radar sets. Kammhuber was promoted to Generalmajor and established his headquarters in a seventeenth-century castle at Zeist near Utrecht. The night-fighter organisation was subordinated to Generaloberst Hubert Weise, responsible for overall command of the air defence of the Reich. During the summer and autumn of 1940, radar-directed nightfighting was in its infancy. Usually fighters were scrambled when early-warning radar stations on the coast reported approaching raiders. After take-off the night-fighters flew to assigned radio beacons, which they orbited until searchlights nearby illuminated an enemy bomber. Then, with his target in sight, the night-fighter pilot closed in for the kill. Known as ‘illuminated night-fighting’ (Helle Nachtjagd), this system achieved some success. The early engagements revealed an important weakness in the system, however. The searchlights were positioned around the bombers’ potential targets, as were the AA guns. There was no effective system of identification, with the result that night-fighters often came under fire from the ground. Quite apart from the loss of men killed or wounded, and in aircraft destroyed or damaged, that made the system manifestly inefficient. The gunners were shooting at fighters, and the fighter pilots had to manoeuvre to avoid the shells, at a time when both should have been engaging enemy bombers. To prevent such ‘friendly fire’ incidents, Kammhuber saw that it was important to designate separate engagement zones for AA guns and night-fighters. He therefore moved his searchlight batteries into
The Instruments 55


the countryside, well clear of the AA gun defences, to form a defensive belt that ran parallel to the coast from Schleswig-Holstein to Liège in Belgium. Raiding aircraft would have to pass through that belt, to reach their targets in Germany. Kammhuber subdivided his searchlight belt into a series of ‘boxes’ about twenty miles wide, each with a radio beacon where a night-fighter would orbit while waiting for the enemy raiders to appear. Having formed his defensive line, Kammhuber set about improving its effectiveness. He saw that any system that relied on searchlights was a slave to the weather. The answer was to use a precision radar on the ground to direct a night-fighter into an attacking position behind an enemy bomber. The Freya was too imprecise for that task, for the ‘blips’ from the night-fighter and the bomber usually merged on the radar screen long before the nightfighter pilot had his quarry in visual range. The Würzburg flakcontrol radar, with its very much higher frequency and superior resolving power, offered a much better prospect. By the end of 1940, production of this radar was getting into its stride and Kammhuber secured a few to direct experimental night interceptions. The initial trials with Würzburg demonstrated the advantages of using radar to direct night-fighters, and Kammhuber requested more sets to equip his defensive line. This now comprised a series of contiguous boxes, forming a barrier which raiding bombers had to cross to reach their targets and again on their return flights. Each night-fighter box was equipped with a Freya and two Würzburg radars, a radio beacon and a ground-control station. The Freya directed one short-range narrow-beam Würzburg to track the nightfighter, while the other tracked the incoming bomber. The Würzburg equipment been designed to direct AA guns and its form of presentation – giving raw range, bearing and elevation information – made it impossible to direct night-fighters from the radar screen. To convert the Würzburg information into a form in which it could be used by the fighter controller, the Luftwaffe developed the so-called ‘Seeburg table’. Located in the headquarters building of each ground-control station, the device looked like a large dais with two flights of steps leading up to a ‘table’ at the centre. The ‘tabletop’ consisted of a horizontal frosted glass screen, bearing an outline map of the area and the Luftwaffe fighter grid.
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Directly beneath this glass screen sat two operators for the lightprojectors. Each received telephoned plots on the aircraft he or she was tracking, from the appropriate Würzburg. One operator projected onto the screen above a red light to indicate the bomber’s position, the other projected a blue spot to indicate the fighter’s position. As the coloured spots of light moved across the frosted glass screen, plotters at the top of the ‘dais’ marked the planes’ respective tracks using a coloured wax crayon. The fighter controller stood over the frosted glass screen, passing radio directions to the fighter pilot to bring him into a position to engage the bomber. The ‘Seeburg table’ became a standard item of equipment at each fighter-control station. This method of close controlled night interceptions was code-named Himmelbett.
Initially Kammhuber positioned his Würzburg radars immediately in front of the searchlight belt. That allowed night-fighter pilots to attempt a radar-controlled interception first. If that failed, they could then resort to the tried and tested ‘illuminated night-fighting’ tactics. The radars thus increased the effective ‘depth’ of the searchlight belt, allowing Kammhuber to redeploy searchlights to extend the line from eastern France to the middle of Denmark.
The Instruments 57
The Seeburg Plotting Table


Yet, although the Würzburg radar gave a useful improvement in the effectiveness of the defensive line, it soon became clear that the device’s range performance fell short of what was needed. All too often, Allied bombers emerged from the line and passed out of Würzburg range before the night-fighter could make a successful interception. After prompting from Kammhuber, Telefunken modified the Würzburg to overcome this deficiency. In the spring of 1941, the company produced a variant of the radar with the diameter of the reflector-dish increased from ten feet to twenty-five feet. That narrowed the width of the radar beam, and enabled the equipment to track aircraft up to forty miles away. The new Telefunken radar was called the Giant Würzburg (Würzburg Riese). Apart from its larger reflector and static mounting, electronically it was little different from the smaller version. During the second half of 1941, the first Giant Würzburg radars entered service to replace the standard sets used for night-fighter control. Following a reorganisation of the night air defences early in 1942, it was decided that, with this influx of new electronic equipment, Kammhuber no longer needed the searchlights in his defensive line. At the time he contested the decision, though later he would admit it was for the best. It forced night-fighter crews to trust their ground controllers to guide them to their targets and, when the crews became accustomed to it the system was more effective than ‘illuminated night-fighting’ could ever have been. During the spring of 1942, Kammhuber’s line was strengthened by the introduction of three new radar devices. The formula of using a larger aerial reflector to narrow the radar beam and increase range, which had worked so well in the case of Würzburg, also succeeded with Freya. The result was the Mammut (‘Mammoth’) built by the I. G. Farben company. This was essentially a Freya with a greatly enlarged reflector ninety feet wide and thirty-five feet high – about the size of a tennis court on its side. The structure was fixed on supporting pylons, and determined the azimuth of aircraft by ‘swinging’ the beam electronically through a limited arc of 100 degrees. The enlarged reflector squashed the radar beam into a narrow pencil, which reached out to aircraft up to 185 miles away. Like its smaller predecessor, Mammut could not measure altitude. The second new radar, the Wassermann, built by the Gema company,
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gave accurate height, range and bearing readings on aircraft up to 175 miles away. This set employed a reflector 130 feet high and 20 feet wide, mounted on a rotating tower. Wassermann was the finest early-warning radar to be produced by either side during World War II. The Luftwaffe installed Mammut and Wassermann sets along the coast of occupied Europe, to extend the range of its earlywarning cover. Those two new sets were both early-warning systems. The third new radar was of an entirely different character: the Lichtenstein lightweight airborne radar, designed for installation in night-fighters. Another Telefunken product, the Lichtenstein operated on frequencies in the 490 MHz band. It had a maximum range of two miles and a minimum range of about 200 yards. That minimum range figure was an important parameter in a night-fighter radar. While the radar transmitted each high powered pulse, the sensitive receiver had to be ‘switched off’ or it would suffer severe damage. That meant the receiver could not pick up echoes from the nearest targets. That dead distance, between the fighter and the closest target that could be seen on radar, was proportional to the length of the transmitted pulse. In fact, the 200-yard minimum range was low for a first-generation airborne radar. The new airborne radar went into production and the first four night-fighters fitted with Lichtenstein arrived at the operational airfield at Leeuwarden in the Netherlands in February 1942. In ser vice, the shortcomings of the device became clear. The entanglement of aerials and reflectors on the aircraft’s nose increased the drag, impaired handling and reduced the top speed of the Ju 88, for example, by 6 mph. Initially, pilots were unwilling to accept those penalties for the privilege of carrying a radar of uncertain reliability. Paradoxically, the main reason for their conservative attitude was the high quality of the ground control using the Giant Würzburg, which usually placed the fighter within visual range of its target. Hauptmann Ludwig Becker and his crew persevered with the Lichtenstein equipment, however. They found that when the set could be coaxed into working properly – and it had its share of teething troubles – it offered considerable advantages, particularly when engaging the British bombers on moonless nights. As Becker’s score of kills accordingly mounted, other crews began to accept the device.
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By March 1942, the German defences were destroying an average of four bombers out of every hundred attacking Germany by night. Of these the fighter defences were responsible for about two-thirds, while AA guns accounted for most of the remainder. By now there were four Geschwader of night-fighters with 265 aircraft, of which 140 might be available on any one night. The force continued its expansion. During April it accepted thirty-three Bf 110s, twenty Ju 88s and thirty Do 217s. Telefunken had already manufactured 275 Lichtenstein radars, and production was running at about sixty sets per month.
On the ground-radar side as well, there was a steady strengthening of Kammhuber’s line. To equip the whole of the defensive line as it then stood, he needed 185 Giant Würzburg sets. By the end of March 1942, Telefunken had delivered about half that number, and the rest were following at the rate of about 30 per month. Although the defences were taking a steadily mounting toll of the British night raiders, Kammhuber’s system of fighter control had an obvious Achilles’ heel. For its success it depended on the Lichtenstein, Freya, Mammut, Wassermann and Giant Würzburg radars, and on effective communications between the fighter pilots
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Himmelbett fighter-control stations at the end of 1942
Interception zone covered by original Himmelbett chain Himmelbett stations
Night-fighter aerodromes
Approximate limit of German early-warning radar cover, against aircraft flying at 10,000 ft


and their ground controllers. In some degree, all these systems were vulnerable to interference. In the next chapter we shall observe how the British intelligence service stripped away the veil of secrecy surrounding that organisation.
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Chapter 3
Discovery
‘The relief offensive for which Britain’s badly harassed Allies have been begging for such a long time has confined itself to the landing of a few parachutists on the coast of northern France. The parachutists were soon forced to make a glorious retreat across the ocean, without having achieved any useful purpose.’
German News Broadcast, 28 February 1942
Throughout 1941 and 1942, RAF Bomber Command was losing a steadily increasing proportion of the planes dispatched against targets in Germany. Yet, until there was detailed knowledge on how the air defences operated, it was impossible to devise appropriate countermeasures. The intelligence sources that had been so useful during the ‘Battle of the Beams’ – the crashed German aircraft, captured aircrew and the analysis of the beams themselves – were now denied, for the RAF bombers were destroyed over enemy territory. Moreover, because the more sensitive items of information concerning the air defence system were usually communicated via landlines, there was little radio traffic on this subject for Bletchley Park to decipher. In consequence, it took many months of hard work to expose the workings of the system Josef Kammhuber had created. In the absence of firm evidence before the war, British scientists had regarded with scepticism the possibility that their German counterparts might also be working on radar, though there was little doubt that they could build such systems once they had the basic idea, since they were known to be advanced in high-frequency radio techniques. In mounting the listening operation with the Graf Zeppelin, the Luftwaffe had concentrated its search for signals on the frequencies used by its radar systems; now British intelligence was taking much the same approach, with a similar lack of success. The first clue that the Germans were working on radar came from the so-called Oslo Report, received via the British Embassy in Oslo in November 1939. Probably sent by a disaffected German scientist,


the document mentioned several military technical systems then under development in Germany. There were references to a remotely-controlled glider carrying an explosive warhead, homing torpedoes and remotely-controlled shells. There was a short description of a warning system able to detect aircraft out to a range of 120 km (75 miles) using ‘pulses reflected by the aircraft’. The report added that, during one of the British air attacks on Wilhelmshaven, the stations covering the northwest coast of Germany had detected the RAF bombers at a range of 120 km. That last statement was greeted with some disbelief. The German fighters’ reaction to these attacks seemed very slow, if there had been such a long advance warning (in fact that was due to other weaknesses in the control and reporting system). Certainly, the RAF fighter reaction would have been much faster.
***
One available source could certainly have confirmed to RAF officers the existence of German radar at this time, had not inter-service secrecy prevented it. In December 1939, the German pocket battleship Graf Spee was scuttled off Montevideo in Uruguay. Five days earlier, the warship had suffered several hits in a battle with Royal Navy cruisers. The German captain had thought his ship could not fight her way back to a friendly port, so to prevent further loss of life he ordered her destruction. The estuary of the River Plate is shallow, however, and when the demolition charges went off the warship sank only about ten feet before she came to rest on the seabed. At first light next morning, a flotilla of sightseeing boats put out from Montevideo to look at the shattered warship. Scores of photographs were taken, to be flashed round the world by news agencies. Most people failed to notice a strange feature on the close-up photographs of the smoking hulk: a structure rather like a bedstead on its side, mounted above the bridge. British naval intelligence sent its own sightseer to Montevideo to look over the warship, radar expert L. Bainbridge Bell. He boarded the wreck and climbed up to the ‘bedstead’ structure – a feat requiring some agility, since the Graf Spee had developed a list. Afterwards, Bainbridge Bell reported that the structure was almost certainly the aerial system for a radar set, probably used for ranging
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the ship’s guns. In this assumption he was correct, this was a Seetakt radar. Armed with that information, naval intelligence officers in London examined other photographs of Graf Spee and observed that the structure was present, though hidden under a canvas cover, on photographs taken as early as 1938. That was a discomforting discovery, for even at the start of 1940 Royal Navy warships had no gun-ranging radars, and no British ship would receive one for well over a year. Bainbridge Bell’s report was pigeonholed in the naval intelligence files, however, and R. V. Jones would not learn of it until 1941.
***
During the early months of 1940, the RAF scientific intelligence service picked up little information that could be connected with Luftwaffe radar systems. Although we now know that these were technically efficient, they were in limited use at that time. At that stage of the war the German strategy was primarily offensive, and the Luftwaffe therefore concentrated on systems like the navigational beams to aid bombers. Radar, as a purely defensive device at that time, had a lower priority. The British forces, being on the defensive, were compelled to adopt the opposite course. In May 1940, as the RAF night bombing offensive against Germany began, a prisoner mentioned that the German Navy had experimented with a ‘radio echo’ device for measuring the range and bearing of distant objects. He said the Luftwaffe was working on similar systems, though these were not so far advanced. From the description, it seemed that the German system bore unmistakable similarities to the British coastal radar chain. There was much here that agreed with the statements made in the Oslo Report. On 5 July, shortly after the reception of the first Knickebein beam signals by a British aircraft, one of R. V. Jones’s intelligence sources passed him the gist of a secret Luftwaffe report dated a week earlier. This stated that German fighter aircraft had been able to intercept British reconnaissance planes on that day because of information from the ‘Freya Meldung’ – the Freya warning. That seemed to confirm that the Germans had some form of aircraft detection system. On learning of Jones’s interest, the source mentioned that a Freya site was operating at Lannion protected by a battery of light
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anti-aircraft guns. Lannion was a small village on the northern coast of Brittany. The report seemed to underline the importance of Freya, for German troops had entered the area only three weeks earlier. There were two obvious ways to follow up: the site should be photographed from the air; also, a listening watch should be maintained for signals that could be traced to the site. Jones also tried a further approach, typical of the methods adopted in the strange craft of military intelligence: he researched the mythological background of the code-name, Freya. Freya was the Nordic goddess of beauty, love and fertility. Her most prized possession was an exquisite necklace called Brisingamen; to acquire this, she had sacrificed her honour and been unfaithful to the husband she loved. Heimdal, watchman of the Nordic gods, guarded Brisingamen. And Heimdal could see a hundred miles in every direction, by day or night. Jones cautiously reported to the Chiefs of Staff:
It is unwise to lay too much stress on this evidence, but these are the only facts that seem to have any relation to our previous knowledge. Actually Heimdal himself would have seemed the best choice for a code-name for RDF [radar] but perhaps it would have been too obvious . . . It is difficult to escape the conclusion therefore that the Freya-Gerät is a form of portable RDF.
This was all good stuff, but it awakened a nagging suspicion in the minds of the War Cabinet. Might the Germans have captured intact a radar set left behind by the British Expeditionary Force in France? How else could the Germans have developed an operational radar system so quickly? On 7 July, Churchill minuted General Ismay, his chief of staff, about the matter:
Ask the Air Ministry whether any RDF [radar] stations fell intact into the hands of the enemy in France. I understand there were two or three. Can I be assured that they were effectively destroyed before the evacuation?
General Ismay made the necessary inquiries, and replied that one radar transmitter had been left behind by the RAF at Boulogne, but this had been thoroughly destroyed. It was doubtful that the Germans could extract useful information from it. One of the Army’s gun-laying radar sets might have been captured in a damaged state, but the others were carefully destroyed. The evacuation ship Crested
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Eagle had been carrying an Army gun-laying radar when she ran ashore near Dunkirk, but a naval party had been put on board to destroy the set. From German sources, we know that their forces had captured a nearly intact British mobile air-warning radar near Boulogne. Far from being impressed with the find, they regarded it as extremely crude and much inferior to the Freya. It was a fair assessment, for at that time the mobile British radars were considerably less effective than their static counterparts. On 14 July, Jones received an intelligence report of a second Freya station in operation, this time at Cap de la Hague on the Cherbourg peninsula. Nine days later, the station played an important part in the operation in which dive-bombers sank the destroyer HMS Delight. At the time, Delight was about twenty miles south of Portland Bill. As she had never been closer than sixty miles from the Freya, and had neither fighter support nor balloons to reveal her position, the action showed that the radar gave cover on lowlevel targets at least as good as that from the latest British equipments. In the second week in August, as the Battle of Britain was about to begin, Jones received the text of a secret Luftwaffe report which stated that Freya was designed to work in conjunction with the fighter defences. Attempts to locate Freya on aerial photographs of the two sites now reported, at Cap de la Hague and at Lannion, met with no success. Photographs were taken from reconnaissance Spitfires flying at 30,000 feet and gave a definition just sufficient to enable the 300foot Knickebein turntables to be picked out. All that could be said for certain was that the Freya had to be smaller than that. In the meantime a radar expert from the TRE, Derek Garrard, set out on a radar hunt on his own account. He filled his car with borrowed receiving equipment, and drove to points along the south coast to look for unusual transmissions. The initiative was rewarded, though his first intercept was not connected with Freya. From a point near Dover he picked up radar transmissions on a frequency of 375 MHz, which could be linked with the shelling of British convoys passing through the Straits of Dover, from gun batteries situated near Calais. In fact, the signals came from a Seetakt radar – the same type as that examined on the Graf Spee many months before.
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Garrard’s find caused a commotion among radar experts in Britain; of those prepared to admit the possibility of the Germans having developed radar, few would accept that they had sets superior to those built in Britain. Yet, here was a German radar, working on a frequency so high as to be almost unusable in Britain, directing coastal gun batteries. If the Germans had learned their radar techniques from a British set captured in France, they had applied their new-found knowledge with remarkable despatch. In the autumn of 1940 the introduction of improved reconnaissance cameras brought a great improvement in the quality of RAF aerial photography. The effect on the hunt for the German radar stations was immediate; on 22 November, a high-flying Spitfire returned with photographs of unprecedented clarity showing the village of Auderville near Cap de la Hague. Just to the west of the village were two unexplained circles side by side, each measuring some twenty feet across, looking like a pair of opera glasses laid lenses down. Dr Charles Frank, a physicist who had recently joined Jones’s staff, examined the photographs through a stereoscope and saw that two consecutive frames did not form a perfect stereo pair, as they should have. A shadow associated with one ‘circle’ had changed slightly in the nine seconds between the first exposure and the second. During that time, a thin wide object on top of one circle had rotated through ninety degrees. In each case the shadow was about two millimetres long, but whereas in the first about a tenth of a millimetre wide, the second was two millimetres wide. The difference was hardly greater than the resolving power of the photographs, but it was enough to establish that here was something worth closer examination. The next obvious move would be to lay on a low-level reconnaissance sortie to photograph the ‘opera glasses’ at Auderville. However, with Britain still under imminent threat of invasion, there were demands with a higher priority on the small force of photographic reconnaissance Spitfires. In the interval, Jones received an Ultra decrypt which mentioned another German aircraft detection device called Würzburg. It appeared that one Freya and one – perhaps two – Würzburg sets were earmarked to go to Romania; also, two Würzburg sets were to be sent to Bulgaria. All had been allocated for coastal defence units. Perhaps, Jones reasoned, this represented the minimum
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number of radars necessary to provide continuous cover along the two nations’ coastlines with the Black Sea. If that was so, he calculated that the range of Freya had to be at least fifty-seven miles, and the range of Würzburg to be at least twenty-three miles. At that time Jones did not know that the two radar systems complemented each other, but although his initial premise was wrong the maximum ranges he had postulated for the two systems were remarkably accurate. He now had clues on two distinct types of aircraft-detection apparatus used by the German forces, though neither had yet been clearly seen or heard. Not until 6 February 1941 was a reconnaissance Spitfire available to take a low-level look at the ‘opera glasses’ at Auderville. The first mission was a failure, however. As the Spitfire sped through the area, the circular objects were missed in the gap between two successive frames of the oblique camera. The second low-level sortie, flown by Flying Officer W. Manifould six days later, was more successful. He returned with a magnificent close-up photograph, which showed that each circle was surmounted by a rotatable aerial-array. Unquestionably, this was a radar station (in fact it was a Freya). Even as Manifould’s photographs were being processed, a listening station in southern England identified pulsed signals coming from the direction of Auderville on 120 MHz. Signals on that frequency had been picked up earlier, but the listeners assumed that they came from the new VHF radios fitted to RAF fighters. Only when Derek Garrard examined the signals on a cathode-ray tube was their true significance recognised. Garrard plotted the bearings on their source, and found that these signals originated not only from Auderville, but also from transmitters situated near Dieppe and Calais. Within four hours of each other, Jones received the low-level photographs and the report of the intercepted radar signals, after a hunt that had lasted more than a year.
***
Within a month of the recognition of the Freya signals, new pulsed signals were picked up on 570 MHz. In the spring of 1941 a special radio-reconnaissance unit, No. 109 Squadron, had begun flying sorties over occupied Europe looking for German radar transmissions. After dark on 8 May one of the unit’s ‘ferret’
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Wellingtons flew a circuitous route taking in the Cherbourg peninsula and Brittany. In the course of the sortie, the plane’s crew obtained rough fixes on nine radar sites. In the months to follow, information on Freya stations came in thick and fast. By the end of October 1941 no fewer than twentyseven of these radars had been located, strung out along the coast between Bordeaux in France and Bodö in Norway. No. 109 Squadron aircraft also brought back scores of fixes on 570 MHz transmitters. Yet the sources of these signals were evidently very small, for they defied all attempts to photograph an example. One spectacular piece of intelligence obtained at this time was a strip of cine film showing a Freya station in operation, with the German crew tracking aircraft targets. Less spectacular, but far more important from the intelligence viewpoint, was the discovery that wireless plots from the Freya units could be overheard in England. When tracking an aircraft, the radar station passed distance-andbearing reports by radio to a central air-defence headquarters. The ‘code’ used to pass the information was relatively simple, and easily broken. For example, on 10 October a listening station in England picked up a Morse transmission which read:
MXF = 114011 = 14E = X =254 = 36 = +
MXF was the radio call-sign of the Freya station; 114011 was the time of the plot in hours, minutes and seconds; 14E was the serial number of the plot. X indicated the number of aircraft present (X meant one, Y meant several and Z meant many); 254 was the bearing from the radar station, and 36 was the range in kilometres British intelligence tapped this source for all it was worth. Reconnaissance aircraft, maintaining an accurate record of their flight path by photographing the ground directly below, flew over German-held territory so that the Freya stations would track them. Listeners in England then recorded the plots on the aircraft’s progress. Afterwards, the bearings and ranges were back-plotted on maps. Several Freya stations were located and identified in this way. Once the various radar stations had been located and identified, the German plotting reports were used to provide up-to-date information on the movements of RAF raiding forces flying over German-held territory and beyond the range of radar stations in Great Britain.
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While this was going on, No. 109 Squadron continued to bring back fixes on the sources of the 570 MHz transmissions. Yet still the device defied all attempts to photograph it. Clearly, it was much smaller than Freya. From agents’ reports, Jones learned that these transmissions were connected with a fire-control device known as the FMG. Towards the end of 1941, news arrived that four FMG sets were operating in the area of Vienna, of all places. Unless Vienna – a city of great beauty but comparatively little military importance – was a radar equipment depot, it seemed reasonable to infer that the FMG device existed in considerable numbers. Another important photographic clue came from the United States, at that time still neutral. The US Embassy in Berlin overlooked the Berlin Zoo district, where a couple of huge concrete flak towers had been built. A photograph arrived on Jones’s desk showing the top of one of the towers; clearly visible was a large dishshaped open-work radar reflector, of a type not seen before. Unfortunately there was nothing nearby to give scale to the object. A few weeks later, a Chinese scientist reported seeing the same device, which he described as paraboloid more than twenty feet in diameter, that could be rotated and elevated. He thought it might be used to direct the anti-aircraft guns. Jones saw that the new radar could not possibly be the 570 MHz radar known to exist in such profusion in German-occupied Europe. Otherwise, its large reflector would have been spotted on aerial photographs long before this. In fact, the Berlin Zoo photograph showed one of the first Giant Würzburg radars to enter service. Still Jones had no picture of the small Würzburg equipment, but the hunt for one was nearing its end. Late in November 1941, Charles Frank was examining a medium-level photograph of the Freya station at Bruneval near Le Havre on the north coast of France. There he saw that a track had been trodden out along the cliff-edge: it ran from the Freya towards a large house, which seemed to serve as a headquarters. Just before it reached the building, the track swung to the right. It ended at a small black object about halfway between the house and the cliff top. Several people had considered it worth their while to tread out a path from the main radar station to the ‘small black object’. Might that object play some part in the working of the Freya? On 3 December Flight Lieutenant Tony Hill, a reconnaissance
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pilot, visited the interpretation centre at Medmenham in Buckinghamshire to discuss the photography of German radar sites. Squadron Leader Claude Wavell knew of Charles Frank’s special interest in the Bruneval radar, and mentioned the mysterious black object to Hill. On the following day, on his own initiative, Hill took off in a Spitfire to look at Bruneval. He swept in low over the cliffs, and was past the emplacement before the startled defenders knew what had happened. On his return, Hill discovered that his camera had failed to function properly. It was a cruel blow – but he had seen the device clearly, and was able to say that it looked ‘like an electric bowl-fire and was about ten feet across’. If Hill was right, this might be the elusive source of the 570 MHz transmissions. On the following day, Hill bravely repeated the performance. This time the camera worked perfectly and the photographs he brought back were among the classics of the war. They showed the device exactly as he had described it, like an electric bowl-fire about ten feet across. Although it now seemed highly probable, the evidence was still not conclusive that this was the source of the 570 MHz signals. Until that was certain, countermeasures could not begin. It is difficult to establish who first suggested the idea of pilfering the device at Bruneval. The notion was so obvious – it lay within 200 yards of the coast – that it might have occurred to several people. At all events, by the beginning of January 1942 such an operation was in the detailed planning stage. Clearly a commando raid launched from the sea would be doomed to failure; the device was situated at the top of high cliffs, protected by a sizeable German garrison. Even if the troops could fight their way to the top of the cliffs without suffering serious casualties, it was unlikely they could do so before the defenders had destroyed the radar. The naval commodore in charge of Combined Operations, Lord Louis Mountbatten, suggested that parachute troops should be used instead. On 21 January the Chiefs of Staff agreed to this, and assigned C Company of the 2nd Parachute Battalion to the operation. An operational squadron of Whitley bombers would transport them to the target area, and a force of light naval craft would evacuate the raiders from the nearby beach when the mission was over. Intensive training began for what the men were told would be ‘a special demonstration exercise’ to be watched by the whole
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of the War Cabinet. The venture received the code-name ‘Biting’. Once the device had been captured, the task of dismembering it was to be undertaken by seven men of the Royal Engineers, commanded by Lieutenant D. Vernon. The eighth man in the team was from the RAF: Flight Sergeant C. Cox, a radar mechanic, who was to go along should his specialist knowledge be required. The dismantling party was given a British gun-laying radar – the nearest available equivalent to the expected German system – on which to practise its dismantling skills. Vernon and Cox then received a special briefing on the anticipated layout of the German set. According to the planned time-table, they would have half an hour with the apparatus; in that time they were asked to make sketches and take photographs of the equipment, then dismantle it systematically starting at the aerial and working backwards through the receiver to the presentation gear. The operating frequency could be established beyond doubt by removing the aerial element from the centre of the ‘bowl’, and measuring it. The next target was the receiver and its associated presentation equipment. These would reveal whether any anti-jamming circuitry was built into the set. The transmitter was also wanted, so British scientists could examine German techniques for generating high powered pulses on ultra-high frequencies. Dr Jones also asked that a couple of prisoners be taken, radar operators if possible. These might be persuaded to reveal information on the methods of operating the radar and aircraft reporting. All German signals equipment bore informative labels and inspection-stamps, and useful general intelligence could be obtained from these. Should the various units prove impossible to dislodge, Jones asked that the labels be torn off and brought back. By the fourth week in February Jones had confirmation from other sources that the ‘bowl-fire’ was a radar code-named Würzburg, and that one of these sets was situated near the Freya station at Bruneval. By then all the military preparations for the operation were complete, but now the weather intervened. On the evening of the 24th the weather was unsuitable, as it was on the next two nights. The timing of the raid was critical; the attack had to take place on the night of a full moon, but also when the tide was on the rise so the assault craft would not be left stranded on the beach. The 27th was the last possible date for the operation for a month or more.
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That evening the forecast was favourable and the Commander-inChief Portsmouth, Admiral Sir William James, signalled: ‘Carry out operation Biting tonight 27th February.’ The twelve converted Whitley bombers of No. 51 Squadron took off from Thruxton near Andover. On board were 119 paratroops and the RAF flight sergeant, sitting huddled together and trussed up in their uncomfortable parachute harnesses. One soldier later wrote: ‘The mugs of hot tea (well laced with rum) we had drunk before taking off began to scream to be let out. In that restricted space and encumbered as we were there was, alas, no way.’ Shortly after midnight on the morning of the 28th, the first sticks of paratroops leapt from their aircraft. Some ten seconds later the men tumbled on to the carpet of virgin snow some 600 yards to the south of the radar site. They hastily shed their parachute harnesses and cocked their weapons, ready to fight for their lives. But the halfexpected rattle of German small-arms fire did not come. Their arrival had passed unnoticed. The sound of the Whitleys’ engines faded into the night, leaving the men feeling very lonely and vulnerable. As the men assembled into their small groups, their next move was not at all warlike: that tea just had to go. Major John Frost, the force commander, later wrote that this ‘was certainly not good drill, as now was the time when a stick of parachutists was most vulnerable . . . but at least it was a gesture of defiance!’ The assault parties now moved on their assigned targets. One, led by Frost himself and comprising fifty men including the dismantling party, crept towards the radar site and the house nearby. A second party, under Lieutenant Timothy, took up covering positions to screen the force from attack from the landward side. The remainder made off to secure the beach and the escape route. Frost’s men silently surrounded the Würzburg, whose silhouette stood out sharply in the moonlight, and the nearby house. If the house was some kind of headquarters, it might be a centre for resistance. Frost himself stole round to the front door with a small party. Satisfied that all was ready, he gave the signal for battle to begin – a long shrill blast on his whistle. With four men at his heels, Frost burst into the house and began searching each room in turn. It proved something of an anti-climax, the only German present died trying to defend one of the upstairs rooms. Outside, a fierce battle was in progress with almost continuous
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automatic fire punctuated by the bangs from exploding handgrenades. Within minutes, all resistance around the Würzburg had been overcome. But, as that fight ceased, another began. There were about a hundred German troops stationed in the vicinity, and Lieutenant Timothy’s covering force found itself being hotly engaged. In the fight to seize the Würzburg, five of its six operators had been killed. The sixth man made off into the darkness, but in his haste he lost his sense of direction and stumbled over the cliff. Fortunately he managed to grab a projecting rock, and with some difficulty he climbed back to the top. He then found himself being helped over the edge by a British paratrooper. In no position to offer resistance, the man quietly surrendered. With the Würzburg secured, Lieutenant Vernon climbed on top of the operating cabin and examined the aerial with the aid of a hand torch. He then photographed the aerial from each angle – an action no sooner made than regretted, for the light from the flashbulbs attracted bullets from several directions. Vernon then summoned his team and ordered one sapper to saw off the aerial element, while the remainder sought to remove the boxed components in the operating cabin. The aerial came away easily, but the boxes defied all attempts to dislodge them using screwdrivers. This was no time for finesse, for the bullets ricocheting off the cabin’s walls were real enough. The men then brought into play their crowbars, and the equipment gave up the unequal struggle. One by one, the units were ripped from the console. The dismantlers had been at work for barely ten minutes out of the planned thirty, when Major Frost saw three lorries approaching with headlights full on. Almost certainly, these were German reinforcements. If the defenders brought into action weapons heavier than the rifles and machine guns they were already using, the raiding force would be at a severe disadvantage. Frost decided to settle for whatever the dismantling party had already secured, and ordered his force to withdraw to the beach. But now he learned that the shoreline was still in German hands. What had gone wrong? Of the forty men detailed to secure the escape route, half had landed more than two miles from the planned dropping-zone. The senior officer present, Lieutenant E. Charteris, took a quick bearing on the lighthouse at Cap d’Antifer and worked
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out his position. He then led his men towards the radar site at a brisk trot, and arrived at the clifftop just as Frost was organising his own force to storm the beach. The combined assault teams rushed the German positions in their path. Now the beach was in British hands and on it lay the wounded, the German prisoners and the items stripped from the Würzburg. Frost told his signallers to call in the naval craft to evacuate the force. It was high time to leave, for on the cliffs on either side of the beach the German forces were becoming increasingly active. After a few minutes, the signallers reported they had had no success in contacting the boats. Frost fired red distress flares, but to no effect. Then, he later wrote, ‘I moved off the beach with my officers to rearrange our defences. It looked as though we were going to be left high and dry, and the thought was hard to bear.’ Just as his troops had begun to take up positions for a final stand, Frost heard a cry: ‘Sir, the boats are coming in!’ He looked back and saw six snub-nosed assault craft sliding to a stop on the beach. With a sigh of relief he ordered his men to embark, while the boats’ crews put down covering fire on the German troops at the top of the cliffs. With the roar of accelerating engines the landing craft backed away from the shore, while the brisk exchange of gunfire continued until the boats were well clear. Safe aboard an assault craft, Major Frost learned the reason for the delay. While he had been signalling, a German destroyer and two patrol boats had passed within a mile of the small British flotilla but noticed nothing. Frost also learned that the pieces of the radar secured by his men were almost exactly what were needed. Mr D. H. Priest, a Telecommunications Research Establishment engineer who had received a temporary commission as a flight lieutenant for the occasion, examined the booty on the boat. Had the coast been clear, he would have landed and climbed to the radar site to look over the Würzburg, but this was not possible. When dawn broke, several Spitfires arrived over the craft and escorted them back to England. The Bruneval operation was successful on almost every count. The raiders had captured most of the radar. Of the three prisoners, one was a radar operator. The paratroops suffered fifteen casualties – two dead, seven wounded and six missing. The dismantling party had done extremely well in the brief time available. The units it brought back included the receiver, the receiver amplifier, the
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modulator – which controlled the timing within the radar – and the transmitter. In addition, there was the sawn-off aerial element. The only unit which Jones had requested and not received was the presentation equipment. If – as had nearly happened – the boxes had proved impossible to tear from their mountings, Jones might have had to make do with the labels alone. So it is interesting to see what could be learned from these. The labels indicated that the manufacturer was Telefunken, a company with factories in the Berlin area. The works numbers were particularly interesting. From previous experience with German serial numbers, Jones had deduced that the number allocated to the first production model of each component was 40,000. The earliest number found on the captured units was 40,144, the latest was 41,093. This suggested that the number of sets of components produced by the date of manufacture of the last item, was 1,093. The earliest inspection date, early November 1940, was stamped on a part of the transmitter; the latest, 19 August 1941, was on the aerial. This did not necessarily mean that 1,093 complete Würzburg sets had been turned out by the latter date, since a proportion of the component units would have served as spares. It was a principle of German design that servicing was facilitated by replacing component units, while the defective ones were returned to a central depot for repair. Assuming that about half the production went into spares, Jones reckoned that around 500 of these radars were available by August 1941, and production was probably running at about 100 sets per month. For him the raid was particularly satisfying, since the intelligence gleaned either confirmed or added to the previous picture. No part of the picture had to be modified or discarded. An important side effect of the raid was that it gave British intelligence added confidence in the accuracy of the information it was receiving. Scientists at the Telecommunications Research Establishment at Swanage made a thorough examination of the Würzburg units. In their view the equipment was considered ‘straightforward and in no respect is it brilliant . . . On the other hand, it must be remembered that the equipment was made in 1940 and designed in 1939 or earlier.’ In 1940, British radar techniques had not been sufficiently advanced to build a set working on 570 MHz with a range of twenty-five miles. While the German radar carried no
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specific anti-jamming circuitry, it could be re-tuned over a narrow range of frequencies to overcome electronic jamming. After the Bruneval horse had been ‘rustled’, the German local defence commanders along the French coast made sure that the stable door was well and truly bolted. The remains of the Würzburg were removed, and a new set was installed in the main Freya compound. Within a few weeks, a dense barbed wire entanglement surrounded the compound. Other radar stations followed this lead. Every German radar station near the coast now became conscious of its vulnerability, and surrounded itself with barbed wire. That greatly helped Jones and his staff; there were several sites which were suspected to contain Würzburg sets, but in each case the existing aerial photographs had failed to show them. Now these sites obligingly ringed themselves with barbed wire – which showed up well on aerial photographs – to confirm the suspicions. Not only in Germany were there repercussions of the Bruneval operation. The success of the raid highlighted a golden opportunity for a retaliatory attack on the Telecommunications Research Establishment, hub of British work on radar and situated near Swanage on the south coast of England. In the spring of 1942 the establishment made a rapid move to Malvern College, well clear of the coast.
***
Two weeks after the Bruneval raid came an event that was to have even greater significance to the development of countermeasures systems in Great Britain. On 11–13 February 1942 the German battlecruisers Scharnhorst and Gneisenau, and a flotilla of smaller ships, had left their temporary base at Brest, run the gauntlet of the defences covering the Straits of Dover, and successfully reached ports in Germany. In doing so they had inflicted a major blow to Britain’s reputation as a naval power. The operation had taken place under the noses of powerful British forces and was a masterpiece of boldness, careful planning and tight security. An important element in the success of the operation was the large scale use of ground jammers. As the warships came within range of the British coastal radars at the eastern end of the Channel, these jammers were switched on simultaneously. Many of the best British radar operators were now serving in the
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Mediterranean theatre, where the heaviest fighting was taking place. Those who replaced them at radars along the south coast of England were less experienced and they reported the clutter on their screens as ‘equipment failure’ or ‘local interference’. As a result commanders did not appreciate the significance of the radar operators’ difficulties until it was too late. This all came out during the far-reaching official inquiry after the event. The escape of the German warships would have significant effects. At the beginning of 1942 several important British radars – those for coast watching, ground-controlled interception, airborne interception, AA gun-control and searchlight-control – all operated on frequencies in the 200 MHz band. A heavy German jamming effort in that part of the spectrum would pay rich dividends if there was a resumption of the night Blitz. Thus, by highlighting that fundamental weakness, the escape of the German warships greatly assisted the British cause. Work began immediately to develop new types of radar that worked in widely different parts of the spectrum. Once those were in place, the Germans would never again have the chance to knock out the radar system with so little effort.
***
During the spring of 1942, British intelligence gradually improved its picture of the Luftwaffe night air defence system. In March, news arrived of an inland Freya station at Nieuwekerken in Belgium, just to the north of the important night-fighter airfield at Saint-Trond. By this time many coastal Freya stations had been located, but inland stations were thought to be a rarity. A high-flying reconnaissance Spitfire was sent to investigate. The photographs it brought back showed a Freya radar set and a cluster of searchlights, but the latter were grouped round a radar with a large circular open-work bowl – like that photographed in the Berlin Zoo. The proximity to the airfield at Saint-Trond strongly suggested that this was some sort of night-fighter control centre. Shortly afterwards this was confirmed. Then an agent reported a night-fighter control centre at Domburg on the Dutch island of Walcheren. And the subsequent high-level photographic reconnaissance revealed a Freya and two ‘Berlin Zoo’ radars there. A more detailed study of the Nieuwekerken site showed there were two ‘Berlin Zoo’ radars there, also. This called
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for a closer look at the new type of radar. On 2 May, a reconnaissance Spitfire ran in fast and low along the Dutch coast and past the Domburg site. Yet again Flight Lieutenant Tony Hill had pulled off a low-level scoop, for he returned with clear photographs of both ‘Berlin Zoo’ type radars. As the Spitfire swept past them, the two radars were pointing in different directions. So Hill’s pictures showed the radar from two quite different angles. Equally important, at one of the sets an operator had been about to climb the ladder to the cabin. He stood watching, helpless, to become a human yardstick when the photographs were analysed. One night two weeks later, a further ploy was adopted to elicit information about the range of this radar. An RAF Beaufighter night-fighter flew towards the Domburg area, closely watched by the British radar station on North Foreland. A German night-fighter rose to intercept, and a long and inconclusive engagement followed. Throughout it, the RAF monitoring service recorded the orders passed to the Luftwaffe pilot. In particular, they noted that he was not permitted to move more than forty miles from the radar station. This was a strong pointer to its maximum range. Soon afterwards, the ‘Berlin Zoo’ radar was identified as the Giant Würzburg. The next major item of intelligence came from a Belgian agent. He had managed to steal from a German headquarters a map showing the deployment of an entire regiment of searchlights. As luck would have it, the map covered the area around Saint-Trond. Marking the station at Nieuwekerken was a lightning flash, as were two more at Zonhoven and Jodoigne, some twenty miles on either side. Might these be fighter-control stations too? And if they were, was twenty miles the standard distance between adjacent sites? Reconnaissance photographs revealed that this was the case. By extrapolating the line, Jones and his staff soon picked out five further night-fighter control stations, strung out at regular intervals along an almost straight line. During the summer of 1942, the clump of flags on the wall map in Jones’s office sprouted shoots to either side of its original starting point in southern Belgium. The great radar hunt was on. Charles Frank christened the defensive belt the ‘Kammhuber Line’, and that appellation caught on in the RAF. Agents were sent to areas previously calculated, to seek out the radar stations. Their catch was good; a paraboloid the size of a
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suburban house could hardly escape being the object of wonder and speculation by the local population. It should be remembered that at this time the ‘man in the street’ had never heard of radar. As a result the descriptive vocabulary of the inhabitants of the Low Countries was seriously strained; ‘inverted umbrella’ and ‘magic mirror’ were typical of the terms used. One Giant Würzburg was talked about so much that it became known as ‘le fameux miroir d’Arsimont’. During their flights over Belgium, Holland and northern France, RAF bombers dropped caged carrier pigeons. The birds’ legs bore labels asking the finder to write in details of any large dish-like structures seen in the area, and then release the pigeons. This method alone assisted in locating three sites previously unknown to Dr Jones. In the next chapter, we shall observe how this information was used to develop the first countermeasures to the Luftwaffe night air defence system.
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81
The rigid airship LZ-130 Graf Zeppelin, the last of these craft to be built, flew an electronic intelligence gathering mission along the east coasts of England and Scotland in August 1939.
Dr Ernst Breuning (right) led the listening team
aboard the Graf Zeppelin. He described to the author how his team picked up signals from the newly erected British radar chain, but misidentified them as coming from a research station in Germany which was conducting experiments to measure the altitude of the ionised layers surrounding the earth.


The huge Knickebein beam-transmitter at Stollberg in Schleswig-Holstein. The aerial array was about 100 feet high, and was supported by railway bogies which ran on a circular track 315 feet in diameter to align the beam on the target.
Heinkel 111 bomber of III/KG 26, the unit which employed the Y-Gerät beam system over Britain in 1940–41. Note the additional aerial for the system, mounted above the fuselage behind the cabin.
Wing Commander Edward Addison commanded No. 80 Wing during the ‘Battle of the Beams’. Later he led No. 100 Group, which provided countermeasures support for bomber operations.


83
Heinkel 111 of Kampfgruppe 100 on the compass swinging base. Note the two additional aerial masts on the rear fuselage, belonging to the X-Gerät beam attack system.
X-Gerät beam transmitter. Dr R. V. Jones played a major role in uncovering the beam systems used by the Luftwaffe during attacks on Great Britain.


84
Left: The first photo to reach Britain showing that the Germans had radar in service. The Graf Spee, pictured in December 1939 after she had been scuttled off Montevideo. The aerial array of the Seetakt radar can be seen (circled) above the bridge.
Below: In November 1940 a reconnaissance aircraft took this photo of two unusual tub-shaped structures (circled and inset) at Auderville near Cherbourg. In February 1941 a Spitfire flew a risky low-altitude mission to get this close-up of the ‘tubs’, each of which was found to house a Freya radar.


85
Messerschmitt Bf 110 night-fighter, with the drag-producing aerial array of the Lichtenstein radar on the nose.
Above: Flight Lieutenant, later Wing Commander, Derek Jackson played a major role in the development of ‘Window’ metal foil strips to jam enemy radars including the Lichtenstein.
Left: Wassermann early
warning radar at Bergen aan Zee in the Netherlands. The surrounding houses give scale to the huge 130-foothigh aerial array.


Above: Generalmajor Josef Kammhuber, architect of the Himmelbett system. Left: Close-up of the Würzburg radar. Designed for use by flak and searchlight batteries, this radar was also used for a short time to direct nightfighters into action.
Reconnaissance photograph of the Würzburg at Bruneval, taken three months before the famous raid.
Low-altitude photo of the Giant Würzburg radar on the island of Walcheren, taken in May 1942.


Following the Bruneval raid, German radar sites were surrounded with dense barbed-wire entanglements, which made them highly conspicuous on aerial photographs. Below: Himmelbett station with a Freya radar (foreground) for long-range search and two narrow-beam Giant Würzburg radars (left and right) to track the movements of the British bomber and the intercepting night-fighter respectively.


H2S indicator in the navigator’s position of a Lancaster bomber.
H2S radar picture of Hamburg (right) compared with a map of the same area. The wide estuary of the River Elbe, pointing at the city from the west, served as a prominent navigation feature on radar.


89
Aerial of a Korfu ground direction-finding station, which tracked RAF bombers by picking up the radiations from H2S.
Generalfeldmarschall Erhard Milch held frequent conferences to discuss each step in the countermeasures battle as it appeared on the German side.
Oberst Dietrich Schwenke headed the Luftwaffe intelligence section responsible for monitoring technical developments by the Western Allies.


90
Generalmajor Joseph Schmid (right) took control of Luftwaffe night-fighter operations after General Kammhuber was ousted from that post following the ‘Window’ debacle in July 1943.
Major Hajo Herrmann (centre) seen with Reichsmarschall Herrmann Göring during an inspection of ‘Wild Boar’ night-fighter pilots. Major Herrmann had been instigator of those new tactics.
Ideal ‘Wild Boar’ conditions. Seen from above, a Lancaster bomber silhouetted against a cloud background by searchlights and fires on the ground, photographed over Berlin during the raid on 16 December 1944.


91
Dr Robert Cockburn led the countermeasures team which designed and put into production many of the British jamming systems.
B-17 Flying Fortress modified as a jamming escort aircraft, before issue to No. 214 Squadron, RAF. The radome under the nose housed the scanner for the H2S radar.
Above right: The 600-pound cylinder of the ‘Jostle’ highpowered VHF communications jammer, mounted on the rear of its special transporting truck. This jammer was fitted to the jamming escort Liberators and Flying Fortresses of No. 100 Group.


92
From the autumn of 1944 Luftwaffe night-fighters needed to be equipped to avoid RAF intruders, as well as engage bombers. This Ju 88 night-fighter carries the nose-mounted aerial array for the SN-2 radar. The blister on top of the cockpit canopy houses the rotating aerial for the Naxos radar-homing and warning receiver . . .
Note the two upward-firing 20-mm cannon fitted in the fuselage . . .
Also the tail mounted aerial for SN-2 to provide warning of Allied intruders closing in from behind.


93
Above: The Jagdschloss fighter-control radar was the first German radar to employ the plan position indicator type of display. To avoid electronic jamming, the radar had provision for the operator to select one of four separate operating frequencies.
Left: Effect of ‘Mandrel’ jamming on the screen of a Jagdschloss radar. The screen is north-aligned. The three large ‘spokes’ in the northwest quadrant point to three aircraft jamming as part of a ‘Mandrel’ screen. The caterpillar-shaped return heading south-southeast has the appearance of a bomber stream, though in fact it is ‘Window’ spoof. The scattered returns in the southeast quadrant come from Luftwaffe night-fighters and No. 100 Group Mosquitoes.


Above: Formation of B-17 bombers being engaged by a flak battery firing though an overcast. Note the effect probably caused by ‘Carpet’ jamming of the fire-control radars: although the bursts appear to be accurate in line, the shells have detonated at the incorrect range and well below the raiders’ altitude. Left: The APQ-9 ‘Carpet III’ jamming equipment, used in large numbers to counter the German Würzburg and Mannheim flak-control radars.
Right: B-29 bombers pictured at one of the islands on the Marianas group. With assistance from radio-countermeasures, these aircraft made devastating attacks on Japanese cities and industrial targets for minimal losses in the spring and summer of 1945.


95
Above: The ‘Tuba’ equipment, designed and built at the Radio Research Laboratory at Harvard, was the largest and most powerful jamming system produced during World War II. The man standing at the base of the aerial gives scale to the special directional array developed for the jammer. ‘Tuba’ became operational too late to have any effect, however.


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‘Little Boy’, the atomic bomb which devastated the Japanese city of Hiroshima. The aerial elements of two of the four radar airburst fuses can be seen mounted on the side of the weapon.
‘Guardian Angel’ B-29, which served as a jamming escort aircraft to provide cover for bombers attacking Japan. Eight jamming aerials are visible on this view of the aircraft.


Chapter 4
Towards the Offensive
‘If you know the enemy and know yourself you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory you will suffer a defeat. If you know neither you will always be beaten.’
General Sun-Tzu
As German troops advanced deeper into the Soviet Union in the summer of 1941, it became clear in Britain that, after almost a year on the defensive, it was time to seize the initiative in the West. Clearly, an invasion of the continent was out of the question in the foreseeable future. For the time being the only readily available means of striking at the German homeland was through RAF Bomber Command. Prime Minister Churchill assured Russian Premier Josef Stalin that, when the weather improved in the spring of 1942, the RAF would launch a heavy air offensive against Germany and: ‘We are continuing to study other measures for taking some of the weight off you.’ Those ‘other measures’ would not materialise for a long time. At that time, however, Bomber Command was going through a period of soul-searching. The fact that the Luftwaffe had found it necessary to develop radio aids to assist accurate navigation and bombing at night had led to some scepticism concerning the results the British bomber crews might themselves be achieving. After all, they had no such aids. One of the doubters was Air Vice-Marshal Robert Saundby, who took up an appointment as Senior Air Staff Officer at Bomber Command headquarters at the end of 1940. He told his staff that, when a force of bombers claimed to have dropped 300 tons of bombs on a certain target, all they could be certain of was that they had ‘exported 300 tons of bombs in its direction’. Professor Lindemann conducted his own investigation into the results of the RAF bombing, based on the photographs brought back by reconnaissance aircraft. The findings were highly disquieting: of the crews who thought they had hit the target, only one in three


had in fact placed their bombs within five miles of it. In the case of targets in the Ruhr industrial area, the figure was as low as one crew in ten. After receiving the report in September 1941, Mr Churchill taxed the Chief of Air Staff with this:
It is an awful thought that perhaps three-quarters of our bombs go astray . . . If we could make it half and half, we should virtually have doubled our bombing power.
As a long-term investment, development began on two radio devices to improve bombing accuracy: Oboe and H2S. These will be described in later chapters. For the moment, the most urgent requirement was for a less ambitious radio aid that would improve basic navigational accuracy at night. Fortunately work on such an aid was well advanced at the Telecommunications Research Establishment and the device, codenamed ‘Gee’, was nearing the service trials stage. The ‘Gee’ (or Grid) system of navigation had been conceived in 1938, and work on the device had begun in earnest in the spring of 1940. ‘Gee’ employed three ground transmitters situated about a hundred miles from each other. These transmitters acted in unison to radiate a train of pulses in a set order. The aircraft carried a special radar receiver, which enabled the navigator to measure the minute time differences between the reception of the various signal pulses. By referring those time differences to a special ‘Gee’ map, the navigator could determine his position to within six miles, while flying up to 400 miles from the furthest transmitter. Closer to the transmitters, the accuracy was even better. In its concept ‘Gee’ was a great improvement over that of Knickebein, its nearest German equivalent. The latter gave an accurate positional fix only at the crossing point for a pair of Lorenz-type beams, whereas ‘Gee’ provided fixes for aircraft anywhere within its area of cover. By the beginning of March 1942, sufficient ‘Gee’ receivers existed to equip about one-third of Bomber Command’s aircraft. The device immediately became popular with crews, who affectionately nicknamed it the ‘Goon box’. The Luftwaffe captured its first ‘Gee’ receiver on 29 March, recovered from the wreckage of a Wellington bomber which came down in the sea off Wilhelmshaven. The device had suffered some salt-water damage, but that water had saved it from complete
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destruction. As the crew had abandoned the Wellington, one of them initiated the explosive detonator intended to destroy the box. The system had a built-in delay to give everyone time to get clear, and before it went off the water had smothered the charges. The procedure adopted by Luftwaffe intelligence from this point on was similar to that used by R. V. Jones and his team in Britain. Intelligence officers and radio experts hunted for any further scraps of information relating to the new British system. Oberst Dietrich Schwenke, the intelligence officer in charge of the Luftwaffe section dealing with equipment captured in the West, discussed the find at a high-level conference held in Berlin on 26 May. He said the remains of this equipment had been recognised in several shot-down British aircraft, but in every case except the Wilhelmshaven incident and a second aircraft which had also crashed in the sea, the units had been smashed beyond repair when the demolition charge went off. Schwenke continued:
The British [have developed] a new system which gives the pilot [sic] his position at all times. The equipment for this is the receiver I have just described. Tests have been carried out on it by Telefunken, but the set was unfortunately not received in good condition. Our experts are still not in complete agreement concerning the technical working of the equipment.
Schwenke went on to describe the working principle of ‘Gee’, and the way it was used. He added that the equipment was being installed as standard in the each of the RAF’s principal heavy-bomber types – the Wellington, the Lancaster, the Stirling and the Halifax. He continued: ‘I think it is being used not so much to find pinpoint targets, as to improve dead-reckoning navigation.’ Once a working receiver became available, Schwenke planned to install it in a Luftwaffe aircraft. It would then be possible to establish the accuracy of the system over Germany, using the British transmissions. The transmitters were known to be located in southeastern England, in positions where they could cover the Ruhr industrial district. Schwenke said the possibility of jamming was under investigation, but first it was necessary to find out exactly how the system worked and the frequencies it used. He said the ground transmissions were fairly powerful, but he thought they could be jammed if more powerful jamming transmitters radiated
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