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the big one
2024-08-27 21:48:20 -05:00
High-accuracy Time and Frequency in VLBI
Katie Pazamickas
kpazamic@peraton.com
Rick Hambly
Rick@cnssys.com
Background – Rick Hambly
• Oscillators and Clocks • What “Clock” Performance Does VLBI Need? • “Absolute Time” (i.e. Clock Accuracy )
The Hydrogen Maser - Katie Pazamickas
• Maser Outputs • Data/Frequency Monitoring • Troubleshooting/Routine Maintenance
GPS Time - Rick Hambly
• Week rollover may mean retiring old GPS receivers • GPS receiver’s quantization error • “Absolute” Receiver Calibration • New developments
Time and Frequency in VLBI
1
Oscillator
• Escapement Wheels & Pendulums • Crystal Oscillators • Cavity Oscillators • Oscillator Locked to Atomic Transition
o Rubidium (6.8 GHz) o Cesium (9.1 GHz) o Hydrogen Maser (1.4 GHz)
Integrator and Display = Clock
• Gears • Electronic Counters • Real Clocks
Time and Frequency in VLBI
Background
Events that occur with a defined
nsec -- minutes Long-Term
seconds - years
2
Time and Frequency in VLBI
Background
3
The Radio Astronomy and Geodesy VLBI community uses Hydrogen Masers at about 50 remote sites around the world.
To achieve ~10° signal coherence for ~1000
seconds at 10 GHz we need the two clocks
(oscillators) at the ends of the interferometer
to maintain relative stability of:
• ≈ [ 10° / (360° * 109 Hz * 103 sec) ]
• ≈ 2.8 * 10-14 @ 1000 sec.
1
Time and Frequency in VLBI
Background
4
In Geodetic applications, the station clocks are modeled at relative levels ~30 psec over a day:
• ≈ [30 * 10-12 / 86400 sec] ≈ 3.5 * 10-16 @ 1 day 2
• A hydrogen maser provides this level of performance.
Time and Frequency in VLBI
Background
55
To correlate data acquired at 16 Mb/s, station timing at
relative levels ~50 nsec or better is needed.
After a few days of inactivity, this requires:
• ≈ [50 * 10-9 / 106 sec]
• H-maser -> ≈ 5 * 10-14 @ 106 sec
3
Since VLBI now defines UT1, VLBI needs to control [UTC(USNO) - UTC(VLBI)] with an ACCURACY (traceable to USNO)
• ≈ 100 nsec to 1 µsec
To detect problems, VLBI should monitor the long-term behavior of the Hydrogen Masers (at least) every hour with PRECISION
• ≈10 to 50 nsec
Time and Frequency in VLBI
Background
66
Time and Frequency in VLBI
Background
7
The only real reason for worrying about “absolute time” is to relate the position of the earth to the position of the stars:
• Generating Sidereal Time to point antennas. • Measuring UT1 (i.e. “Sundial Time”) to see changes
due to redistribution of mass in/on the earth over long periods of time (a.k.a. “The Reference Frame”) • Knowing the position of the earth with respect to the moon, planets and satellites. • Making the correlation and data analysis jobs easier
Time and Frequency in VLBI
Background
8
At the stations this means that we will need to pay attention to timing elements like
• Frequency Standard and Station Timing • The lengths of all signal & clock cables • The geometry of the feed/receiver to the antenna. • Calibration of instrumental delays inside the
receiver and backend. • The care with which system changes are reported to
the correlators and the data analysts.
Time and Frequency in VLBI
Background
9
The Real Signal Path
* Note -- If the axes don’t intersect, then an “offset axis” model of the antenna is used
Time and Frequency in VLBI
Background
10
CONTROL ROOM
H-Maser
Phase Cal Ground Unit: Monitors Cable Length
Changes
Counter
UP DOWN
This is the “clock” that is
used to analyze VLBI data
IF
Time and Frequency in VLBI
Background
ANTENNA
Cable Length Transponder
5/10 MHz
Divide by n
1/5/10 MHz
Pulse Generator
1 Pulse/μsec
Microwave Receiver
Quasar
11
This is the “clock” the correlator uses
to make fringes
H-Maser
5/10 MHz
Clock in Mk5 or Mk6 (XCube) Formatter
5/10 MHz
Mark 5 or Mark 6 (XCube)
Recorder
Clipper/ Sampler
Down Converter
Time and Frequency in VLBI
Background
IF From Microwave
Receiver
IF Distributor
12
 Compare two distant clocks by observing the same GPS satellite(s) at the same time (also called Common View)
 Requires some inter-visibility between sites  Requires some near-real-time communication  Links you directly to the “Master Clock” on the other end at ~1 nsec level
 Use Geodetic GPS receivers (i.e. as an extension of the IGS network)
 Requires high quality, probably dual frequency, receiver but it’s hard to gain
access to the internal clock.  Requires transferring ~1 MB/day of data from site  Requires fairly extensive computations using dual-frequency data to get
~300 psec results with ionosphere corrections  Allows Geodetic community to use VLBI Site (and H-Maser) for geodesy  Difficult to obtain “Real Time” clock pulses!
 Use the Broadcast GPS Timing Signals as a clock
 Yields “Real Time” 10-30 nsec results with low cost hardware  Single Frequency L1 only (for now) suffers from ionospheric error
Time and Frequency in VLBI
Background
13
• Start with a good timing receiver, like the CNS Clock II with the Synergy SSR (u-blox) front end.
• Average the positioning data for 1 to 2 days to determine the station’s coordinates. This should be good to <5 meters. If the site has been accurately surveyed, use these values.
• Lock the receiver’s position to this average. • Make sure that your Time-Interval Counter (TIC) is triggering
cleanly. Start the counter with the 1 PPS signal from the “house” atomic clock and stop with the GPS receiver’s 1PPS. • Average the individual one/second TIC readings over ~5 minutes (300 seconds).
• These steps are semi-automated in Tac32Plus & GPSTime.
Time and Frequency in VLBI
Background
14
From: Roberto Ambrosini, Tom Clark, Brian Corey, and Ed Himwich To: All IVS Stations Date: 1 May 2014
We recommend the following practices for management of the 1 PPS derived from the Maser and used as the station 1 PPS. Its synchronization with UTC as derived from the GPS 1 PPS offers a common timing reference for all VLBI stations worldwide. We refer to the difference in the epochs of the Maser and GPS 1 PPS signals, as measured by a counter, as the Maser/GPS offset, regardless of which signal occurs later.
Because it is evident that crossing zero time for the Maser/GPS offset should be carefully avoided (the counter would read the complement of one second of the desired delay, arithmetic processing of data by the counter not being recommended), we recommend keeping the offset at a small but significant distance from zero and its drift rate positive.
We also recommend keeping the time and frequency retuning of the Maser at a minimum, typically no more than once in a year.
This procedure offers: less work at the station, better modelling of the long term drift of the Maser, and a better chance to identify jumps in the offset.
Here follow some practical recommendations for the Maser/GPS offset:
(1) Either the Maser 1 PPS or GPS 1 PPS can occur first.
(2) The offset should be significantly, at least a few microseconds, different from zero.
(3) The offset should not be too large, a useful upper limit might be on the order of 100 microseconds.
(4) The offset should be growing slowly, typically less than 0.1 microseconds/day.
(5) The offset should not be adjusted unnecessarily, no more often than once per year if possible.
(6) Items (2)-(5) are only recommendations and may not be feasible in some situations and do not need to replace existing successful practice at any station. However to the extent it is reasonable, stations should align themselves with these practices.
Time and Frequency in VLBI
Background
15
Recommendation (1) is a recognition that different stations have different preferences on which 1 PPS occurs first: Maser or GPS.
Recommendations (2)-(4) are intended to minimize both the need to re-tune the Maser and the chances of the offset going through zero.
Recommendation (5) is intended to make it easier to relate the offset data from one experiment to another.
For completeness, the following requirements (as opposed to recommendations) are listed for the FS log recorded offset between GPS and formatter 1PPS signals, the "GPS/FM offset". These requirements are necessary to allow correct interpretation of the offset data downstream. Please note that these requirements deal with the GPS/FM offset, which is related to, but different from Maser/GPS offset discussed above. In addition to the GPS/FM offset, stations can, and are encouraged to, record (appropriately labelled) additional available clock offset data, including the Maser/GPS offset, in their FS logs or separately.
The requirements for the GPS/FM offset recorded in the FS logs:
(7) The offset is positive and small, i.e. close to (but not too close to) zero and NOT close to one second. If the recommendations (2)-(4) for the Maser/GPS offset above are used for that offset, they are likely to also be true for the GPS/Maser offset as well. In any event, the GPS/FM offset should not cross zero.
(8) The offset is recorded with either of two possible commands depending on how the counter is connected. The connections should be chosen to agree with (7) and: (A) If the counter is started by the GPS 1 PPS, use the "gps-fmout" command. This should be the case if the formatter output 1 PPS (usually determined by the Maser) is late. (B) If the counter is started by the fmout 1 PPS, use the "fmout-gps" command. This should be the case if the GPS 1 PPS is late. It will be necessary to change which command is used if which signal is late changes. This should not be needed if recommendations (2)-(4) for the Maser/GPS offset are followed.
(9) The offset counter does not use arithmetical processing. It just reports the "raw" difference in time between the start and stop signal. So for example, the small positive offset in (7) is not achieved by subtracting the raw difference from 1 second.
(10) The offset counter does not use averaging. This allows immediate detection of jumps. Averaging can be applied in post processing of the data.
(11) The offset must be measured at least once per scan in MIDOB. Additional measurements are acceptable as well.
Time and Frequency in VLBI
Background
16
Time and Frequency in VLBI
H-Maser
17
Time and Frequency in VLBI
H-Maser
Credit: Microsemi MHM2010 Manual 18
Credit: Microsemi MHM2010 Manual
Time and Frequency in VLBI
H-Maser
19
Sigma Tau MHM 2010
• 2 5MHz • Maser Data • 2 10 MHz • Sync Port • 2 1PPS
NR Maser
• 4 5MHz • 2 1 PPS • Maser Data
Time and Frequency in VLBI
H-Maser
20
CNS Clock II or
Original CNS Clock
or
Time and Frequency in VLBI
53132A
H-Maser
53230A
21
Maser Data Monitoring
Frequency Data – Tac32Plus
Time and Frequency in VLBI
H-Maser
22
This data set shows the H-maser frequency error of about 7*10-14
Time and Frequency in VLBI
H-Maser
23
However, a more detailed look at the data set shows an old GPS receiver with known data issues. This GPS receiver should be replaced.
Time and Frequency in VLBI
H-Maser
24
For comparison, this data set shows the CNS HP5065 Rubidium frequency error of about 1.6*10-12
Time and Frequency in VLBI
H-Maser
25
• Hydrogen Pressure • Microprocessor batteries • Magnetics/Degaussing • Vacion pumps • Hydrogen gas • Frequency corrections
Time and Frequency in VLBI
H-Maser
26
• Power Outages
• Temperature instabilitiesheater currents
• Loss of IF/VCO • Backup Batteries
• Microprocessor Failure
• Power Supplies
• Fuses
Time and Frequency in VLBI
H-Maser
27
• Week rollover may mean retiring old GPS receivers (Motorola VP, UT+, etc.)
• “We have legacy equipment using the Oncore VP. We have found that the VP receivers have a cutoff date after which the date reverts back 1024 weeks.” The compile date of v10.0 was 24‐Sep‐1999 => rollover is 10‐May‐2019.
• GPS receiver’s quantization error (“sawtooth”).
• “Absolute” Receiver Calibration
• New developments • The SSR-M8T GNSS receiver • Tac32Plus updates • CNS Clock II improvements (NTP, Oscillator, PPS)
Time and Frequency in VLBI
GPS Time
28
microseconds (normalized)
0.040 0.030 0.020
~26 nsec p-to-p
Rx A - Motorola M12+ V2.0 vs. USNO
Data logged by Tac32Plus, Aug 8, 2002 UTC (Day 220). ©2002 CNS Systems, Inc., plotted by Richard M. Hambly RED = Raw 1PPS BLUE = Sawtooth Corrected Data
0.010
0.000
-0.010
-0.020
-0.030
-0.040 01:00:00
01:01:00
01:02:00
01:03:00
~1.5 to 3 nsec RMS noise (after applying quantization correction)
COPYRIGHT 1991-2002 MOTOROLA INC. SFTW P/N # 61-G10268A SOFTWARE VER # 2 SOFTWARE REV # 0 SOFTWARE DATE AUG 14 200 MODEL # P283T12NR5 HWDR P/N # 2 SERIAL # P030XY MANUFACTUR DATE 2G13
01:04:00 01:05:00 01:06:00 Time(UTC)
01:07:00
01:08:00
01:09:00
01:10:00
Time and Frequency in VLBI
GPS Time
29
LOs RF STUFF
DSP STUFF Samplers Correlators Integrators
CRYSTAL
These are derived f rom the same 1/F Signal source, so they are locked to each other.
MASTER OSCILLATOR
& CLOCK
1/F Clock Edg e
Unless 1/F is a "perfect" multiple of 1second, the 1PPS w ill have a saw tooth "w alk"
Freq = F
IN Looooonnnngggg Counter
1 PPS Clock Edg e
Computer
START REGISTER STOP REGISTER
LATCH
Serial message tells error +/- 1 nsec
1PPS OUT
RS232
• For the older VP, UT+ Oncore, F=9.54 MHz, so the 1/F quantization error has a range of +/- 52 nsec (104 nsec peak-to-peak).
• The M12+ & M12M have F ≈ 40 MHz, so the quantization error has been reduced to +/- 12.5 nsec (25 nsec).
• SSR-M8T has F ≈ 30.72 * 2 = 61.44 MHz, so the quantization error has been reduced to +/- 8 nsec (16 nsec).
Time and Frequency in VLBI
GPS Time
30
• When the formatter (Mark 5/6 sampler) needs to be reset, you have to feed it a 1PPS timing pulse to restart the internal VLBI clock. After it is started, it runs smoothly at a rate defined by the Maser’s 5/10 MHz.
• The AVERAGE of the 1PPS pulses from the GPS receiver is “correct”, but any single pulse can be in error by ±52, ±13, or ±8 nsec because of the quantization error.
• Once you have restarted the formatter with the noisy 1 PPS signal, you then measure the actual (GPS minus Formatter) time that you actually achieved.
• Or, you can use the 1PPS from a CNS Clock II which has the quantization error removed.
Time and Frequency in VLBI
GPS Time
31
GPS Timing Receiver
1PPS with quantization noise
Programmable Delay Line with 250 psec steps
(Dallas/Maxim DS1023-25)
Serial Data
PIC Microprocessor generates the correction for the NEXT 1PPS tick
“Clean” 1PPS
RS-232
This technique is used in CNS Clock II units with revision levels A through K, although with a variety of different delay line types and step values.
Time and Frequency in VLBI
GPS Time
32
Serial Data
“Clean” 1PPS RS-232
CNS Clock II
with SSR-M8F
DAC
Primary
Reference
Oscillator
(OCXO)
PPS Generator
This technique is used in CNS Clock II units, beginning with revision L. Among other benefits, this provides holdover with synchronized date and time for better NTP and IRIG-B performance. The PPS stability is better than the delay line version.
Time and Frequency in VLBI
GPS Time
33
1994 – 2004: the TAC
• Data available on RS-232, USB,
Ethernet, RS-485 and solid
state relay ports.
• Ethernet NTP Server.
Available Since January 2005, now at Revision L • TNC GPS Antenna Connector.
• Buffered 1 PPS outputs.
• GPS Steered OCXO
10 (or 5) MHz output.
• High Performance PPS.
• IRIG-B
• Good holdover performance
Time and Frequency in VLBI
GPS Time
34
Revision
CNS Clock (original) CNS Clock II
Rev A Rev B Rev C Rev D Rev E Rev F Rev G Rev H Rev I Rev J Rev K Rev L
Serial Number Range
Internal Receiver Delay (nanoseconds)
801001 801455
8
n/a
404001 n/a
404029 404056 404109 404160 404266 404319 404345 404372 404399
n/a
404028 n/a
404055 404108 404159 404265 404138 404344 404371 404398
53 (estimated) 53 (estimated) 53 (estimated) 53 (estimated) 53 (estimated) 53 (estimated) 53 (estimated) 97 (estimated) 97 (estimated)
97 54
• The current CNS Clock internal delay is set using the table in the latest instruction manual, available online. This offset is removed by setting the parameter in Tac32Plus or GPSTime.
• For the CNS Clock II, revision B and up, performs quantization correction in hardware, so the software correction should be set to “Off”.
Time and Frequency in VLBI
GPS Time
35
Microseconds
0.050 0.040 0.030
RED = Raw 1PPS
CNS Clock II with M12M JB6430 V1.1 with 0.15nsec/div Delay Line Hardware vs. Software 1PPS Corrections
Data logged by Tac32Plus, April 19, 2009 UTC (Day 109). ©2009 CNS Systems, Inc., plot by Richard M. Hambly
GREEN= Hardware Corrected Data BLUE = Software Sawtooth Corrected Data Violet = Correction Difference
0.020
0.010
0.000
-0.010
-0.020 0.00
-0.030 0.00
-0.040
-0.050
-00.0.0600
2.8 nsec RMS
9.0 nsec RMS
2.5 nsec RMS 0.7 nsec RMS
-0.070 00:00
00:01
00:02
00:03
00:04
00:05 Time (UTC)
00:06
00:07
Time and Frequency in VLBI
GPS Time
00:08
00:09
00:10
36
Calibrating the UTC Offset (ACCURACY) of M12+ receivers with 2.0 Firmware.
We observed that the “Oncore” firmware evolution from 5.x ⇒ 6.x ⇒ 8.x ⇒ 10.x has been accompanied by about 40 nsec of “DC” timing offsets. Motorola tasked CNS to calibrate the new M12+ receiver in 2002.
Tac32Plus software simultaneously processes data from four Time Interval Counters and four CNS Clocks, writing
12 logs continuously.
Time Interval Counters compare the 1PPS from each CNS Clock
(M12+) against the USNO’s UTC time tick.
This is the “Gold Standard” “A” receiver that we used for subsequent calibrations.
Time and Frequency in VLBI
GPS Time
37
• Motorola quit the GPS business in 2005. The M12 design was
licensed to iLotus in Singapore. The current variant is the
M12M.
• Anticipating the need for an M12 replacement, Synergy
Systems LLC and CNS Systems, Inc. developed the SSR series of
receivers. These are form, fit, and function replacements for
the M12 using u-blox GPS modules.
• The latest version of this new receiver has improved hardware,
firmware and the u-blox M8T/F GNSS module that supports
multiple satellite systems. This is standard in the latest CNS
Clock II product.
• CNS has an upgrade kit for the original TAC and CNS Clock units
that replaces the obsolete Motorola VP and UT+ receivers with
the latest SSR-M8T+ board.
Time and Frequency in VLBI
GPS Time
38
An iLotus M-12M receiver. The M12+ looks the same
The Synergy SSR-M8T Receiver
The u-blox LEA-M8T module
Time and Frequency in VLBI
GPS Time
39
“Gold” Motorola
M12+
iLotus M12-M
Synergy LEA-6Ts u-blox u-blox Moto Native Emul . Cmds
Time and Frequency in VLBI
GPS Time
Maser 1PPS Distributor
Four HP53132 Counters
40
8.55
8.50
10 days of 1 minute averages of Sigma-Tau 1pps tick to each of 4 rcvrs.
8.45
Maser rate ~ 27.3 nsec/day Clock offsets ~ 8 μsec
8.40
GPS LATE TO MASER 1PPS TICK, uSec
8.35
A: MOTOROLA M12+ "Gold Standard"
8.30
B: MOTOROLA/iLOTUS M12M C: u-blox 6T (Motorola Emulator)
8.25
D: u-blox 6T (u-blox Native)
8.20
8.15
8.10 8/17/12 0:00
8/19/12 0:00
Time and Frequency in VLBI
8/21/12 0:00
8/23/12 0:00
GPS Time
8/25/12 0:00
8/27/12 0:00
8/29/12 0:00
41
Time and Frequency in VLBI
GPS Time
42
Time and Frequency in VLBI
GPS Time
43
1. Small, low cost GPS receivers can provide timing needed for VLBI anywhere in the world. This is not a new statement, it’s been true since the 1990’s! See www.cnssys.com under the “Publications” tab for “Timing for VLBI” notes from the IVS TOWs for more details.
2. Existing designs based on Motorola/iLotus M12s should have no problem in making the change to u-blox by using the Synergy SSR-M8T/M8F receivers.
3. The Synergy SSR receiver with either the uBlox LEA-M8T/M8F
(GNSS) is a superior product. In fact, the u-blox we tested were
a factor ~5 BETTER than the M12’s in all tests. When used in
the CNS Clock II with its quantization correction circuit, the UTC
offset is set based on a chart in the latest manual. Just plug that
value into Tac32Plus or GPSTime and all is good.
Time and Frequency in VLBI
GPS Time
44
Motorola UT+ receivers, used in some original CNS Clocks have failed due to the week rollover event. These now provide the wrong date and time.
A receiver upgrade kit is available for original TAC and CNS Clock units.
Replaces old Motorola VP and UT+ with new SSRM8T (u-blox) receivers.
Time and Frequency in VLBI
GPS Time
45
Agilent announced “End-of-Life” for the 53131 and 53132 counters that have been the standard VLBI Time Interval Counter. These use a simple RS232 printer port interface. Tac32Plus was built around this capability.
Agilent is recommending the 53230A as their suggested replacement for the 131/132. This is the counter that CNS is now using. Berkeley Nucleonics offers their Model 1104 as an alternative.
Both these counters use Ethernet ports for control and data. This allows Tac32Plus and GPSTime to implement setup commands and collect data, simplifying station operation and interface wiring.
Time and Frequency in VLBI
GPS Time
46
Time and Frequency in VLBI
CNS Clock II
HP/Agilent 53132A
Serial Port
Agilent 53230A Ethernet
Tac32Plus
Berkeley Nucleonics Model 1104 Ethernet
Note: GPS time vs. HP5065A Rubidium CNS Systems’ time standard
GPS Time
47
Time and Frequency in VLBI
TIC Setup is simple 53132A vs.
and familiar
BN1105
GPS Time
53132A vs. 53230A
48
• Support for the TAPR TICC time interval counter. • Add satellite constellation selection for SSR receivers: GPS,
GLONASS, Galileo, Beidou, QZSS and/or SBAS (WAAS, etc.). • Implement the Leap Indicator (LI) sub-field in the first word of the
NTP protocol message. • Enable dynamic mode settings for SSR (u-blox) receivers. Auto
select based on navigation vs. position hold and self-survey. • Improve restart after receiver power interruption. • Improve startup after initial installation. • Improved the firmware upload capability for the SSR Plus series
receivers. • Additional support for native u-blox mode. • Many minor changes and bug fixes. • See https://www.cnssys.com/Tac32Plus/Tac32Plus.php
Time and Frequency in VLBI
GPS Time
49
• MultiPlatform executables, especially Linux.
Time and Frequency in VLBI
GPS Time
50
CNS Clock II: • TCP/IP or UDP/IP data interface. • Internal Web page setup. • Expanded IRIG capabilities. • Firmware updates using Ethernet.
Contact Rick Hambly: rick@cnssys.com
Time and Frequency in VLBI
GPS Time
51
Time and Frequency in VLBI
GPS Time
52