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IS-GPS-200E 8 June 2010
GLOBAL POSITIONING SYSTEM WING (GPSW) SYSTEMS ENGINEERING & INTEGRATION INTERFACE SPECIFICATION IS-GPS-200 Revision E
Navstar GPS Space Segment/Navigation User Interfaces
AUTHENTICATED BY: ____________________________________
DAVID B. GOLDSTEIN, Col Chief Engineer
Global Positioning Systems Wing
DISTRIBUTION STATEMENT A.: Approved for Public Release; Distribution is Unlimited.
DESCRIPTION ICD-GPS-200, Initial Release
ICD-GPS-200A
DATE 25 Jan 1983 25 Sep 1984
ICD-GPS-200B
30 Nov 1987
ICD-GPS-200C
10 Oct 1993
IS-GPS-200D
7 Dec 2004
IRN-200D-001
7 Mar 2006
IS-GPS-200E
8 June 2010
DISTRIBUTION A. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED
APPROVALS
AUTHORIZED SIGNATURES Signature on File
REPRESENTING
GPS Wing (GPSW) Space & Missiles Center (SMC) – GPSW LAAFB
DATE 8 June 2010
INTERFACE SPECIFICATION
THIS DOCUMENT SPECIFIES TECHNICAL REQUIREMENTS AND NOTHING HEREIN CONTAINED SHALL BE DEEMED TO ALTER THE TERMS OF ANY CONTRACT OR PURCHASE ORDER BETWEEN ALL PARTIES AFFECTED.
ICD TITLE
Navstar GPS Space Segment/Navigation User Interfaces
Prepared by: Science Applications International Corporation
GPSW SE&I 300 N. Sepulveda Blvd., Suite 3000.
El Segundo, CA 90245
CODE IDENT NO. 4WNC1
DOCUMENT NO. IS-GPS-200
REV
SHEET I
E
ii
IS-GPS-200E
8 June 2010
REV
NC A
B C C C D
IRN200D-
001 E N/A
REVISION RECORD
DESCRIPTION
DOCUMENT DATE
Initial Release
25 Jan 1983
Incorporates IRN-200NC-001, IRN-200NC-002, and IRN200NC-003
25 Sep 1984
Incorporates IRN-200A-001A
30 Nov 1987
Incorporates IRN-200B-001 thru IRN-200B-007
10 Oct 1993
Re-formatted in Microsoft Word 6.0 in GEMS compatible format 10 Oct 1993
Changed distribution status to Public Release
25 Sep 1997
Incorporates IRN-200C-001 thru IRN-200C-005R1, change ICDGPS-200 to IS-GPS-200, introduce and specify the requirements of Improved Clock and Ephemeris (ICE) message for L2 C signal, and other additional updates
7 Dec 2004
Adds additional PRN sequences to Section 6
7 Mar 2006
GPS IIIA Incorporations SE&I Tech Pubs
8 Jun 2010 29 July 2010
APPROVED
12 Jan 1996 20 Oct 1997 23 Nov 2004 9 Mar 2006 8 June 2010
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TABLE OF CONTENTS
1. INTRODUCTION ..................................................................................................................................................1 1.1 Scope ........................................................................................................................................................1 1.2 IS Approval and Changes.........................................................................................................................1
2. APPLICABLE DOCUMENTS..............................................................................................................................2 2.1 Government Documents...........................................................................................................................2 2.2 Non-Government Documents...................................................................................................................2
3. REQUIREMENTS .................................................................................................................................................3 3.1 Interface Definition ..................................................................................................................................3 3.2 Interface Identification .............................................................................................................................3 3.2.1 Ranging Codes ........................................................................................................................3 3.2.2 NAV Data ............................................................................................................................... 9 3.2.3 L1/L2 Signal Structure .......................................................................................................... 10 3.3 Interface Criteria..................................................................................................................................... 12 3.3.1 Composite Signal .................................................................................................................. 12 3.3.2 PRN Code Characteristics ..................................................................................................... 17 3.3.3 Navigation Data .................................................................................................................... 33 3.3.4 GPS Time and SV Z-Count...................................................................................................35
4. NOT APPLICABLE ............................................................................................................................................. 37 5. NOT APPLICABLE ............................................................................................................................................. 38 6. NOTES................................................................................................................................................................... 39
6.1 Acronyms ............................................................................................................................................... 39 6.2 Definitions .............................................................................................................................................. 42
6.2.1 User Range Accuracy............................................................................................................42 6.2.2 SV Block Definitions ............................................................................................................ 42 6.2.3 Operational Interval Definitions............................................................................................43 6.2.4 GPS Week Number ............................................................................................................... 44 6.2.5 L5 Civil Signal ...................................................................................................................... 44 6.3 Supporting Material................................................................................................................................44 6.3.1 Received Signals ................................................................................................................... 44 6.3.2 Extended Navigation Mode (Block II/IIA) ........................................................................... 46 6.3.4 Block IIA Mode (Block IIR/IIR-M)......................................................................................47 6.3.5 Autonomous Navigation Mode ............................................................................................. 47 6.3.6 PRN Code sequences expansion ........................................................................................... 47 6.3.7 Pre-Operational Use .............................................................................................................. 57 10. APPENDIX I. LETTERS OF EXCEPTION...................................................................................................58 10.1 Scope .................................................................................................................................................... 58
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10.2 Applicable Documents ......................................................................................................................... 58 10.3 Letters of Exception ............................................................................................................................. 58 20. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA, D(t) ........................................... 67 20.1 Scope .................................................................................................................................................... 67 20.2 Applicable Documents. ........................................................................................................................ 67
20.2.1 Government Documents......................................................................................................67 20.2.2 Non-Government Documents ............................................................................................. 67 20.3 Requirements........................................................................................................................................68 20.3.1 Data Characteristics ............................................................................................................ 68 20.3.2 Message Structure ............................................................................................................... 68 20.3.3 Message Content ................................................................................................................. 79 20.3.4 Timing Relationships ........................................................................................................118 20.3.5 Data Frame Parity ............................................................................................................. 125 30. APPENDIX III. GPS NAVIGATION DATA STRUCTURE FOR CNAV DATA, DC(t)..........................128 30.1 Scope .................................................................................................................................................. 128 30.2 Applicable Documents. ...................................................................................................................... 128 30.2.1 Government Documents....................................................................................................128 30.2.2 Non-Government Documents ........................................................................................... 128 30.3 Requirements......................................................................................................................................128 30.3.1 Data Characteristics .......................................................................................................... 128 30.3.2 Message Structure ............................................................................................................. 129 30.3.3 Message Content ............................................................................................................... 129 30.3.4 Timing Relationships ........................................................................................................175 30.3.5 Data Frame Parity ............................................................................................................. 176
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LIST OF FIGURES
Figure 3-1.
Generation of P-, C/A-Codes and Modulating Signals ...................................................................18
Figure 3-2.
X1A Shift Register Generator Configuration..................................................................................19
Figure 3-3.
X1B Shift Register Generator Configuration .................................................................................. 20
Figure 3-4.
X2A Shift Register Generator Configuration..................................................................................20
Figure 3-6.
P-Code Generation .......................................................................................................................... 23
Figure 3-7.
P-Code Signal Component Timing ................................................................................................. 24
Figure 3-8.
G1 Shift Register Generator Configuration .................................................................................... 27
Figure 3-9.
G2 Shift Register Generator Configuration .................................................................................... 27
Figure 3-10. Example C/A-Code Generation ...................................................................................................... 28
Figure 3-11. C/A-Code Timing Relationships.....................................................................................................29
Figure 3-12. L2 CM-/L2 CL-Code Timing Relationships ................................................................................... 31
Figure 3-13. L2 CM/L2 CL Shift Register Generator Configuration .................................................................. 32
Figure 3-14. Convolutional Encoder ................................................................................................................... 34
Figure 3-15. Convolutional Transmit/Decoding Timing Relationships............................................................... 34
Figure 3-16. Time Line Relationship of HOW Message .....................................................................................36
Figure 6-1.
User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR) ............................ 45
Figure 10.3-1. Letters of Exception. ........................................................................................................................... 59
Figure 10.3-2. Letters of Exception (continued)........................................................................................................60
Figure 10.3-3. Letters of Exception (continued)........................................................................................................61
Figure 10.3-4. Letters of Exception (continued)........................................................................................................62
Figure 10.3-5. Letters of Exception (continued)........................................................................................................63
Figure 10.3-6. Letters of Exception (continued)........................................................................................................64
Figure 10.3-7. Letters of Exception (continued)........................................................................................................65
Figure 10.3-8. Letters of Exception (continued)........................................................................................................66
Figure 20-1. Data Format (sheet 1 of 11) ............................................................................................................. 69
Figure 20-1. Data Format (sheet 2 of 11) ............................................................................................................ 70
Figure 20-1. Data Format (sheet 3 of 11) ............................................................................................................ 71
Figure 20-1. Data Format (sheet 4 of 11) ............................................................................................................ 72
Figure 20-1. Data Format (sheet 5 of 11) ............................................................................................................ 73
Figure 20-1. Data Format (sheet 6 of 11) ............................................................................................................ 74
Figure 20-1. Data Format (sheet 7 of 11) ............................................................................................................ 75
Figure 20-1. Data Format (sheet 8 of 11) ............................................................................................................ 76
Figure 20-1. Data Format (sheet 9 of 11) ............................................................................................................ 77
Figure 20-1. Data Format (sheet 10 of 11) .......................................................................................................... 78
Figure 20-1. Data Format (sheet 11 of 11) .......................................................................................................... 79
Figure 20-2. TLM and HOW Formats ................................................................................................................. 81
Figure 20-3. Sample Application of Correction Parameters ................................................................................ 89
Figure 20-4. Ionospheric Model ........................................................................................................................ 115
Figure 20-5. Example Flow Chart for User Implementation of Parity Algorithm ............................................. 127
Figure 30-1. Message Type 10 - Ephemeris 1 ................................................................................................... 130
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Figure 30-2. Figure 30-3. Figure 30-4. Figure 30-5. Figure 30-6. Figure 30-7. Figure 30-8. Figure 30-9. Figure 30-10. Figure 30-11. Figure 30-12. Figure 30-13. Figure 30-14. Figure 30-15. Figure 30-16.
Message Type 11 - Ephemeris 2 ................................................................................................... 131 Message Type 30 - Clock, IONO & Group Delay ........................................................................ 132 Message Type 31 - Clock & Reduced Almanac ........................................................................... 133 Message Type 32 - Clock & EOP ................................................................................................. 134 Message Type 33 - Clock & UTC.................................................................................................135 Message Type 34 - Clock & Differential Correction .................................................................... 136 Message Type 35 - Clock & GGTO.............................................................................................. 137 Message Type 36 - Clock & Text ................................................................................................. 138 Message Type 37 - Clock & Midi Almanac ................................................................................. 139 Message Type 12 - Reduced Almanac .......................................................................................... 140 Message Type 13 – Clock Differential Correction ....................................................................... 141 Message Type 14 – Ephemeris Differential Correction ................................................................ 142 Message Type 15 - Text................................................................................................................143 Reduced Almanac Packet Content ................................................................................................ 160 Differential Correction Data Packet .............................................................................................. 168
vii
IS-GPS-200E
8 June 2010
LIST OF TABLES
Table 3-I. Table 3-II. Table 3-III. Table 3-IV.
Table 3-Va.
Table 3-Vb. Table 3-Vc.
Table 3-VI. Table 3-VII. Table 6-I Table 6-II. Table 20-I. Table 20-II. Table 20-III. Table 20-IV. Table 20-V. Table 20-VI. Table 20-VII. Table 20-VIII. Table 20-IX. Table 20-X. Table 20-XI. Table 20-XII. Table 20-XIII. Table 20-XIV. Table 30-I. Table 30-I. Table 30-II. Table 30-III. Table 30-IV. Table 30-V. Table 30-VI. Table 30-VII. Table 30-VIII. Table 30-IX. Table 30-X. Table 30-XI.
Code Phase Assignments ..................................................................................................................5 Code Phase Assignments (IIR-M, IIF, and subsequent blocks only) ................................................7 Signal Configuration ....................................................................................................................... 11 Composite L1 Transmitted Signal Phase ........................................................................................ 14
Received Minimum RF Signal Strength for Block IIA, IIR, IIR-M, IIF and III Satellites (20.46 MHz Bandwidth) ............................................................................................................................ 15 Received Minimum RF Signal Strength for GPS III (30.69 MHz Bandwidth) .............................. 15 Space Service Volume (SSV) Received Minimum RF Signal Strength for GPS III and Subsequent Satellites over the Bandwidth Specified in 3.3.1.1 – GEO Based Antennas...................................15 P-Code Reset Timing ...................................................................................................................... 25 Final Code Vector States.................................................................................................................26 Additional C/A-/P-Code Phase Assignments..................................................................................49 Additional L2 CM-/L2 CL-Code Phase Assignments.....................................................................55 Subframe 1 Parameters ...................................................................................................................85 Ephemeris Data Definitions ............................................................................................................ 91 Ephemeris Parameters ..................................................................................................................... 92 Elements of Coordinate Systems .................................................................................................... 93 Data IDs and SV IDs in Subframes 4 and 5 .................................................................................... 99 Almanac Parameters ..................................................................................................................... 100 NAV Data Health Indications ....................................................................................................... 102 Codes for Health of SV Signal Components.................................................................................103 UTC Parameters ............................................................................................................................ 106 Ionospheric Parameters ................................................................................................................. 107 IODC Values and Data Set Lengths (Block II/IIA) ..................................................................... 120 IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF/IIIA) ................................................121 Reference Times ........................................................................................................................... 124 Parity Encoding Equations ............................................................................................................ 126 Message Types 10 and 11 Parameters...........................................................................................148 Message Types 10 and 11 Parameters (2 of 2).............................................................................149 Elements of Coordinate System .................................................................................................... 150 Clock Correction and Accuracy Parameters ................................................................................. 153 Group Delay Differential Parameters............................................................................................156 Midi Almanac Parameters.............................................................................................................159 Reduced Almanac Parameters.......................................................................................................160 Earth Orientation Parameters ........................................................................................................ 162 Application of EOP Parameters ..................................................................................................... 163 UTC Parameters ............................................................................................................................ 166 Differential Correction Parameters ............................................................................................... 169 GPS/GNSS Time Offset Parameters ............................................................................................. 174
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1. INTRODUCTION
1.1 Scope. This Interface Specification (IS) defines the requirements related to the interface between the Space Segment (SS) of the Global Positioning System (GPS) and the navigation User Segment (US) of the GPS for radio frequency (RF) link 1 (L1) and link 2 (L2).
1.2 IS Approval and Changes. The Interface Control Contractor (ICC) designated by the government is responsible for the basic preparation, approval coordination, distribution, retention, and Interface Control Working Group (ICWG) coordination of the IS in accordance with GP-03-001. The Navstar GPS Wing (GPSW) is the necessary authority to make this IS effective. The GPSW administers approvals under the auspices of the Configuration Control Board (CCB), which is governed by the appropriate GPSW Operating Instruction (OI). Military organizations and contractors are represented at the CCB by their respective segment member. All civil organizations and public interest are represented by the Department of Transportation representative of the GPSW.
A proposal to change the approved version of this IS can be submitted by any ICWG participating organization to the GPSW and/or the ICC. The ICC is responsible for the preparation of the change paper and change coordination, in accordance with GP-03-001. The ICC prepares the change paper as a Proposed Interface Revision Notice (PIRN) and is responsible for coordination of PIRNs with the ICWG. The ICWG coordinated PIRN must be submitted to the GPSW CCB for review and approval.
The ICWG review period for all Proposed Interface Revisions Notices (PIRNs) is 45 days after receipt by individual addressees. A written request to extend the review period may be submitted to the ICC for consideration.
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2. APPLICABLE DOCUMENTS
2.1 Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS navigation User Segment, and form a part of this IS to the extent specified herein.
Specifications Federal Military Other Government Activity Standards Federal Military Other Publications
None None None
None None
GP-03-001 (GPS Interface Control Working Group Charter)
2.2 Non-Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS Navigation User Segment and form a part of this IS to the extent specified herein.
Specifications None
Other Publications None
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3. REQUIREMENTS
3.1 Interface Definition. The interface between the GPS Space Segment (SS) and the GPS navigation User Segment (US) includes two RF links, L1 and L2. Utilizing these links, the space vehicles (SVs) of the SS shall provide continuous earth coverage signals that provide to the US the ranging codes and the system data needed to accomplish the GPS navigation (NAV) mission. These signals shall be available to a suitably equipped user with RF visibility to an SV.
3.2 Interface Identification. The carriers of L1 and L2 are typically modulated by one or more bit trains, each of which normally is a composite generated by the modulo-2 addition of a pseudo-random noise (PRN) ranging code and the downlink system data (referred to as NAV data).
3.2.1 Ranging Codes. Three PRN ranging codes are transmitted: the precision (P) code which is the principal navigation ranging code; the Y-code, used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated; and the coarse/acquisition (C/A) code which is used for acquisition of the P (or Y) code (denoted as P(Y)) and as a civil ranging signal. Code-division-multiple-access techniques allow differentiating between the SVs even though they may transmit at the same frequencies. The SVs will transmit intentionally "incorrect" versions of the C/A and the P(Y) codes where needed to protect the users from receiving and utilizing anomalous navigation signals. These two "incorrect" codes are termed non-standard C/A (NSC) and non-standard Y (NSY) codes.
For Block IIR-M, IIF, and subsequent blocks of SVs, two additional PRN ranging codes are transmitted. They are the L2 civil-moderate (L2 CM) code and the L2 civil-long (L2 CL) code. The SVs will transmit intentionally "incorrect" versions of the L2 CM and L2 CL codes where needed to protect the users from receiving and utilizing anomalous navigation signals. These "incorrect" codes are termed non-standard L2 CM (NSCM) and non-standard L2 CL (NSCL) codes. The SVs shall also be capable of initiating and terminating the broadcast of NSCM and/or NSCL code(s) independently of each other, in response to CS command.
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3.2.1.1 P-Code. The PRN P-code for SV ID number i is a ranging code, Pi(t), of 7 days in length at a chipping rate of 10.23 Mbps. The 7 day sequence is the modulo-2 sum of two sub-sequences referred to as X1 and X2i; their lengths are 15,345,000 chips and 15,345,037 chips, respectively. The X2i sequence is an X2 sequence selectively delayed by 1 to 37 chips thereby allowing the basic code generation technique to produce a set of 37 mutually exclusive P-code sequences of 7 days in length. Of these, 32 are designated for use by SVs and 5 are reserved for other purposes (e.g. ground transmitters, etc.). Assignment of these code phase segments by SV-ID number (or other use) is given in Table 3-I. Additional PRN P-code sequences with assigned PRN numbers are provided in Section 6.3.5.2, Table 6-I.
3.2.1.2 Y-Code. The PRN Y-code is used in place of the P-code when the A-S mode of operation is activated.
3.2.1.3 C/A-Code. The PRN C/A-Code for SV ID number i is a Gold code, Gi(t), of 1 millisecond in length at a chipping rate of 1023 Kbps. The Gi(t) sequence is a linear pattern generated by the modulo-2 addition of two subsequences, G1 and G2i, each of which is a 1023 chip long linear pattern. The epochs of the Gold code are synchronized with the X1 epochs of the P-code. As shown in Table 3-I, the G2i sequence is a G2 sequence selectively delayed by pre-assigned number of chips, thereby generating a set of different C/A-codes. Assignment of these by GPS PRN signal number is given in Table 3-I. Additional PRN C/A-code sequences with assigned PRN numbers are provided in Section 6.3.5.1, Table 6-I.
3.2.1.4 L2 CM-Code (IIR-M, IIF, and subsequent blocks). The PRN L2 CM-code for SV ID number i is a ranging code, CM,i(t), which is 20 milliseconds in length at a chipping rate of 511.5 Kbps. The epochs of the L2 CM-code are synchronized with the X1 epochs of the P-code. The CM,i(t) sequence is a linear pattern which is short cycled every count of 10230 chips by resetting with a specified initial state. Assignment of initial states by GPS PRN signal number is given in Table 3-II. Additional PRN L2 CM-code sequence pairs are provided in Section 6.3.5.3, Table 6-II.
3.2.1.5 L2 CL-Code (IIR-M, IIF, and subsequent blocks). The PRN L2 CL-code for SV ID number i is a ranging code, CL,i(t), which is 1.5 seconds in length at a chipping rate of 511.5 Kbps. The epochs of the L2 CL-code are synchronized with the X1 epochs of the P-code. The CL,i(t) sequence is a linear pattern which is generated using the same code generator polynomial as the one used for CM,i(t). However, the CL,i(t) sequence is short cycled by resetting with a specified initial state every code count of 767250 chips. Assignment of initial states by GPS PRN signal number is given in Table 3-II. Additional PRN L2 CL-code sequence pairs are provided in Section 6.3.5.3, Table 6-II.
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Table 3-I. Code Phase Assignments (sheet 1 of 2)
SV GPS PRN Code Phase Selection
ID No.
Signal No.
C/A(G2i)****
(X2i)
Code Delay
Chips
C/A
P
First 10 Chips Octal*
C/A
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
11
11
12
12
13
13
14
14
15
15
16
16
17
17
18
18
19
19
2
6
1
3
7
2
4
8
3
5
9
4
1
9
5
2 10
6
1
8
7
2
9
8
3 10
9
2
3
10
3
4
11
5
6
12
6
7
13
7
8
14
8
9
15
9 10
16
1
4
17
2
5
18
3
6
19
5
1
6
2
7
3
8
4
17
5
18
6
139
7
140
8
141
9
251
10
252
11
254
12
255
13
256
14
257
15
258
16
469
17
470
18
471
19
1440 1620 1710 1744 1133 1455 1131 1454 1626 1504 1642 1750 1764 1772 1775 1776 1156 1467 1633
First 12 Chips
Octal P
4444 4000 4222 4333 4377 4355 4344 4340 4342 4343
4343
* In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). ** C/A codes 34 and 37 are common.
*** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). **** The two-tap coder utilized here is only an example implementation that generates a limited set
of valid C/A codes. = "exclusive or"
NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above.
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Table 3-I. Code Phase Assignments (sheet 2 of 2)
SV GPS PRN Code Phase Selection
ID Signal
No.
No.
C/A(G2i)**** (X2i)
Code Delay
Chips
C/A
P
First 10 Chips Octal*
C/A
20
20
21
21
22
22
23
23
24
24
25
25
26
26
27
27
28
28
29
29
30
30
31
31
32
32
***
33
***
34**
***
35
***
36
***
37**
4
7
20
5
8
21
6
9
22
1
3
23
4
6
24
5
7
25
6
8
26
7
9
27
8 10
28
1
6
29
2
7
30
3
8
31
4
9
32
5 10
33
4 10
34
1
7
35
2
8
36
4 10
37
472
20
473
21
474
22
509
23
512
24
513
25
514
26
515
27
516
28
859
29
860
30
861
31
862
32
863
33
950
34
947
35
948
36
950
37
1715 1746 1763 1063 1706 1743 1761 1770 1774 1127 1453 1625 1712 1745 1713 1134 1456 1713
First 12 Chips
Octal P
4343
4343
* In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). ** C/A codes 34 and 37 are common.
*** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). **** The two-tap coder utilized here is only an example implementation that generates a limited
set of valid C/A codes. = "exclusive or"
NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above.
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Table 3-II. Code Phase Assignments (IIR-M, IIF, and subsequent blocks only) (sheet 1 of 2)
SV ID
GPS PRN
Initial Shift Register State
End Shift Register State (Octal)
No.
Signal No.
(Octal)
L2 CM
L2 CL
L2 CM *
L2 CL **
1
1
742417664
624145772
552566002
267724236
2
2
756014035
506610362
034445034
167516066
3
3
002747144
220360016
723443711
771756405
4
4
066265724
710406104
511222013
047202624
5
5
601403471
001143345
463055213
052770433
6
6
703232733
053023326
667044524
761743665
7
7
124510070
652521276
652322653
133015726
8
8
617316361
206124777
505703344
610611511
9
9
047541621
015563374
520302775
352150323
10
10
733031046
561522076
244205506
051266046
11
11
713512145
023163525
236174002
305611373
12
12
024437606
117776450
654305531
504676773
13
13
021264003
606516355
435070571
272572634
14
14
230655351
003037343
630431251
731320771
15
15
001314400
046515565
234043417
631326563
16
16
222021506
671511621
535540745
231516360
17
17
540264026
605402220
043056734
030367366
18
18
205521705
002576207
731304103
713543613
19
19
064022144
525163451
412120105
232674654
* Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters).
NOTE: There are many other available initial register states which can be used for other signal transmitters including any additional SVs in future.
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Table 3-II. Code Phase Assignments (IIR-M, IIF, and subsequent blocks only) (sheet 2 of 2)
SV ID
GPS PRN Initial Shift Register State (Octal) End Shift Register State (Octal)
No.
Signal No.
L2 CM
L2 CL
L2 CM *
L2 CL **
20
20
120161274
266527765 365636111
641733155
21
21
044023533
006760703 143324657
730125345
22
22
724744327
501474556 110766462
000316074
23
23
045743577
743747443 602405203
171313614
24
24
741201660
615534726 177735650
001523662
25
25
700274134
763621420 630177560
023457250
26
26
010247261
720727474 653467107
330733254
27
27
713433445
700521043 406576630
625055726
28
28
737324162
222567263 221777100
476524061
29
29
311627434
132765304 773266673
602066031
30
30
710452007
746332245 100010710
012412526
31
31
722462133
102300466 431037132
705144501
32
32
050172213
255231716 624127475
615373171
***
33
500653703
437661701 154624012
041637664
***
34
755077436
717047302 275636742
100107264
***
35
136717361
222614207 644341556
634251723
***
36
756675453
561123307 514260662
257012032
***
37
435506112
240713073 133501670
703702423
* Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters).
NOTE: There are many other available initial register states which can be used for other signal transmitters including any additional SVs in future.
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3.2.1.6 Non-Standard Codes. The NSC, NSCM, NSCL, and NSY codes, used to protect the user from tracking anomalous navigation signals, are not for utilization by the user and, therefore, are not defined in this document.
3.2.2 NAV Data. The NAV data, D(t), includes SV ephemerides, system time, SV clock behavior data, status messages and C/A to P (or Y) code handover information, etc. The 50 bps data is modulo-2 added to the P(Y)and C/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers. For a given SV, the data train D(t), if present, is common to the P(Y)- and C/A- codes on both the L1 and L2 channels. The content and characteristics of the NAV data, D(t), are given in Appendix II of this document.
For Block IIR-M, Block IIF, and subsequent blocks of SVs, civil navigation (CNAV) data, DC(t), also includes SV ephemerides, system time, SV clock behavior, status messages, etc. The DC(t) is a 25 bps data stream which is encoded by a rate ½ convolutional encoder. When selected by ground command, the resulting 50 sps symbol stream is modulo-2 added to the L2 CM-code; the resultant bit-train is combined with L2 CL-code using chip by chip timedivision multiplexing method (i.e. alternating between L2 CM data and L2 CL chips); the multiplexed bit-train is used to modulate the L2 carrier. The content and characteristics of the CNAV data, DC(t), are given in Appendix III of this document.
During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, Block IIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t). Moreover, the NAV data, D(t), can be used in one of two different data rates which are selectable by ground command. D(t) with a data rate of 50 bps can be commanded to be modulo-2 added to the L2 CM-code, or D(t) with a symbol rate of 50 symbols per second (sps) (rate ½ convolutional encoding of 25 bps NAV data) can be commanded to be modulo-2 added to the L2 CM-code. The resultant bit-train is combined with L2 CL-code using chip by chip time-division multiplexing method (i.e. alternating between L2 CM data and L2 CL chips). This multiplexed bit-train is used to modulate the L2 carrier.
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3.2.3 L1/L2 Signal Structure. The L1 consists of two carrier components which are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. One bit train is the modulo-2 sum of the P(Y)-code and NAV data, D(t), while the other is the modulo-2 sum of the C/A-code and the NAV data, D(t). For Block II/IIA and IIR, the L2 is BPSK modulated by only one of those two bit trains; the bit train to be used for L2 modulation is selected by ground command. A third modulation mode is also selectable on the L2 channel by ground command: it utilizes the P(Y)-code without the NAV data as the modulating signal. For a particular SV, all transmitted signal elements (carriers, codes and data) are coherently derived from the same onboard frequency source.
For Block IIR-M, Block IIF, and subsequent blocks of SVs, the L2 consists of two carrier components. One carrier component is BPSK modulated by the bit train which is the modulo-2 sum of the P(Y)-code with or without NAV data D(t), while the other is BPSK modulated by any one of three other bit trains which are selectable by ground command. The three possible bit trains are: (1) the modulo-2 sum of the C/A-code and D(t); (2) the C/A-code with no data and; (3) a chip-by-chip time multiplex combination of bit trains consisting of the L2 CM-code with DC(t) and the L2 CL-code with no data. The L2 CM-code with the 50 sps symbol stream of DC(t) is time-multiplexed with L2 CL-code at a 1023 kHz rate as described in paragraph 3.2.2. The first L2 CM-code chip starts synchronously with the end/start of week epoch.
During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, Block IIR-M may modulo-2 add the NAV data, D(t), to the L2 CM-code instead of CNAV data, DC(t). In such configuration, the data rate of D(t) may be 50 bps (i.e. without convolution encoding) or it may be 25 bps. The D(t) of 25 bps shall be convolutionally encoded resulting in 50 sps.
The different configurations and combinations of codes/signals specified in this section are shown in Table 3-III.
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Table 3-III.
Signal Configuration
SV Blocks
In-Phase*
L1 Quadrature-Phase*
In-Phase*
L2** Quadrature-Phase*
Block II/IIA/IIR P(Y) D(t)
C/A D(t)
Block IIR-M*** P(Y) D(t)
C/A D(t)
Block IIRM/IIF/IIIA
P(Y) D(t)
C/A D(t)
P(Y) D(t) or
P(Y) or
C/A D(t)
P(Y) D(t) or
P(Y)
P(Y) D(t) or
P(Y)
Not Applicable
L2 CM D(t) with L2 CL or
L2 CM D (t) with L2 CL or
C/A D(t) or C/A
L2 CM DC(t) with L2 CL or
C/A D(t) or C/A
Notes:
1) The configuration identified in this table reflects only the content of Section 3.2.3 and does not show all available codes/signals on L1/L2.
2) It should be noted that there are no flags or bits in the navigation message to directly indicate which signal option is broadcast for L2 Civil (L2 C) signal.
= ―exclusive-or‖ (modulo-2 addition) D(t) = NAV data at 50 bps
D (t) = NAV data at 25 bps with FEC encoding resulting in 50 sps DC(t) = CNAV data at 25 bps with FEC encoding resulting in 50 sps
* Terminology of ―in-phase‖ and ―quadrature-phase‖ is used only to identify the relative phase quadrature relationship of the carrier components (i.e. 90 degrees offset of each other).
** The two carrier components on L2 may not have the phase quadrature relationship. They may be broadcast on same phase (ref. Section 3.3.1.5).
*** Possible signal configuration for Block IIR-M only during the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal. See paragraph 3.2.2.
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3.3 Interface Criteria. The criteria specified in the following define the requisite characteristics of the SS/US interface for the L1 and L2.
3.3.1 Composite Signal. The following criteria define the characteristics of the composite signals.
3.3.1.1 Frequency Plan. For Block IIA, IIR, IIR-M, and IIF satellites, the requirements specified in this IS shall pertain to the signal contained within two 20.46 MHz bands; one centered about the L1 nominal frequency and the other centered about the L2 nominal frequency (see Table 3-Vb). For Block III and subsequent satellites, the requirements specified in this IS shall pertain to the signal contained within two 30.69 MHz bands; one centered about the L1 nominal frequency and the other centered about the L2 nominal frequency (see Table 3-Vc). The carrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency source within the SV. The nominal frequency of this source -- as it appears to an observer on the ground -- is 10.23 MHz. The SV carrier frequency and clock rates -- as they would appear to an observer located in the SV -- are offset to compensate for relativistic effects. The clock rates are offset by f/f = -4.4647E-10, equivalent to a change in the P-code chipping rate of 10.23 MHz offset by a f = -4.5674E-3 Hz. This is equal to 10.2299999954326 MHz. The nominal carrier frequencies (f0) shall be 1575.42 MHz, and 1227.6 MHz for L1 and L2, respectively.
3.3.1.2 Correlation Loss. The correlation loss is defined as the difference between the signal power received in the bandwidth defined in 3.3.1.1 (excluding signal combining loss) and the signal power recovered in an ideal correlation receiver of the same bandwidth using an exact replica of the waveform within an ideal sharp-cutoff filter bandwidth, whose bandwidth corresponds to that specified in 3.3.1.1 and whose phase is linear over that bandwidth.
The total allowable correlation loss due to SV modulation and filtering imperfections, which is a function of signal, shall be:
Code
C/A & L2C L1P(Y) & L2P(Y)
Correlation Loss (IIF and prior SVs)
0.6 dB 0.6 dB
Correlation Loss (III SVs) 0.3 dB 0.6 dB
3.3.1.3 Carrier Phase Noise. The phase noise spectral density of the unmodulated carrier shall be such that a phase locked loop of 10 Hz one-sided noise bandwidth shall be able to track the carrier to an accuracy of 0.1 radians rms.
3.3.1.4 Spurious Transmissions. In-band spurious transmissions, from the SV, shall be at or below -40 dBc over the respective bands specified in 3.3.1.1. In-band spurious transmissions are defined as transmissions within the bands specified in 3.3.1.1 which are not expressly components of the L1 and L2 signals.
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3.3.1.5 Signal Component Phasing.
3.3.1.5.1 Phase Quadrature. The two L1 carrier components modulated by the two separate bit trains (C/A-code plus data and P(Y)-code plus data) shall be in phase quadrature (within ±100 milliradians) with the C/A signal carrier lagging the P signal by 90 degrees. Referring to the phase of the P carrier when Pi(t) equals zero as the "zero phase angle", the P(Y)- and C/A-code generator output shall control the respective signal phases in the following manner: when Pi(t) equals one, a 180-degree phase reversal of the P-carrier occurs; when Gi(t) equals one, the C/A carrier advances 90 degrees; when the Gi(t) equals zero, the C/A carrier shall be retarded 90 degrees (such that when Gi(t) changes state, a 180-degree phase reversal of the C/A carrier occurs). The resultant nominal composite transmitted signal phases as a function of the binary state of only the two modulating signals are as shown in Table 3-IV.
For Block IIR-M, IIF, and subsequent blocks of SVs, the two L2 carrier components shall be either in phase quadrature or in the same phase (within ±100 milliradians) – see paragraph 3.3.1.5.3 for additional information. The civil signal carrier component is modulated by any one of three (IIF) or four (IIR-M) different bit trains as described in paragraph 3.2.3. The resultant composite transmitted signal phases will vary as a function of the binary state of the modulating signals as well as the signal power ratio and phase quadrature relationship. Beyond these considerations, additional carrier components in Block IIR-M, IIF, and subsequent blocks of SVs will result in composite transmitted signal phase relationships other than the nominal special case of Table 3-IV. The current phase relationship of the two L2 carrier components (L2C and L2P(Y)) shall be indicated by means of bit 273 of the CNAV Type 10 Message (See section 30.3.3), where zero indicates phase quadrature, with the L2C lagging the L2P(Y) by 90 degrees, and one indicates that L2C and L2P(Y) are in-phase. If the CNAV message is not available, then the L2C and L2P(Y) shall be fixed in phase quadrature.
3.3.1.5.2 Phase Crosstalk. For Block IIF, the crosstalk between the C/A, when selected, and P(Y) signals shall not exceed –20 dB in the L1 and L2. The crosstalk is the relative power level of the undesired signal to the desired reference signal.
3.3.1.5.3 Phase Continuity. While the satellite is broadcasting standard C/A, P(Y), and L2C codes with data that indicates that C/A, P(Y), and L2C signal health (respectively) is OK, there will not be any commanded operation causing an intentional phase discontinuity. This does not apply to phase discontinuities caused by signal modulation. Prior to health data being available on L2C, satellites will be set unhealthy using the non-standard code.
3.3.1.6 User-Received Signal Levels. The SV shall provide L1 and L2 navigation signal strength at end-of-life (EOL), worst-case, in order to meet the minimum levels specified in Table 3-V. Any combining operation done by
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the SV and associated loss is compensated by an increase in SV transmitted power and thus transparent to the user segment. The minimum received power is measured at the output of a 3 dBi linearly polarized user receiving antenna (located near ground) at worst normal orientation, when the SV is above a 5-degree elevation angle. The received signal levels are observed within the in-band allocation defined in para. 3.3.1.1.
The Block IIF SV shall provide L1 and L2 signals with the following characteristic: the L1 off-axis relative power (referenced to peak transmitted power) shall not decrease by more than 2 dB from the Edge-of-Earth (EOE) to nadir, nor more than 10 dB from EOE to 20 degrees off nadir, and no more than 18 dB from EOE to 23 degrees off nadir; the L2 off-axis power gain shall not decrease by more than 2 dB from EOE to nadir, and no more than 10 dB from EOE to 23 degrees off nadir; the power drop off between EOE and 23 degrees shall be in a monotonically decreasing fashion.
The Block III SV shall provide L1 and L2 signals with the following characteristic: the L1 off-axis relative power (referenced to peak transmitted power) shall not decrease by more than 2 dB from the Edge-of-Earth (EOE) to nadir; the L2 off-axis power gain shall not decrease by more than 2 dB from EOE to nadir; the power drop off between EOE and ±26 degrees shall be in a monotonically decreasing fashion. Additional related data is provided as supporting material in paragraph 6.3.1.
Table 3-IV. Composite L1 Transmitted Signal Phase ** (Block II/IIA and IIR SVs Only)
Nominal Composite L1 Signal Phase*
0° -70.5° +109.5° 180°
Code State
P
C/A
0
0
1
0
0
1
1
1
* Relative to 0, 0 code state with positive angles leading and negative angles lagging. ** Based on the composite of two L1 carrier components with 3 dB difference in the power levels of the two.
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Table 3-Va.
Received Minimum RF Signal Strength for Block IIA, IIR, IIR-M, IIF and III Satellites (20.46 MHz Bandwidth)
SV Blocks IIA/IIR
IIR-M/IIF III
Channel
L1 L2 L1 L2 L1 L2
P(Y) -161.5 dBW -164.5 dBW -161.5 dBW -161.5 dBW -161.5 dBW -161.5 dBW
Signal
C/A or L2 C -158.5 dBW -164.5 dBW -158.5 dBW -160.0 dBW -158.5 dBW -158.5 dBW
Table 3-Vb. Received Minimum RF Signal Strength for GPS III (30.69 MHz Bandwidth)
SV Blocks III
Channel
L1 L2
P(Y) -161.5 dBW -161.5 dBW
Signal
C/A or L2 C -158.5 dBW -158.5 dBW
3.3.1.6.1 Space Service Volume (SSV) User-Received Signal Levels. The SV shall provide L1 and L2 navigation signal strength at end-of-life (EOL), worst-case, in order to meet the minimum levels specified in Table 3-Vc. The minimum received power is measured at the output of a 0 dBi right-hand circularly polarized (i.e. 0 dB axial ratio) user receiving antenna at normal orientation, at the off-nadir angles defined in Table 3-Vc. The received signal levels are observed within the in-band allocation defined in paragraph. 3.3.1.1.
Table 3-Vc. Space Service Volume (SSV) Received Minimum RF Signal Strength for GPS III and Subsequent Satellites over the Bandwidth Specified in 3.3.1.1 – GEO Based Antennas
SV Blocks
Channel
Off Axis Angle Relative To Nadir
Signal
P(Y)
C/A or L2 C
III and
L1
23.5 deg
-187.0 dBW*
-184.0 dBW*
Subsequent
L2
26.0 deg
-186.0 dBW
-183.0 dBW
* Over 99.5% of the solid angle inside a cone with its apex at the SV and measured from 0 degrees at the center of the Earth
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3.3.1.7 Equipment Group Delay. Equipment group delay is defined as the delay between the signal radiated output of a specific SV (measured at the antenna phase center) and the output of that SV's on-board frequency source; the delay consists of a bias term and an uncertainty. The bias term is of no concern to the US since it is included in the clock correction parameters relayed in the NAV data, and is therefore accounted for by the user computations of system time (reference paragraphs 20.3.3.3.3.1, 30.3.3.2.3). The uncertainty (variation) of this delay as well as the group delay differential between the signals of L1 and L2 are defined in the following.
3.3.1.7.1 Group Delay Uncertainty. The effective uncertainty of the group delay shall not exceed 3.0 nanoseconds (95% probability).
3.3.1.7.2 Group Delay Differential. The group delay differential between the radiated L1 and L2 signals (i.e. L1 P(Y) and L2 P(Y), L1 P(Y) and L2C) is specified as consisting of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay shall not exceed 15.0 nanoseconds. The random plus non-random variations about the mean shall not exceed 3.0 nanoseconds (95% probability), when including consideration of the temperature and antenna effects during a vehicle orbital revolution. Corrections for the bias components of the group delay differential are provided to the US in the NAV/CNAV message using parameters designated as TGD (reference paragraph 20.3.3.3.3.2) and Inter-Signal Correction (ISC) (reference paragraph 30.3.3.3.1.1).
3.3.1.7.3 Space Service Volume Group Delay Differential. The group delay differential between the radiated L1 and L2 signals with respect to the Earth Coverage signal for users of the Space Service Volume are provided in TBD.
3.3.1.8 Signal Coherence. All transmitted signals for a particular SV shall be coherently derived from the same on-board frequency standard. On the L1 carrier, the chip transitions of the modulating signals, C /A and L1P(Y), and on the L2 carrier the chip transitions of L2P(Y) and L2C, shall be such that the average time difference between the chips on the same carrier do not exceed 10 nanoseconds. The variable time difference shall not exceed 1 nanosecond (95% probability), when including consideration of the temperature and antenna effect changes during a vehicle orbital revolution. Corrections for the bias components of the time difference are provided to the US in the CNAV message using parameters designated as ISCs (reference paragraph 30.3.3.3.1.1).
3.3.1.9 Signal Polarization. The transmitted signal shall be right-hand circularly polarized (RHCP). For the angular range of 13.8 degrees from nadir, L1 ellipticity shall be no worse than 1.2 dB for Block II/IIA and shall be no worse than 1.8 dB for Block IIR/IIR-M/IIF/IIIA SVs. L2 ellipticity shall be no worse than 3.2 dB for Block
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II/IIA SVs and shall be no worse than 2.2 dB for Block IIR/IIR-M/IIF/IIIA over the angular range of 13.8 degrees from nadir.
3.3.2 PRN Code Characteristics. The characteristics of the P-, L2 CM-, L2 CL-, and the C/A-codes are defined below in terms of their structure and the basic method used for generating them. Figure 3-1 depicts a simplified block diagram of the scheme for generating the 10.23 Mbps Pi(t) and the 1.023 Mbps Gi(t) patterns (referred to as Pand C/A-codes respectively), and for modulo-2 summing these patterns with the NAV bit train, D(t), which is clocked at 50 bps. The resultant composite bit trains are then used to modulate the signal carriers.
3.3.2.1 Code Structure. The Pi(t) pattern (P-code) is generated by the modulo-2 summation of two PRN codes, X1(t) and X2(t - iT), where T is the period of one P-code chip and equals (1.023E7)-1 seconds, while i is an integer from 1 through 37. This allows the generation of 37 unique P(t) code phases (identified in Table 3-I) using the same basic code generator.
The linear Gi(t) pattern (C/A-code) is the modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The latter sequence is selectively delayed by an integer number of chips to produce many different G(t) patterns (defined in Table 3-I).
The CM,i(t) pattern (L2 CM-code) is a linear pattern which is reset with a specified initial state every code count of 10230 chips. Different initial states are used to generate different CM,i(t) patterns (defined in Table 3-II).
The CL,i(t) pattern (L2 CL-code) is also a linear pattern but with a longer reset period of 767250 chips. Different initial states are used to generate different CL,i(t) patterns (defined in Table 3-II).
For a given SV-ID, two different initial states are used to generate different CL,i(t) and CM,i(t) patterns.
Section 6.3.5 provides a selected subset of additional P-, L2 CM-, L2 CL-, and the C/A-code sequences with assigned PRN numbers.
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Z-COUNT
ZCOUNTER
RESET COMMAND GENERATOR
REMOT COMMAN
EPOCH RESET
EPOC DETECT
X1 EPOCH
EPOCH RESET
50 Hz 20
X1 GENERATO
1.023
MHz 10
GOLD CODE GENERATOR
1 KHz
EPOCH DETECT
X1(t)
X2i(t) COD
SELEC DEVIC
RECLOCKING DEVICE
Gi(t) Pi(t)
Gi(t) D(t)
FORMATTED DATA
D(t)
DATA
ENCODER
X2 GENERATO
10.23 MHz FREQUENCY
SOURCE
Pi(t) D(t) Pi(t)
Figure 3-1. Generation of P-, C/A-Codes and Modulating Signals
3.3.2.2 P-Code Generation. Each Pi(t) pattern is the modulo-2 sum of two extended patterns clocked at 10.23 Mbps (X1 and X2i). X1 itself is generated by the modulo-2 sum of the output of two 12-stage registers (X1A and X1B) short cycled to 4092 and 4093 chips respectively. When the X1A short cycles are counted to 3750, the X1 epoch is generated. The X1 epoch occurs every 1.5 seconds after 15,345,000 chips of the X1 pattern have been generated. The polynomials for X1A and X1B, as referenced to the shift register input, are:
X1A: 1 + X6 + X8 + X11 + X12, and X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12.
Samples of the relationship between shift register taps and the exponents of the corresponding polynomial, referenced to the shift register input, are as shown in Figures 3-2, 3-3, 3-4 and 3-5.
The state of each generator can be expressed as a code vector word which specifies the binary sequence constant of each register as follows: (a) the vector consists of the binary state of each stage of the register, (b) the stage 12 value appears at the left followed by the values of the remaining states in order of descending stage numbers, and (c) the shift direction is from lower to higher stage number with stage 12 providing the current output. This code vector convention represents the present output and 11 future outputs in sequence. Using this convention, at each X1 epoch, the X1A shift register is initialized to code vector 001001001000 and the X1B shift register is initialized
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to code vector 010101010100. The first chip of the X1A sequence and the first chip of the X1B sequence occur simultaneously in the first chip interval of any X1 period.
The natural 4095 chip cycles of these generating sequences are shortened to cause precession of the X1B sequence with respect to the X1A sequence during subsequent cycles of the X1A sequence in the X1 period. Reinitialization of the X1A shift register produces a 4092 chip sequence by omitting the last 3 chips (001) of the natural 4095 chip X1A sequence. Re-initialization of the X1B shift register produces a 4093 chip sequence by omitting the last 2 chips (01) of the natural 4095 chip X1B sequence. This results in the phase of the X1B sequence lagging by one chip for each X1A cycle in the X1 period.
The X1 period is defined as the 3750 X1A cycles (15,345,000 chips) which is not an integer number of X1B cycles. To accommodate this situation, the X1B shift register is held in the final state (chip 4093) of its 3749th cycle. It remains in this state until the X1A shift register completes its 3750th cycle (343 additional chips). The completion of the 3750th X1A cycle establishes the next X1 epoch which re-initializes both the X1A and X1B shift registers starting a new X1 cycle.
STAGE NUMBERS
POLYNOMIAL X1A: 1 + X 6 + X 8 + X 11 + X 12
1
2
3
4
5
6
7
8
9
10
11
12
0
0
0
1
0
0
1
0
0
1
0
0
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
11
12
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-2. X1A Shift Register Generator Configuration
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STAGE NUMBERS
POLYNOMIAL X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12
1
2
3
4
5
6
7
8
9
10
11
12
0
0
1
0
1
0
1
0
1
0
1
0
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
11
12
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-3. X1B Shift Register Generator Configuration
POLYNOMIAL X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12
STAGE NUMBERS
1
2
3
4
5
6
7
8
9
10
11
12
1
0
1
0
0
1
0
0
1
0
0
1
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
11
12
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-4. X2A Shift Register Generator Configuration
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STAGE NUMBERS
POLYNOMIAL X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12
1
2
3
4
5
6
7
8
9
10
11
12
0
0
1
0
1
0
1
0
1
0
1
0
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
11
12
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-5.
X2B Shift Register Generator Configuration
The X2i sequences are generated by first producing an X2 sequence and then delaying it by a selected integer number of chips, i, ranging from 1 to 37. Each of the X2i sequences is then modulo-2 added to the X1 sequence thereby producing up to 37 unique P(t) sequences.
The X2A and X2B shift registers, used to generate X2, operate in a similar manner to the X1A and X1B shift registers. They are short-cycled, X2A to 4092 and X2B to 4093, so that they have the same relative precession rate as the X1 shift registers. X2A epochs are counted to include 3750 cycles and X2B is held in the last state at 3749 cycle until X2A completes its 3750th cycle. The polynomials for X2A and X2B, as referenced to the shift register input, are:
X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12, and X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12.
(The initialization vector for X2A is 100100100101 and for X2B is 010101010100).
The X2A and X2B epochs are made to precess with respect to the X1A and X1B epochs by causing the X2 period to be 37 chips longer than the X1 period. When the X2A is in the last state of its 3750th cycle and X2B is in the
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last state of its 3749th cycle, their transitions to their respective initial states are delayed by 37 chip time durations.
At the beginning of the GPS week, X1A, X1B, X2A and X2B shift registers are initialized to produce the first chip of the week. The precession of the shift registers with respect to X1A continues until the last X1A period of the GPS week interval. During this particular X1A period, X1B, X2A and X2B are held when reaching the last state of their respective cycles until that X1A cycle is completed (see Table 3-VI). At this point, all four shift registers are initialized and provide the first chip of the new week.
Figure 3-6 shows a functional P-code mechanization. Signal component timing is shown in Figure 3-7, while the end-of-week reset timing and the final code vector states are given in Tables 3-VI and 3-VII, respectively.
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10.23 MHz
6, 8, 11, 12
RESUME
C
R
I
REGISTER
1 6
12
SET X1A EPOCH A
4092 DECODE
X1 EPOCH
3750
CLOCK CONTROL
HALT
C
X1B
R
I
REGISTER
1
12
1, 2, 5, 8, 9, 10, 11, 12
CLOCK CONTROL
HALT
C
X2A
R
I
REGISTER
1
12
SET X1B EPOCH
3749
Z-COUNTER 403,200
B
4093 DECODE
7 DAY RESET
END/WEEK
REGISTER INPUTS
3750
SET X2A EPOCH C
C - CLOCK I - INPUT R - RESET TO
INITIAL CONDITIONS ON NEXT CLOCK
1, 3, 4, 5, 7, 8, 9, 10, 11, 12
4092 DECODE
START/WEEK
RESUME
X2
EPOCH
37
ENABLE
A
HALT
END/WEEK
X1 Pi
B
CLOCK CONTROL
C
X2B
R
I
REGISTER
1 2
12
SET X2B EPOCH
3749
C
X2 i
1 i
2, 3, 4, 8, 9, 12
4093 DECODE
X2
SHIFT
REGISTER
Figure 3-6. P-Code Generation
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0
1
2
3
0
1
2
3
0
X1 EPOCHS
37 Chips
74 Chips
X2 EPOCHS *
P Epoch
TIME 0
1.5 sec 3.0 sec 4.5 sec 7 days
* Does not include any offset due to PRN delay.
Figure 3-7. P-Code Signal Component Timing
14 days
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Table 3-VI.
P-Code Reset Timing
(Last 400 sec of 7-day period) **
X1A-Code 1
Code Chip
X1B-Code 345
X2A-Code 1070
X2B-Code 967
3023
3367
4092
3989
3127
3471
4092
4093
3749
4093
4092
4093
4092*
4093
4092
* Last Chip of Week. ** Does not include any X2 offset due to PRN delay.
4093
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Code X1A X1B X2A X2B
Table 3-VII.
Chip Number
4091 4092 4092 4093 4091 4092 4092 4093
Final Code Vector States
Vector State
100010010010 000100100100 100101010101 001010101010 111001001001 110010010010 000101010101 001010101010
Vector State for 1st Chip following Epoch 001001001000
010101010100
100100100101
010101010100
NOTE: First Chip in each sequence is output bit whose leading edge occurs simultaneously with the epoch.
3.3.2.3 C/A-Code Generation. Each Gi(t) sequence is a 1023-bit Gold-code which is itself the modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The G2i sequence is formed by effectively delaying the G2 sequence by an integer number of chips. The G1 and G2 sequences are generated by 10-stage shift registers having the following polynomials as referred to in the shift register input (see Figures 3-8 and 3-9).
G1 = X10 + X3 + 1, and G2 = X10 + X9 + X8 + X6 + X3 + X2 + 1.
The initialization vector for the G1 and G2 sequences is 1111111111. The G1 and G2 shift registers are initialized at the P-coder X1 epoch. The G1 and G2 registers are clocked at 1.023 MHz derived from the 10.23 MHz P-coder clock. The initialization by the X1 epoch phases the 1.023 MHz clock to insure that the first chip of the C/A code begins at the same time as the first chip of the P-code.
The effective delay of the G2 sequence to form the G2i sequence may be accomplished by combining the output of two stages of the G2 shift register by modulo-2 addition (see Figure 3-10). However, this two-tap coder implementation generates only a limited set of valid C/A codes. Table 3-I contains a tabulation of the G2 shift register taps selected and their corresponding P-code X2i and PRN signal numbers together with the first several chips of each resultant PRN code. Timing relationships related to the C/A code are shown in Figure 3-11.
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STAGE NUMBERS
POLYNOMIAL G1: 1 + X 3 + X 10
INPUT
1
2
3
4
5
6
7
8
9
10
1
1
1
1
1
1
1
1
1
1
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-8. G1 Shift Register Generator Configuration
POLYNOMIAL G2: 1 + X 2 + X 3 +X 6 + X 8 + X 9 + X 10
STAGE NUMBERS
INPUT
1
2
3
4
5
6
7
8
9
10
1
1
1
1
1
1
1
1
1
1
OUTPUT
0
1
2
3
4
5
6
7
8
9
10
INITIAL CONDITIONS
SHIFT DIRECTION
TAP NUMBERS
Figure 3-9. G2 Shift Register Generator Configuration
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X1 EPOCH
10.23 MHz SYNCH
10
SYNCH
G EPOCH 20
1 Kbps
1023 DECODE
3
10
I
S
G1
G1
REGISTER
C
2 3 6 8 9 10
I
C
G2
REGISTER
S
G i
G2i PHASE SELECT
LOGIC
50 bps TO DATA ENCODER
REGISTER INPUTS
C I S -
CLOCK INPUT SET ALL ONES
Figure 3-10. Example C/A-Code Generation
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X1 Epoch @ 2/3 bps
1023
1023
1023 BIT Gold Code @ 1023 Kbps
1023
1023
1023
etc.
1 msec
0
1
2
18
19
0
Gold Code Epochs @ 1000/sec
Data @ 50 cps 20 msec
Figure 3-11. C/A-Code Timing Relationships
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3.3.2.4 L2 CM-/L2 CL-Code Generation. Each CM,i(t) pattern (L2 CM-code) and CL,i(t) pattern (L2 CL-code) are generated using the same code generator polynomial each clocked at 511.5 Kbps. Each pattern is initiated and reset with a specified initial state (defined in Table 3-II). CM,i(t) pattern is reset after 10230 chips resulting in a code period of 20 milliseconds, and CL,i(t) pattern is reset after 767250 chips resulting in a code period of 1.5 seconds. The L2 CM and L2 CL shift registers are initialized at the P-coder X1 epoch. The first L2 CM-code chip starts synchronously with the end/start of week epoch. Timing relationships related to the L2 CM -/L2 CLcodes are shown in Figure 3-12.
The maximal polynomial used for L2 CM- and L2 CL-codes is 1112225171 (octal) of degree 27. The L2 CM and L2 CL code generator is conceptually described in Figure 3-13 using modular-type shift register generator.
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0
End/start of week
1.5 second
X1 Epoch @ 2/3 bps
767250
767250 BIT L2 CL-Code @ 511.5 Chips
Kbps
1
2
3
4
10230
10230
10230 10230
10230 BIT L2 CM-Code @ 511.5 Kbps
73
74
10230 10230
1
etc. 75 10230 etc. 20 msec
Data @ 50 cps
L2 CM @ 511.5 Kbps
L2 CL @ 511.5 Kbps
L2 C @ 1023 Kbps Figure 3-12. L2 CM-/L2 CL-Code Timing Relationships
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Figure 3-13. L2 CM/L2 CL Shift Register Generator Configuration
DELAY NUMBERS
POLYNOMIAL:
1 + X3 + X4 +X5 + X6 + X9 + X11 + X13 + X16 + X19 + X21 + X24 + X27
32
3
3
2
3
3
2
2
3
1
1
1
3
OUTPUT
INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG) SHIFT DIRECTION
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3.3.3 Navigation Data. The content and format of the NAV data, D(t), and the CNAV data, DC(t), are given in Appendices II and III, respectively, of this document.
3.3.3.1 Navigation Data Modulation (L2 CM). For Block IIR-M, Block IIF, and subsequent blocks of SVs, the CNAV bit train, DC(t), is rate ½ encoded and, thus, clocked at 50 sps. The resultant symbol sequence is then modulo-2 added to the L2 CM-code. During the initial period of Block IIR-M SVs operation, prior to Initial Operational Capability of L2 C signal, and upon ground command, the NAV bit train, D(t), at one of two data rates, may be modulo-2 added to the L2 CM-code instead of CNAV data, DC(t), as further described in Section 3.2.2.
3.3.3.1.1 Forward Error Correction. The CNAV bit train, DC(t), will always be Forward Error Correction (FEC) encoded by a rate 1/2 convolutional code. For Block IIR-M, the NAV bit train, D(t), can be selected to be convolutionally encoded. The resulting symbol rate is 50 sps. The convolutional coding will be constraint length 7, with a convolutional encoder logic arrangement as illustrated in Figure 3-14. The G1 symbol is selected on the output as the first half of a 40-millisecond data bit period.
Twelve-second navigation messages broadcast by the SV are synchronized with every eighth of the SV's P(Y)-code X1 epochs. However, the navigation message is FEC encoded in a continuous process independent of message boundaries (i.e. at the beginning of each new message, the encoder registers illustrated in Figure 3-14 contains the last six bits of the previous message).
Because the FEC encoding convolves successive messages, it is necessary to define which transmitted symbol is synchronized to SV time, as follows. The beginning of the first symbol that contains any information about the first bit of a message will be synchronized to every eighth X1 epoch (referenced to end/start of week). The users’ convolutional decoders will introduce a fixed delay that depends on their respective algorithms (usually 5 constraint lengths, or 35 bits), for which they must compensate to determine system time from the received signal. This convolutional decoding delay and the various relationships with the start of the data block transmission and SV time are illustrated in Figure 3-15.
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DATA INPUT (25 BPS)
G2 (133 OCTAL)
G1 (171 OCTAL)
OUTPUT SYMBOLS (50 SPS)
(ALTERNATING G1/G2)
SYMBOL CLOCK
Figure 3-14. Convolutional Encoder
ENCODED DATA BLOCK TRANSMITTED ON L2
ENCODED DATA BLOCK
RECEIVED BY USER
EARLY
DATA BLOCK DECODED BY
USER
USER’S DECODING DELAY
DOWNLINK DELAY
LATER
SV 12 SECOND EPOCHS
Figure 3-15. Convolutional Transmit/Decoding Timing Relationships
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3.3.4 GPS Time and SV Z-Count. GPS time is established by the Control Segment and is referenced to Coordinated Universal Time (UTC) as maintained by the U.S. Naval Observatory (UTC(USNO)) zero time-point defined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time is one week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be a continuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is an inherent but bounded drift rate between the UTC and GPS time scales. The OCS shall control the GPS time scale to be within one microsecond of UTC (modulo one second).
The NAV data contains the requisite data for relating GPS time to UTC. The accuracy of this data during the transmission interval shall be such that it relates GPS time (maintained by the MCS of the CS) to UTC (USNO) within 90 nanoseconds (one sigma). This data is generated by the CS; therefore, the accuracy of this relationship may degrade if for some reason the CS is unable to upload data to a SV. At this point, it is assumed that alternate sources of UTC are no longer available, and the relative accuracy of the GPS/UTC relationship will be sufficient for users. Range error components (e.g. SV clock and position) contribute to the GPS time transfer error, and under normal operating circumstances (two frequency time transfers from SV(s) whose navigation message indicates a URA of eight meters or less), this corresponds to a 97 nanosecond (one sigma) apparent uncertainty at the SV. Propagation delay errors and receiver equipment biases unique to the user add to this time transfer uncertainty.
In each SV the X1 epochs of the P-code offer a convenient unit for precisely counting and communicating time. Time stated in this manner is referred to as Z-count, which is given as a binary number consisting of two parts as follows:
a. The binary number represented by the 19 least significant bits of the Z-count is referred to as the time of week (TOW) count and is defined as being equal to the number of X1 epochs that have occurred since the transition from the previous week. The count is short-cycled such that the range of the TOW-count is from 0 to 403,199 X1 epochs (equaling one week) and is reset to zero at the end of each week. The TOWcount's zero state is defined as that X1 epoch which is coincident with the start of the present week. This epoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight is defined as 0000 hours on the UTC scale which is nominally referenced to the Greenwich Meridian. Over the years the occurrence of the "zero state epoch" may differ by a few seconds from 0000 hours on the UTC scale since UTC is periodically corrected with leap seconds while the TOW-count is continuous without such correction. To aid rapid ground lock-on to the P-code signal, a truncated version of the TOW-count, consisting of its 17 most significant bits, is contained in the hand-over word (HOW) of the L1 and L2 NAV data (D(t)) stream; the relationship between the actual TOW-count and its truncated HOW version is illustrated by Figure 3-16.
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b. The most significant bits of the Z-count are a binary representation of the sequential number assigned to the current GPS week (see paragraph 6.2.4).
X1 EPOCHS
P(Y)-CODE EPOCH (END/START OF WEEK)
1.5 sec
403,192
403,196
0 123 4 567 8
403,199
DECIMAL EQUIVALENTS OF ACTUAL TOW COUNTS
SUBFRAME EPOCHS
6 sec
100,799
0
1
2
3
DECIMAL EQUIVALENTS OF HOW-MESSAGE TOW COUNTS
NOTES:
1. TO AID IN RAPID GROUND LOCK-ON THE HAND-OVER WORD (HOW ) OF EACH SUBFRAME CONTAINS A TRUNCATED TIME-OF-WEEK (TOW) COUNT
2. THE HOW IS THE SECOND WORD IN EACH SUBFRAME (REFERENCE PARAGRAPH 20.3.3.2).
3. THE HOW-MESSAGE TOW COUNT CONSISTS OF THE 17 MSBs OF THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME.
4. TO CONVERT FROM THE HOW-MESSAGE TOW COUNT TO THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME, MULTIPLY BY FOUR.
5. THE FIRST SUBFRAME STARTS SYNCHRONOUSLY WITH THE END/START OF WEEK EPOCH.
Figure 3-16. Time Line Relationship of HOW Message 36
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4. NOT APPLICABLE
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5. NOT APPLICABLE
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6. NOTES 6.1 Acronyms
AI AODO A-S Autonav BPSK CDC CNAV cps CRC CS DC dBc
dBi dBW DN EAROM ECEF ECI EDC EOE EOL ERD FEC GGTO GNSS GPS GPSW HOW ICC ID IERS IODC IODE
-
Availability Indicator
-
Age of Data Offset
-
Anti-Spoofing
-
Autonomous Navigation
-
Bi-Phase Shift Key
-
Clock Differential Correction
-
Civil Navigation
-
cycles per second
-
Cyclic Redundancy Check
-
Control Segment
-
Differential Correction
-
Power ratio of a signal to a (unmodulated) carrier signal,
expressed in decibels
-
Decibel with respect to isotropic antenna
-
Decibel with respect to 1 W
-
Day Number
-
Electrically Alterable Read-Only Memory
-
Earth-Centered, Earth-Fixed
-
Earth-Centered, Inertial
-
Ephemeris Differential Correction
-
Edge-of-Earth
-
End of Life
-
Estimated Range Deviation
-
Forward Error Correction
-
GPS/GNSS Time Offset
-
Global Navigation Satellite System
-
Global Positioning System
-
Global Positioning System Wing
-
Hand-Over Word
-
Interface Control Contractor
-
Identification
-
International Earth Rotation and Reference Systems Service
-
Issue of Data, Clock
-
Issue of Data, Ephemeris
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IRM IRP IS ISC LSB LSF L2 C L2 CL L2 CM MCS MSB NAV NDUS NMCT NSC NSCL NSCM NSY OBCP OCS PPS PRN RF RMS SA SEP SPS sps SS SSV SV SVN TBD TBS TLM TOW
-
IERS Reference Meridian
-
IERS Reference Pole
-
Interface Specification
-
Inter-Signal Correction
-
Least Significant Bit
-
Leap Seconds Future
-
L2 Civil Signal
-
L2 Civil-Long Code
-
L2 Civil-Moderate Code
-
Master Control Station
-
Most Significant Bit
-
Navigation
-
Nudet Detection User Segment
-
Navigation Message Correction Table
-
Non-Standard C/A-Code
-
Non-Standard L2 CL-Code
-
Non-Standard L2 CM-Code
-
Non-Standard Y-code
-
On-Board Computer Program
-
Operational Control System
-
Precise Positioning Service
-
Pseudo-Random Noise
-
Radio Frequency
-
Root Mean Square
-
Selective Availability
-
Spherical Error Probable
-
Standard Positioning Service
-
symbols per second
-
Space Segment
-
Space Service Volume
-
Space Vehicle
-
Space Vehicle Number
-
To Be Determined
-
To Be Supplied
-
Telemetry
-
Time Of Week
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UE URA URE US USNO UTC WGS 84 WN WNe
-
User Equipment
-
User Range Accuracy
-
User Range Error
-
User Segment
-
U.S. Naval Observatory
-
Coordinated Universal Time
-
World Geodetic System 1984
-
Week Number
-
Extended Week Number
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6.2 Definitions
6.2.1 User Range Accuracy. User Range Accuracy (URA) is a statistical indicator of the GPS ranging accuracy obtainable with a specific signal and SV. Whether the integrity status flag is 'off' or 'on', 4.42 times URA bounds instantaneous URE under all conditions with 1 -1e-5 per hour probability. When the integrity status flag is 'on', 5.73 times URA bounds instantaneous URE under all conditions with 1-1e-8 per hour probability. Integrity properties of the URA are specified with respect to the upper bound values of the URA index.
Note #1: URA applies over the curve fit interval that is applicable to the NAV data from which the URA is read, for the worst-case location within the intersection of the satellite signal and the terrestrial service volume.
Note #2: The URA for a particular signal may be represented by a single parameter in the NAV data or by more than one parameter representing components of the total URA. Specific URA parameters and formulae for calculating the total URA for a signal are defined in the applicable Space Segment to Navigation User Segment ICD's.
6.2.1.1 Integrity Assured URA. When the integrity assurance monitoring is available, as indicated by the ―integrity status flag‖ being set to ―1‖, the URA value is chosen such that the probability of the ―actual‖ URE exceeding a threshold is met (see section 3.5.3.10 for probability values). The URA value is conveyed to the user in the form of URA index values. The URA index represents a range of values; for integrity assurance applications.
6.2.1.2 User Differential Range Accuracy. User Differential Range Accuracy (UDRA) is a statistical indicator of the GPS ranging accuracy obtainable with a specific signal and SV after the application of the associated differential corrections (DC parameters).
6.2.2 SV Block Definitions. The following block definitions are given to facilitate discussion regarding the capability of the various blocks of GPS satellites to support the SV-to-US interface.
6.2.2.1 Developmental SVs. The original concept validation satellites developed by Rockwell International and designated as satellite vehicle numbers (SVNs) 1-11 are termed "Block I" SVs. These SVs were designed to provide 3-4 days of positioning service without contact from the CS. These SVs transmitted a configuration code of 000 (reference paragraph 20.3.3.5.1.4). There are no longer any active Block I SVs in the GPS constellation. The last Block I SV was decommissioned in 1995.
6.2.2.2 Operational SVs. The operational satellites are designated Block II, Block IIA, Block IIR, Block IIR-M, Block IIF and Block III SVs. Characteristics of these SVs are provided below. Modes of operation for these SVs and accuracy of positioning services provided are described in paragraphs 6.3.2 through 6.3.4. These SVs transmit
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configuration codes as specified in paragraph 20.3.3.5.1.4. The navigation signal provides no direct indication of the type of the transmitting SV.
6.2.2.2.1 Block II SVs. The first block of full scale operational SVs developed by Rockwell International are designated as SVNs 13-21 and are termed "Block II" SVs. These SVs were designed to provide 14 days of positioning service without contact from the CS.
6.2.2.2.2 Block IIA SVs. The second block of full scale operational SVs developed by Rockwell International are designated as SVNs 22-40 and are termed "Block IIA" SVs. These SVs are capable of providing 60 days of positioning service without contact from the CS.6.2.2.2.3 Block IIR SVs. The block of operational replenishment SVs developed by Lockheed Martin are designated as SVNs 41-61 and are termed "Block IIR" SVs. These SVs have the capability of storing at least 60 days of navigation data with current memory margins, while operating in a IIA mode, to provide positioning service without contact from the CS for that period. (Contractual requirements for these SVs specify transmission of correct data for only 14 days to support short-term extended operations while in IIA mode.) The IIR SV will provide a minimum of 60 days of positioning service without contact from the CS when operating in autonomous navigation (Autonav) mode.
6.2.2.2.4 Block IIR-M SVs. The subset of operational replenishment SVs developed by Lockheed Martin which are ―Modernized‖ configuration of ―Block IIR‖ SVs are termed ―Block IIR-M‖.
6.2.2.2.5 Block IIF SVs. The block of operational replenishment SVs developed by Boeing are designated as SVNs 62-73 and are termed ―Block IIF‖ SVs. This is the first block of operational SVs that transmit the L5 Civil signal. These SVs will provide at least 60 days of positioning service without contact from the CS.
6.2.2.2.6 Block IIIA SVs. The block of operational replenishment SVs are designated as SVNs 74-81. This is the first block of operational SVs that transmit the L1C signal. These SVs will provide at least 60 days of positioning service without contact from the CS.
6.2.3 Operational Interval Definitions. The following three operational intervals have been defined. These labels will be used to refer to differences in the interface definition as time progresses from SV acceptance of the last navigation data upload.
6.2.3.1 Normal Operations. The SV is undergoing normal operations whenever the fit interval flag (reference paragraph 20.3.3.4.3.1) is zero.
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6.2.3.2 Short-term Extended Operations. The SV is undergoing short-term extended operations whenever the fit interval flag is one and the IODE (reference paragraph 20.3.4.4) is less than 240.
6.2.3.3 Long-term Extended Operations. The SV is undergoing long-term extended operations whenever the fit interval flag is one and the IODE is in the range 240-255.
6.2.4 GPS Week Number. The GPS week numbering system is established with week number zero (0) being defined as that week which started with the X1 epoch occurring at midnight UTC(USNO) on the night of January 5, 1980/ morning of January 6, 1980. The GPS week number continuously increments by one (1) at each end/start of week epoch without ever resetting to zero. Users must recognize that the week number information contained in the Nav Message may not necessarily reflect the current full GPS week number (see paragraphs 20.3.3.3.1.1, 20.3.3.5.1.5, 20.3.3.5.2.4, and 30.3.3.1.1.1).
6.2.5 L5 Civil Signal. L5 is the GPS downlink signal at a nominal carrier frequency of 1176.45 MHz. The L5 signal is only available on Block IIF and subsequent blocks of SVs and the signal is specified/described in interface specification IS-GPS-705.
6.3 Supporting Material
6.3.1 Received Signals. The guaranteed minimum user-received signal levels are defined in paragraph 3.3.1.6. As additional supporting material, Figure 6-1 illustrates an example variation in the minimum received power of the near-ground user-received L1 and L2 signals from Block II/IIA/IIR SVs as a function of SV elevation angle.
Higher received signals levels can be caused by such factors as SV attitude errors, mechanical antenna alignment errors, transmitter power output variations due to temperature variations, voltage variations and power amplifier variations, and due to a variability in link atmospheric path loss. For Block II/IIA and IIR SVs, the maximum received signal levels as a result of these factors is not expected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) and C/A components of the L1 channel, nor -158.0 dBW for either signal on the L2 channel. For Block IIR-M and IIF SVs, the maximum received signal levels as a result of these factors is not expected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) and C/A components of the L1 channel and L2 channel. In addition, due to programmable power output capabilities of Block IIR-M and IIF SVs, under certain operational scenarios, individual signal components of Block IIR-M/IIF SVs may exceed the previously stated maximum but are not expected to exceed -150 dBW.
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RECEIVED POWER AT 3dBi LINEARLY POLARIZED ANTENNA (dBW)
-155.5
-158.5 -161.5
C/A - L1 P - L1
-164.5
P - L2 or C/A - L2
0o 5o
20 o
40 o
60 o
USER ELEVATION ANGLE (DEG)
80 o
90 o
100 o
Figure 6-1. User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR)
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6.3.2 Extended Navigation Mode (Block II/IIA). The Block II and IIA SVs are capable of being uploaded by the CS with a minimum of 60 days of navigation data to support a 60 day positioning service. Due to memory retention limitations, the Block II SVs may not transmit correct data for the entire 60 days but are guaranteed to transmit correct data for at least 14 days to support short-term extended operations. Under normal conditions the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this IS. During normal operations, the SVs will have a user range error that is at or below a level required to support a positioning accuracy of 16 meters spherical error probable (SEP). In addition, the almanac data, UTC parameters and ionospheric data will be maintained current to meet the accuracy specified in this IS.
If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV will individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV's last upload) as defined in paragraphs 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As time from upload continues through these three operational intervals, the user range error of the SV will increase, causing a positioning service accuracy degradation. The rate of accuracy degradation is slow over the short-term extended operations interval, such that at the end of this interval (approximately 14 days after upload) the US will be able to achieve a positioning accuracy of 425 meters SEP. The rate of accuracy degradation increases in the long-term extended interval, such that by the 180th day after the last upload, the positioning errors will have grown to 10 kilometers SEP. During these intervals the URA will continue to provide the proper estimate of the user range errors.
During short-term and long-term extended operations (approximately day 2 through day 62 after an upload), the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of last upload.
6.3.3 Extended Navigation Mode (Block IIIA). The Block IIIA SVs shall be capable of being uploaded by the CS with a minimum of 60 days of data to support a 60 day positioning service. Under normal conditions, the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this IS.
If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV shall individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As time from upload continues through these three operational intervals, the user range error (URE) of the SV will increase, causing a positioning service accuracy degradation.
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6.3.4 Block IIA Mode (Block IIR/IIR-M). The Block IIR/IIR-M SVs, when operating in the Block IIA mode, will perform similarly to the Block IIA SVs and have the capability of storing at least 60 days of navigation data, with current memory margins, to provide positioning service without contact from the CS for that period (through short-term and long-term extended operations). (Contractual requirements for these SVs specify transmission of correct data for only 14 days to support short-term extended operations while in IIA mode.) Under normal conditions, the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this IS.
If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV will individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As time from upload continues through these three operational intervals, the user range error (URE) of the SV will increase, causing a positioning service accuracy degradation.
6.3.5 Autonomous Navigation Mode. The Block IIR/IIR-M, Block IIF, and directional crosslink-capable Block III SV in conjunction with a sufficient number of other Block IIR/IIR-M, Block IIF or directional crosslink-capable Block III SVs, operates in an Autonav mode when commanded by the CS. Each Block IIR/IIR-M/IIF/directional crosslink-capable III SV in the constellation determines its own ephemeris and clock correction parameters via SVto-SV ranging, communication of data, and on-board data processing which updates data uploaded by the CS. In the Autonav mode the Block IIR/IIR-M/IIF/directional crosslink-capable III SV will maintain normal operations as defined in paragraph 6.2.3.1 and as further described within this IS, and will have a URE of no larger than 6 meters, one sigma for Block IIR/IIR-M. URE of 6 meters, one sigma, is expected to support 16 meter SEP accuracy under a nominal position dilution of precision. If the CS is unable to upload the SVs, the Block IIR/IIR-M/IIF/directional crosslink-capable III SVs will maintain normal operations for period of at least 60 days after the last upload.
In the Autonav mode, the almanac data, UTC parameters and ionospheric data are still calculated and maintained current by the CS and uploaded to the SV as required. If the CS is unable to upload the SVs, the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of the last upload.
6.3.6 PRN Code sequences expansion. The additional PRN sequences provided in this section are for information only. The additional PRN sequences identified in this section are not applicable to Block II/IIA, IIR/IIR -M, IIF SVs. In addition, the current valid range for GPS PRN signal number for C/A- and P-code is 1 – 37 as specified in Table 3-I. The PRN sequences provided in this section are for other L1/L2 signal applications, such as Satellite Based Augmentation System (SBAS) satellite signals, and potential use in the future by GPS.
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6.3.6.1 Additional C/A-code PRN sequences. The PRN C/A-code is described in Section 3.2.1.3 and 36 legacy C/A-code sequences are assigned by SV-ID number in Table 3-I. An additional set of 173 C/A-code PRN sequences are selected and assigned with PRN numbers in this section as shown in Table 6-I. Among the 173 additional sequences; PRN numbers 38 through 63 are reserved for future GPS SVs; PRN numbers 64 through 119 are reserved for future Ground Based Augmentation System (GBAS) and other augmentation systems; PRN numbers 120 through 158 are reserved for SBAS; and PRN numbers 159 through 210 are reserved for other Global Navigation Satellite System (GNSS) applications. For GPS application, the CNAV data, Dc(t), will be modulo-2 added to the C/A-code sequences of PRN numbers 38 through 63. Any assignment of a C/A-code PRN number and its code sequence for any additional SV and/or other L1/L2 signal applications, such as SBAS satellite signals, will be selected from the sequences of Table 6-I and will be approved, controlled, and managed by the GPSW.
It should be noted that, in Table 6-I, the C/A-code sequences are identified by ―G2 Delay‖ and ―Initial G2 Setting‖ which is not the same as the method used in Table 3-I. The two-tap coder implementation method referenced and used in Table 3-I is not used in Table 6-I due to its limitation in generating C/A-code sequences. The ―G2 Delay‖ specified in Table 6-I may be accomplished by using the ―Initial G2 Setting‖ as the initialization vector for the G2 shift register of Figure 3-9.
6.3.6.2 Additional P-Code PRN sequences. The PRN P-code set of 37 mutually exclusive sequences are described in Section 3.2.1.1, and assignment of these code segments by SV-ID number is given in Table 3-I. An additional set of 173 P-code PRN sequences are described in this section. Among the 173 additional sequences; PRN numbers 38 through 63 are reserved for future GPS SVs; PRN numbers 64 through 119 are reserved for future GBAS and other augmentation systems; and PRN numbers 120 through 210 are reserved for other future applications. For GPS application, the CNAV data, Dc(t), which may include additional future military message types, will be modulo-2 added to the P-code sequences of PRN numbers 38 through 63. The P-code PRN numbers and their code sequences defined in Table 6-I are not for general use and will be approved, controlled, and managed by the GPSW.
6.3.6.2.1 Additional P-code Generation. The generation of 37 mutually exclusive P-code PRN sequences are described in Section 3.3.2.2. The additional set of 173 P-code PRN sequences are generated by circularly shifting each of the original 37 sequences (over one week) by an amount corresponding to 1, 2, 3, 4, or 5 days. The additional sequences are therefore time shifted (i.e. offset) versions of the original 37 sequences. These offset Pcode PRN sequences, Pi(t), are described as follows:
Pi(t) = Pi-37x(t – xT), where i is an integer from 38 to 210, x is an integer portion of (i-1)/37, and T is defined to equal 24 hours.
As an example, P-code sequence for PRN 38 would be the same sequence as PRN 1 shifted 24 hours into a week (i.e. 1st chip of PRN 38 at beginning of week is the same chip for PRN 1 at 24 hours after beginning of week). The
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complete list of the additional P-code PRN assignments is shown in Table 6-I. Any assignment of a P-code PRN number and its code sequence for any additional SV and/or other L1/L2 signal applications will be selected from the sequences of Table 6-I.
Table 6-I
Additional C/A-/P-Code Phase Assignments (sheet 1 of 6)
PRN Signal No. *
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
G2 Delay (Chips)
67 103
91 19 679 225 625 946 638 161 1001 554 280 710 709 775 864 558 220 397 55 898 759 367 299 1018
C/A
Initial G2 Setting
(Octal)**
0017 0541 1714 1151 1651 0103 0543 1506 1065 1564 1365 1541 1327 1716 1635 1002 1015 1666 0177 1353 0426 0227 0506 0336 1333 1745
First 10 Chips (Octal)**
1760 1236 0063 0626 0126 1674 1234 0271 0712 0213 0412 0236 0450 0061 0142 0775 0762 0111 1600 0424 1351 1550 1271 1441 0444 0032
X2 Delay (Chips)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
P
P-code Relative Delay (Hours)
***
P1(t-24) P2(t-24) P3(t-24) P4(t-24) P5(t-24) P6(t-24) P7(t-24) P8(t-24) P9(t-24) P10(t-24) P11(t-24) P12(t-24) P13(t-24) P14(t-24) P15(t-24) P16(t-24) P17(t-24) P18(t-24) P19(t-24) P20(t-24) P21(t-24) P22(t-24) P23(t-24) P24(t-24) P25(t-24) P26(t-24)
First 12 Chips (Octal)
3373 3757 3545 5440 4402 4023 4233 2337 3375 3754 3544 3440 5402 2423 5033 2637 3135 5674 4514 2064 5210 2726 5171 2656 5105 2660
* PRN sequences 38 through 63 are reserved for GPS.
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000).
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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Table 6-I
Additional C/A-/P-Code Phase Assignments (sheet 2 of 6)
PRN Signal
No.
64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
G2 Delay (Chips)
729 695 780 801 788 732
34 320 327 389 407 525 405 221 761 260 326 955 653 699 422 188 438 959 539 879 677 586 153 792 814 446
C/A
Initial G2 Setting
(Octal)**
0254 1602 1160 1114 1342 0025 1523 1046 0404 1445 1054 0072 0262 0077 0521 1400 1010 1441 0365 0270 0263 0613 0277 1562 1674 1113 1245 0606 0136 0256 1550 1234
First 10 Chips (Octal)**
1523 0175 0617 0663 0435 1752 0254 0731 1373 0332 0723 1705 1515 1700 1256 0377 0767 0336 1412 1507 1514 1164 1500 0215 0103 0664 0532 1171 1641 1521 0227 0543
X2 Delay (Chips)
27 28 29 30 31 32 33 34 35 36 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
P
P-code Relative Delay (Hours)
***
P27(t-24) P28(t-24) P29(t-24) P30(t-24) P31(t-24) P32(t-24) P33(t-24) P34(t-24) P35(t-24) P36(t-24) P37(t-24) P1(t-48) P2(t-48) P3(t-48) P4(t-48) P5(t-48) P6(t-48) P7(t-48) P8(t-48) P9(t-48) P10(t-48) P11(t-48) P12(t-48) P13(t-48) P14(t-48) P15(t-48) P16(t-48) P17(t-48) P18(t-48) P19(t-48) P20(t-48) P21(t-48)
First 12 Chips (Octal)
5112 4667 2111 5266 4711 4166 2251 5306 4761 2152 5247 5736 2575 3054 3604 3520 5472 4417 2025 3230 5736 4575 2054 3204 3720 5572 4457 4005 2220 3332 3777 3555
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000)
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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Table 6-I
Additional C/A-/P-Code Phase Assignments (sheet 3 of 6)
PRN Signal
No.
96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
G2 Delay (Chips)
264 1015
278 536 819 156 957 159 712 885 461 248 713 126 807 279 122 197 693 632 771 467 647 203 145 175
52 21 237 235
C/A
Initial G2 Setting
(Octal)**
0260 1455 1535 0746 1033 1213 0710 0721 1763 1751 0435 0735 0771 0140 0111 0656 1016 0462 1011 0552 0045 1104 0557 0364 1106 1241 0267 0232 1617 1076
First 10 Chips (Octal)**
1517 0322 0242 1031 0744 0564 1067 1056 0014 0026 1342 1042 1006 1637 1666 1121 0761 1315 0766 1225 1732 0673 1220 1413 0671 0536 1510 1545 0160 0701
X2 Delay (Chips)
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14
P
P-code Relative Delay (Hours)
***
P22(t-48) P23(t-48) P24(t-48) P25(t-48) P26(t-48) P27(t-48) P28(t-48) P29(t-48) P30(t-48) P31(t-48) P32(t-48) P33(t-48) P34(t-48) P35(t-48) P36(t-48) P37(t-48) P1(t-72) P2(t-72) P3(t-72) P4(t-72) P5(t-72) P6(t-72) P7(t-72) P8(t-72) P9(t-72) P10(t-72) P11(t-72) P12(t-72) P13(t-72) P14(t-72)
First 12 Chips (Octal)
3444 3400 5422 2433 3037 5635 2534 5074 4614 2124 5270 2716 5165 4650 2106 5261 2752 5147 4641 2102 5263 2713 3167 3651 3506 5461 4412 2027 5231 2736
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000)
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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Table 6-I
Additional C/A-/P-Code Phase Assignments (sheet 4 of 6)
PRN Signal
No.
126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155
G2 Delay (Chips)
886 657 634 762 355 1012 176 603 130 359 595
68 386 797 456 499 883 307 127 211 121 118 163 628 853 484 289 811 202 1021
C/A
Initial G2 Setting
(Octal)**
1764 0717 1532 1250 0341 0551 0520 1731 0706 1216 0740 1007 0450 0305 1653 1411 1644 1312 1060 1560 0035 0355 0335 1254 1041 0142 1641 1504 0751 1774
First 10 Chips (Octal)**
0013 1060 0245 0527 1436 1226 1257 0046 1071 0561 1037 0770 1327 1472 0124 0366 0133 0465 0717 0217 1742 1422 1442 0523 0736 1635 0136 0273 1026 0003
X2 Delay (Chips)
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 1 2 3 4 5 6 7
P
P-code Relative Delay (Hours)
***
P15(t-72) P16(t-72) P17(t-72) P18(t-72) P19(t-72) P20(t-72) P21(t-72) P22(t-72) P23(t-72) P24(t-72) P25(t-72) P26(t-72) P27(t-72) P28(t-72) P29(t-72) P30(t-72) P31(t-72) P32(t-72) P33(t-72) P34(t-72) P35(t-72) P36(t-72) P37(t-72) P1(t-96) P2(t-96) P3(t-96) P4(t-96) P5(t-96) P6(t-96) P7(t-96)
First 12 Chips (Octal)
3175 5654 2504 5060 2612 3127 5671 4516 4065 4210 4326 4371 2356 5345 4740 2142 5243 2703 5163 4653 4107 4261 4312 2525 3070 5616 2525 3070 3616 3525
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000)
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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Table 6-I
Additional C/A-/P-Code Phase Assignments (sheet 5 of 6)
PRN Signal
No.
156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185
G2 Delay (Chips)
463 568 904 670 230 911 684 309 644 932
12 314 891 212 185 675 503 150 395 345 846 798 992 357 995 877 112 144 476 193
C/A
Initial G2 Setting
(Octal)**
0107 1153 1542 1223 1702 0436 1735 1662 1570 1573 0201 0635 1737 1670 0134 1224 1460 1362 1654 0510 0242 1142 1017 1070 0501 0455 1566 0215 1003 1454
First 10 Chips (Octal)**
1670 0624 0235 0554 0075 1341 0042 0115 0207 0204 1576 1142 0040 0107 1643 0553 0317 0415 0123 1267 1535 0635 0760 0707 1276 1322 0211 1562 0774 0323
X2 Delay (Chips)
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
P
P-code Relative Delay (Hours)
***
P8(t-96) P9(t-96) P10(t-96) P11(t-96) P12(t-96) P13(t-96) P14(t-96) P15(t-96) P16(t-96) P17(t-96) P18(t-96) P19(t-96) P20(t-96) P21(t-96) P22(t-96) P23(t-96) P24(t-96) P25(t-96) P26(t-96) P27(t-96) P28(t-96) P29(t-96) P30(t-96) P31(t-96) P32(t-96) P33(t-96) P34(t-96) P35(t-96) P36(t-96) P37(t-96)
First 12 Chips (Octal)
5470 4416 4025 4230 4336 2375 5354 2744 5140 4642 4103 2263 5313 2767 5151 2646 3101 5662 4513 2067 3211 3726 3571 3456 3405 3420 5432 4437 2035 5234
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000)
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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Table 6-I.
Additional C/A-/P-Code Phase Assignments (sheet 6 of 6)
PRN Signal
No.
186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210
G2 Delay (Chips)
109 445 291
87 399 292 901 339 208 711 189 263 537 663 942 173 900
30 500 935 556 373
85 652 310
C/A
Initial G2 Setting
(Octal)**
1665 0471 1750 0307 0272 0764 1422 1050 1607 1747 1305 0540 1363 0727 0147 1206 1045 0476 0604 1757 1330 0663 1436 0753 0731
First 10 Chips (Octal)**
0112 1306 0027 1470 1505 1013 0355 0727 0170 0030 0472 1237 0414 1050 1630 0571 0732 1301 1173 0020 0447 1114 0341 1024 1046
X2 Delay (Chips)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
P
P-code Relative Delay (Hours)
***
P1(t-120) P2(t-120) P3(t-120) P4(t-120) P5(t-120) P6(t-120) P7(t-120) P8(t-120) P9(t-120) P10(t-120) P11(t-120) P12(t-120) P13(t-120) P14(t-120) P15(t-120) P16(t-120) P17(t-120) P18(t-120) P19(t-120) P20(t-120) P21(t-120) P22(t-120) P23(t-120) P24(t-120) P25(t-120)
First 12 Chips (Octal)
5067 2611 5126 4671 4116 2265 5310 2766 5151 2646 3101 3662 5513 4467 4011 4226 4331 4376 2355 5344 4740 2142 5243 2703 5163
** In the octal notation for the first 10 chips of the C/A-code or the initial settings as shown in this table, the first digit (1/0) represents a "1" or ―0‖, respectively, for the first chip and the last three digits are the
conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 38 are: 1111110000)
*** Pi(t-N): P-code sequence of PRN number i shifted by N hours. See Section 6.3.5.2.1.
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6.3.6.3 Additional L2 CM-/L2 CL-Code PRN sequences. The PRN L2 CM-code and L2 CL-code are described in Sections 3.2.1.4 and 3.2.1.5, respectively, and 37 L2 CM-/L2 CL-code sequence pairs are assigned by SV-ID number in Table 3-II. An additional set of 78 L2 CM-/L2 CL-code PRN sequence pairs are selected and assigned with PRN numbers in this section as shown in Table 6-II. Among the 78 additional sequences, PRN numbers 38 through 63 are reserved for future GPS SVs, and PRN numbers 159 through 210 are reserved for other GNSS applications. PRN allocations do not exist for numbers 64 through 158 for L2 CM-/L2 CL-code. Any assignment of a L2 CM-/L2 CL-code PRN number and its code sequence pair for any additional SV and/or other L2 signal applications will be selected from the sequences of Table 6-II and will be approved, controlled, and managed by the GPSW.
Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 1 of 3)
PRN Signal No. ***
Initial Shift Register State (Octal)
L2 CM
L2 CL
End Shift Register State (Octal)
L2 CM *
L2 CL **
38
771353753
39
226107701
40
022025110
41
402466344
42
752566114
43
702011164
44
041216771
45
047457275
46
266333164
47
713167356
48
060546335
49
355173035
50
617201036
51
157465571
52
767360553
53
023127030
54
431343777
55
747317317
56
045706125
57
002744276
58
060036467
59
217744147
60
603340174
61
326616775
62
063240065
63
111460621
101232630 132525726 315216367 377046065 655351360 435776513 744242321 024346717 562646415 731455342 723352536 000013134 011566642 475432222 463506741 617127534 026050332 733774235 751477772 417631550 052247456 560404163 417751005 004302173 715005045 001154457
453413162 637760505 612775765 136315217 264252240 113027466 774524245 161633757 603442167 213146546 721323277 207073253 130632332 606370621 330610170 744312067 154235152 525024652 535207413 655375733 316666241 525453337 114323414 755234667 526032633 602375063
463624741 673421367 703006075 746566507 444022714 136645570 645752300 656113341 015705106 002757466 100273370 304463615 054341657 333276704 750231416 541445326 316216573 007360406 112114774 042303316 353150521 044511154 244410144 562324657 027501534 521240373
* Short cycled period = 10230 ** Short cycled period = 767250 *** PRN sequences 38 through 63 are reserved for GPS.
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PRN Signal
No.
Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 2 of 3)
Initial Shift Register State (Octal)
End Shift Register State (Octal)
L2 CM
L2 CL
L2 CM *
L2 CL **
159
604055104
160
157065232
161
013305707
162
603552017
163
230461355
164
603653437
165
652346475
166
743107103
167
401521277
168
167335110
169
014013575
170
362051132
171
617753265
172
216363634
173
755561123
174
365304033
175
625025543
176
054420334
177
415473671
178
662364360
179
373446602
180
417564100
181
000526452
182
226631300
183
113752074
184
706134401
185
041352546
186
664630154
187
276524255
188
714720530
189
714051771
190
044526647
605253024 063314262 066073422 737276117 737243704 067557532 227354537 704765502 044746712 720535263 733541364 270060042 737176640 133776704 005645427 704321074 137740372 056375464 704374004 216320123 011322115 761050112 725304036 721320336 443462103 510466244 745522652 373417061 225526762 047614504 034730440 453073141
425373114 427153064 310366577 623710414 252761705 050174703 050301454 416652040 050301251 744136527 633772375 007131446 142007172 655543571 031272346 203260313 226613112 736560607 011741374 765056120 262725266 013051476 144541215 534125243 250001521 276000566 447447071 000202044 751430577 136741270 257252440 757666513
044547544 707116115 412264037 223755032 403114174 671505575 606261015 223023120 370035547 516101304 044115766 704125517 406332330 506446631 743702511 022623276 704221045 372577721 105175230 760701311 737141001 227627616 245154134 040015760 002154472 301767766 226475246 733673015 602507667 753362551 746265601 036253206
* Short cycled period = 10230 ** Short cycled period = 767250
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Table 6-II. Additional L2 CM-/L2 CL-Code Phase Assignments (sheet 3 of 3)
PRN Signal No.
Initial Shift Register State (Octal)
L2 CM
L2 CL
End Shift Register State (Octal)
L2 CM *
L2 CL **
191
207164322
533654510
606512137
202512772
192
262120161
377016461
734247645
701234023
193
204244652
235525312
415505547
722043377
194
202133131
507056307
705146647
240751052
195
714351204
221720061
006215430
375674043
196
657127260
520470122
371216176
166677056
197
130567507
603764120
645502771
123055362
198
670517677
145604016
455175106
707017665
199
607275514
051237167
127161032
437503241
200
045413633
033326347
470332401
275605155
201
212645405
534627074
252026355
376333266
202
613700455
645230164
113771472
467523556
203
706202440
000171400
754447142
144132537
204
705056276
022715417
627405712
451024205
205
020373522
135471311
325721745
722446427
206
746013617
137422057
056714616
412376261
207
132720621
714426456
706035241
441570172
208
434015513
640724672
173076740
063217710
209
566721727
501254540
145721746
110320656
210
140633660
513322453
465052527
113765506
* Short cycled period = 10230 ** Short cycled period = 767250
6.3.7 Pre-Operational Use. Before any new signal or group of signals (e.g., L2C, L5, M, L1C, etcetera) is declared operational, the availability of and/or the configuration of the broadcast signal or group of signals may not comply with all requirements of the relevant IS or ICD. For example, the pre-operational broadcast of L2C signals from the IIR-M satellites did not include any NAV or CNAV data as required by IS-GPS-200. Pre-operational use of any
new signal or group of signals is at the users own risk.
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10. APPENDIX I. LETTERS OF EXCEPTION
10.1 Scope. Approval of this document, as well as approval of any subsequent changes to the document, can be contingent upon a "letter of exception". This appendix depicts such "letters of exception" when authorized by the GPSW.
10.2 Applicable Documents. The documents listed in Section 2.0 shall be applicable to this appendix.
10.3 Letters of Exception. Any letter of exception which is in force for the revision of the IS is depicted in Figure 10.3-1 – 10.3-8.
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Lockheed Martin Space Systems Company Space & Strategic Missiles Valley Forge Operations P.O. Box 8555 Philadelphia, PA 19101
26 May 2003 GPS IIR-CM-MOD-147
SMC/CZK 2420 VELA WAY, SUITE 1467 LOS ANGELES AFB CA 90245-4659
Attention:
Mr. David Smith
Subject:
GPS Block IIR Modernization Contract F04701-00-C-0006
Review and approval of ICD-GPS-PIRN-200C-007B, dated 08 November 2003, post 9
April 2003 CCB (L2C = -160).
Reference:
1) PCOL# 03-012, dated 22 May 03; F04701-00-C-0006; REQUEST FOR IMPACTS DUE TO IMPLEMENTING PROPOSED CHANGES TO PIRN-200C-007 REVISION B
Dear Mr. Smith:
Lockheed Martin Space Systems Company has been asked to review and comment on changes made to ICD-GPS-PIRN-200C-007B at the JPO CCB boarded on or about 09 April 2003. It is our understanding that the ONLY change made to the 08 November 2002 of the subject ICD is L2C for IIR-M SVs changed
from –161.4 dBW to –160.0 dBW.
Based on that change, Lockheed Martin takes exception to IIR-M L2 C signal power specified in Table 3III. Per Lockheed Martin contract requirements as specified in SS-SS-500, Rev. A, dated 14 May 2001,
LMSSC calculates links using: 0-dBi circularly polarized user receiving antenna (located) near ground when the SV is above a 5°
elevation angle Atmospheric loss of 0.5 dB at edge of earth Assumes SV antenna gains are averaged about azimuth
Using the assumptions as specified in paragraph 3.3.1.6 of PIRN-200C-007B, the GPS IIRM SVs provide a minimum receive signal of -161.4 dBW for L2 C signal. Lockheed Martin therefore takes exception to -
160 dBW for L2C of PIRN-200c-007B. Formal request for cost and schedule impacts should come through the JPO Contracting Officer.
To change from -161.4 dBW to -160.0 dBW would have to be analyzed and coordinated between Lockheed Martin and ITT. If such a change were technically possible, there would be impacts to L-Band
level testing, SV level testing, test scripts, Specs, OOH, and various ICDs. These impacts would be in both cost and schedule.
Figure 10.3-1. Letters of Exception.
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GPS IIR-CM-MOD-147 Page 2
Currently, there is an ongoing effort between Lockheed Martin, Boeing, Arinc, Aerospace, and the JPO concerning signal flexibility under the ConOps study. Lockheed Martin recommends, based on the
outcome and direction of this effort, that an impact to the ICD-200 change be included in the resulting request for ROMs for Flex Power implementation.
Note that if Lockheed Martin has taken earlier exception to a change in any requirements in a previous revision of this document, Lockheed Martin continues to take exception to that change. A letter explicitly
stating that the exception is no longer valid will accomplish the retraction of an exception.
Should you have any questions, please contact Martin O’Connor at (610) 354-7866 for technical concerns, or the undersigned at (610) 354-7989 for contractual matters.
Very truly yours,
LOCKHEED MARTIN CORPORATION
Signature on file
Brent B. Achee II GPS Block IIR Deputy Program Director
xc:
Capt. K. Eggehorn
Mary Guyes
Soon Yi, ARINC
J. Windfelder, DCMC
Figure 10.3-2. Letters of Exception (continued).
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Lockheed Martin Space Systems Company Space & Strategic Missiles Valley Forge Operations P.O. Box 8555 Philadelphia, PA 19101
27 September 2004 GPS IIR-CM-3023, Rev A
ARINC 2250 E. Imperial Highway, Suite 450
El Segundo, CA 90245-3546
Attention:
Mr. Soon K. Yi
Subject:
Review of IS-GPS-200 Rev D
Reference:
1) Contract F04701-89-C-0073 2) IS-GPS-200D, dated 09 July 2004
Dear Mr. Yi:
Lockheed Martin Space Systems Company has reviewed the subject version of IS-GPS-200D, dated 09 July 2004. It is Lockheed Martin’s understanding that the JPO and ARINC are in the process of
incorporating major changes to ICD-200C, eliminating multiple Letters of Exception, and change the Interface Control Document to an Interface Specification (IS). With this in mind, Lockheed Martin is
rescinding all previous letters of exception: 1. GPS IIR-CM-1046, dated 17 August 1994 2. GPS IIR-CM-MOD-0097, dated 08 May 2002
3. GPS IIR-CM-2837, dated 26 May 2003 4. GPS IIR-CM-MOD-0177, dated 16 March 2004
Lockheed Martin would like to establish this correspondence for the review of IS-GPS-200 as the baseline letter of exception. Lockheed Martin is taking exception to: 1. L2CNAV 2. IIR-M L2C Signal Power, as defined in Table 3-V
The original Letter of Exception, dated 09 September 2004 listed IODC as an exception. Lockheed Martin has been able to verify this exception no longer exists. This revision to the LOE should therefore
be used in it’s place. Specific reasoning for these exceptions are documented in the attached table.
Lockheed Martin is also submitting technical comments identified herein. If this document is approved at JPO CCB, LMSSC will expect a letter from JPO requesting cost and schedule impacts to implement these
out-of-scope requirements on the IIR and IIR-M contracts.
Per discussions with ARINC, telecons with the JPO, and the IS-200D review directions, it is Lockheed Martin’s understanding that the once this document is Configuration Controlled by the JPO, ICD-200 will
be removed from Lockheed Martin’s contract with the government and replace with IS-200. The approved IS-200 will contain this LOE and Lockheed Martin will be notified in writing as to changes that
occurred as part of the CCB process for concurrence to said changes
Figure 10.3-3. Letters of Exception (continued).
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Should you have any questions, please contact Marty O’Connor at (610) 354-7866 for technical concerns, or the undersigned at (610) 354-2569 for contractual matters.
Very truly yours,
LOCKHEED MARTIN CORPORATION
Signature on file
Paul E. Ruffo, CPCM Manager of Contracts GPS Block IIR, IIR-M, III
xc:
Mary Guyes
A. Trader
J. Windfelder, DCMA
Capt. Brian Knight
Figure 10.3-4. Letters of Exception (continued).
62
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Figure 10.3-5. Letters of Exception (continued).
63
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Figure 10.3-6. Letters of Exception (continued).
64
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Figure 10.3-7. Letters of Exception (continued). 65
IS-GPS-200E 8 June 2010
Figure 10.3-8. Letters of Exception (continued). 66
IS-GPS-200E 8 June 2010
20. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA, D(t)
20.1 Scope. This appendix describes the specific GPS navigation (NAV) data structure denoted as D(t). When transmitted as part of the NAV data, D(t), the specific data structure of D(t) shall be denoted by data ID number 2, represented by the two-bit binary notation as 01.
20.2 Applicable Documents.
20.2.1 Government Documents. In addition to the documents listed in paragraph 2.1, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein.
Specifications Federal Military Other Government Activity Standards Federal Military Other Publications
None None None
None None
GP-03-001 (GPS Interface Control Working Group Charter)
20.2.2 Non-Government Documents. In addition to the documents listed in paragraph 2.2, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein.
Specifications None
Other Publications None
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20.3 Requirements
20.3.1 Data Characteristics. The data stream shall be transmitted by the SV on the L1 and L2 channels at a rate of 50 bps. In addition, upon ground command, the data stream shall be transmitted by the Block IIR-M SV on the L2 CM channel at a rate of 25 bps using FEC encoding resulting in 50 sps.
20.3.2 Message Structure. As shown in Figure 20-1, the message structure shall utilize a basic format of a 1500 bit long frame made up of five subframes, each subframe being 300 bits long. Subframes 4 and 5 shall be subcommutated 25 times each, so that a complete data message shall require the transmission of 25 full frames. The 25 versions of subframes 4 and 5 shall be referred to herein as pages 1 through 25 of each subframe. Each subframe shall consist of ten words, each 30 bits long; the MSB of all words shall be transmitted first.
Each subframe and/or page of a subframe shall contain a telemetry (TLM) word and a handover word (HOW), both generated by the SV, and shall start with the TLM/HOW pair. The TLM word shall be transmitted first, immediately followed by the HOW. The HOW shall be followed by eight data words. Each word in each frame shall contain parity (reference Section 20.3.5).
Block II and IIA SVs are designed with sufficient memory capacity for storing at least 60 days of uploaded NAV data. However, the memory retention of these SVs will determine the duration of data transmission. Block IIR SVs have the capability, with current memory margin, to store at least 60 days of uploaded NAV data in the Block IIA mode and to store at least 60 days of CS data needed to generate NAV data on-board in the Autonav mode. Block IIIA SVs have the capability to support operation for at least 60 days without contact from the CS. Alternating ones and zeros will be transmitted in words 3 through 10 in place of the normal NAV data whenever the SV cannot locate the requisite valid control or data element in its on-board computer memory. The following specifics apply to this default action: (a) the parity of the affected words will be invalid, (b) the two trailing bits of word 10 will be zeros (to allow the parity of subsequent subframes to be valid -- reference paragraph 20.3.5), (c) if the problem is the lack of a data element, only the directly related subframe(s) will be treated in this manner, (d) if a control element cannot be located, this default action will be applied to all subframes and all subframes will indicate ID = 1 (Block II/IIA only) (i.e., an ID-code of 001) in the HOW (reference paragraph 20.3.3.2) (Block IIR/IIR-M, IIF, and IIIA SVs indicate the proper subframe ID for all subframes). Certain failures of control elements which may occur in the SV memory or during an upload will cause the SV to transmit in non-standard codes (NSC and NSY) which would preclude normal use by the US. Normal NAV data transmission will be resumed by the SV whenever a valid set of elements becomes available.
Block II/IIA SVs are uploaded with a minimum of 60 days of NAV data. However, the EAROM retentivity for Block II SVs is designed and guaranteed for only 14 days. Therefore, Block II SV memory is most likely to fail
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IS-GPS-200E
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sometime during long-term extended operations after repeated write operations. In the case of memory failure, the SV will transmit alternating ones and zeros in word 3-10 as specified in the above paragraph. The EAROM retentivity for Block IIA SVs is designed and guaranteed for at least 60 days.
The memory retentivity is guaranteed for at least 60 days for SVs subsequent to Block IIA.
Although the data content of the SVs will be temporarily reduced during the upload process, the transmission of valid NAV data will be continuous. The data capacity of specific operational SVs may be reduced to accommodate partial memory failures.
SUBFRAME NO.
1
PAGE
NO.
1
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS WORD 2
3 SECONDS WORD 3
71
WORD 4
31
61
73 77 83 91
MSB FIRST WORD 5
121
TLM
HOW
WN
N/A
22 BITS
C P
22 BITS
t P
10
BITS
P
23 BITS***
P
24 BITS***
P
C/A OR P ON L2 - 2 BITS URA INDEX - 4 BITS SV HEALTH - 6 BITS
2 MSBs
L2 P DATA FLAG - 1 BIT IODC - 10 BITS TOTAL
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
MSB FIRST WORD 10
151
181
197
211 219
241
271
T GD
t oc
a f2
af1
a f0
1
N/A
24 BITS***
P
16
P
P 8
P
t P
BITS*** 8 BITS
16 BITS
BITS 16 BITS
22 BITS
8 LSBs
IODC - 10 BITS TOTAL
*** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 1 of 11)
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SUBFRAME NO.
2
PAGE
NO.
1
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
31
61 69
91
107
MSB FIRST WORD 5
121
TLM
HOW
IODE
C rs
n
N/A
C P
tP 8
P
8 P
24 BITS
P
22 BITS
22 BITS
BITS 16 BITS
16 BITS BITS
M0 - 32 BITS TOTAL
MSBs
LSBs
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS WORD 7
3 SECONDS WORD 8
WORD 9
MSB FIRST WORD 10
151
167
181
211
227
241
271
287
C UC
C US
toe
8 P
24 BITS
P
8 P
24 BITS
P
t P
2
N/A
16 BITS BITS
16 BITS BITS
16 BITS
e - 32 BITS TOTAL
MSBs
LSBs
A - 32 BITS TOTAL
MSBs
LSBs
FIT INTERVAL FLAG - 1 BIT AODO - 5 BITS
P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 2 of 11)
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SUBFRAME NO.
3
PAGE NO.
N/A
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
1 TLM
22 BITS
31
C P
HOW 22 BITS
61
77
91
t P
Cic 16 BITS
8 P BITS
24 BITS
MSB FIRST
WORD 5
121
137
C is P
16 BITS
8 P BITS
0 - 32 BITS TOTAL
MSBs
LSBs i0 - 32 BITS TOTAL
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
MSB FIRST WORD 10
151
181
211
241
271 279
3
N/A
24 BITS
C rc P
16 BITS
8 P BITS
24 BITS
P 24 BITS
IODE P 8
BITS
IDOT 14 BITS
t P
LSBs
MSBs i 0 - 32 BITS TOTAL
LSBs
- 32 BITS TOTAL
P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 3 of 11)
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SUBFRAME NO.
5
PAGE NO.
1 THRU
24
WORD 1 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
61
31
63 69
91 99
TLM 22 BITS
C P
HOW 22 BITS
t P
e 16 BITS
t oa P 8
BITS
i 16 BITS
MSB FIRST WORD 5
121
P
8 P
16 BITS BITS
DATA ID - 2 BITS SV ID - 6 BITS
SV HEALTH
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS WORD 7
3 SECONDS WORD 8
WORD 9
MSB FIRST WORD 10
151
181
211
241
271 279
290
1
A P
0 P
M 0
P
P
t P
5
THRU
24
24 BITS
24 BITS
24 BITS
24 BITS
af0 - 11 BITS TOTAL af1 - 11 BITS TOTAL
8 MSBs
P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED NOTE: PAGES 2, 3, 4, 5, 7, 8, 9 & 10 OF SUBFRAME 4 HAVE THE SAME FORMAT AS PAGES 1 THROUGH 24 OF SUBFRAME 5
Figure 20-1. Data Format (sheet 4 of 11)
3 LSBs
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SUBFRAME NO.
5
PAGE NO.
25
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
1 TLM
22 BITS
31
HOW C P
22 BITS
61
63 69
91
121
SV HEALTH
SV HEALTH
t P
t oa 8
WNa 8
P
6 BITS/SV
P
6 BITS/SV
P
SV SV SV SV SV SV SV SV
BITS BITS
1 2 3 4
5 678
DATA ID- 2 BITS SV (PAGE) ID- 6 BITS
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS WORD 7
3 SECONDS
WORD 8
WORD 9
MSB FIRST WORD 10
151
181
211
241
271 277
SV HEALTH
SV HEALTH
SV HEALTH
SV HEALTH
5
25
6 BITS/SV
P
6 BITS/SV
P
6 BITS/SV
P
6 BITS/SV
P
SV SV SV SV SV SV SV SV SV SV SV SV SV SV SV SV
16 BITS ** t P
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
6 BITS ***
** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 5 of 11)
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SUBFRAME NO.
PAGE
NO.
1
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
61
31
63 69
91
MSB FIRST WORD 5
121
TLM
HOW
1, 6, 11,
C P
t P
4
16 & 21
22 BITS
22 BITS
16
P
24 BITS***
P
24 BITS***
P
BITS***
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
WORD 6 151
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
181
211
241 249
MSB FIRST WORD 10
271
4
1, 6, 11, 16 & 21
24 BITS***
P
24 BITS***
P
24 BITS***
P 8***
16
P
22 BITS**
t P
BITS BITS***
** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 6 of 11)
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WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
SUBFRAME NO.
PAGE
NO.
1
61
31
63 69
91
121
TLM
HOW
4
12, 19, 20, 22, 23 & 24
22 BITS
C P
22 BITS
t P
16
P
24 BITS***
P
24 BITS***
P
BITS***
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
WORD 6 151
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
181
211
241 249
MSB FIRST WORD 10
271
4
12, 19, 20, 22, 23 & 24
24 BITS***
P
24 BITS***
P
24 BITS***
P 8*** 16 BITS** P
22 BITS**
t P
BITS
** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 7 of 11)
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SUBFRAME NO.
4
PAGE NO.
18
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
1 TLM
22 BITS
31
C P
HOW 22 BITS
61 63 69 77
91 99 107
121 129 137
t P
0
1
2
3
0
1
2
3
8
8 P8
8
8 P 8
8
8 P
BITS BITS
BITS BITS BITS
BITS BITS BITS
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
MSB FIRST WORD 10
151
181
211 219 227
241 249 257
271 279
4
18
A 1 P
P 8
tot WN t
tLS
8
8 P8
8
DN 8
P
t LSF 8
14 BITS **
t P
24 BITS
24 BITS
BITS BITS BITS
BITS BITS BITS
BITS
A 0 - 32 BITS TOTAL
MSBs
LSBs
** RESERVED FOR SYSTEM USE P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
WNLSF
Figure 20-1. Data Format (sheet 8 of 11)
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SUBFRAME NO.
4
PAGE NO.
25
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
1 TLM
22 BITS
31
C P
HOW 22 BITS
61
t P
63 69
91
121
A-SPOOF &
A- SPOOF &
A- SPOOF &
SV CONFIG
SV CONFIG
SV CONFIG
SV SV SV SV P SV SV SV SV SV SV P SV SV SV SV SV SV P
12 3 4
5 6 7 8 9 10
11 12 13 14 15 16
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
MSB FIRST WORD 10
227
151
181
211
229
241
271
A- SPOOF &
A- SPOOF &
A-SPOOF &
SV HEALTH
SV HEALTH
SV CONFIG
SV CONFIG
SV CONFIG
6 BITS/SV
6 BITS/SV
4
25
SV SV SV SV SV SV P SV SV SV SV SV SV P SV SV SV SV SV P SV SV SV SV P SV SV SV
t P
17 18 19 20 21 22
23 24 25 26 27 28
29 30 31 32 25
26 27 28 29
30 31 32
2 BITS **
** RESERVED FOR SYSTEM USE P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
SV HEALTH - 6 BITS 4 BITS **
Figure 20-1. Data Format (sheet 9 of 11)
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SUB FRAME NO.
4
4
PAGE NO.
13
WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
1 TLM
22 BITS
31
C P
HOW 22 BITS
61 69
t P
63
71
91
121
E
E
E
E
E
E
E
E
E
E
E
E
E
R
R
R
R
R
R
R
R
R
R
R
R
R
D
D
D
D
D
D
D
D
D
D
D
D
D
1
2
3
3
4
5
6
7
7
8
9
1
1
6
6 2P
4
6
6
6
2P 4
6
6
0
1 P
B
B
M
L
B
B
B
M
L
B
B
6
2
I
I
S
T
T
B
S
S
S
S
I
I
I
S
B
T
T
T
B
S
S
S
S
S
S
I
I
B
M
B
T
T
I
S
S
S
S
T
B
S
S
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
AVAILABILITY INDICATOR - 2 BITS
WORD 6
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
MSB FIRST WORD 10
13
151
181
211
241
271
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
3
1
2
3
4 5P
5
6
7
P 8
9
9
0
1
2
3P 3
4
5
6
7 P
7
8
9
0 tP
4
6
6
6
2
4
6
6
6
2
4
6
6
6
2
4
6
6
6
2
4
6
6
6
L
B
S
I
B
T
S
S
B
B
M
I
I
S
T
T
B
S
S
S
L
B
S
I
B
T
S
S
B
B
M
I
I
S
T
T
B
S
S
S
L
B
B
B
M
S
I
I
I
S
B
T
T
T
B
S
S
S
S
S
L
B
B
B
M
S
I
I
I
S
B
T
T
T
B
S
S
S
S
S
L
B
B
B
S
I
I
I
B
T
T
T
S
S
S
S
P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5)
C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 10 of 11)
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WORD 1
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 2
WORD 3
WORD 4
MSB FIRST WORD 5
SUBFRAME NO.
PAGE
NO.
1
61
31
63 69
91
121
TLM
HOW
4
14, 15
C P
t P
& 17**
22 BITS
22 BITS
16
P
24 BITS**
P
24 BITS**
P
BITS**
DATA ID - 2 BITS SV (PAGE) ID - 6 BITS
WORD 6 151
DIRECTION OF DATA FLOW FROM SV
150 BITS
3 SECONDS
WORD 7
WORD 8
WORD 9
181
211
241
MSB FIRST WORD 10
271
4
14, 15
24 BITS**
P
24 BITS**
P
24 BITS**
P
24 BITS**
P
22 BITS**
t P
& 17**
** THE INDICATED PORTIONS OF WORDS 3 THROUGH 10 OF PAGES 14 AND 15 ARE RESERVED FOR SYSTEM USE, WHILE THOSE OF PAGE 17 ARE RESERVED FOR SPECIAL MESSAGES PER PARAGRAPH 20.3.3.5.1.10
P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24. BIT 23 IS THE INTEGRITY STATUS FLAG AND BIT 24 IS RESERVED
Figure 20-1. Data Format (sheet 11 of 11)
20.3.3 Message Content. The format and contents of the TLM word and the HOW, as well as those of words three through ten of each subframe/page, are described in the following subparagraphs. The timing of the subframes and
pages is covered in Section 20.3.4.
20.3.3.1 Telemetry Word. Each TLM word is 30 bits long, occurs every six seconds in the data frame, and is the first word in each subframe/page. The format shall be as shown in Figure 20-2. Bit 1 is transmitted first. Each TLM word shall begin with a preamble, followed by the TLM message, the integrity status flag, one reserved bit, and six parity bits. The TLM message contains information needed by the precise positioning service (PPS) user (authorized user) and by the CS, as described in the related SS/CS interface documentation.
Bit 23 of each TLM word is the Integrity Status Flag (ISF). A "0" in bit position 23 indicates that the conveying signal is provided with the legacy level of integrity assurance. That is, the probability that the instantaneous URE of the conveying signal exceeds 4.42 times the upper bound value of the current broadcast URA index, for more than 5.2 seconds, without an accompanying alert, is less than 1E-5 per hour. A "1" in bit-position 23 indicates that the conveying signal is provided with an enhanced level of integrity assurance. That is, the probability that the
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instantaneous URE of the conveying signal exceeds 5.73 times the upper bound value of the current broadcast URA index, for more than 5.2 seconds, without an accompanying alert, is less than 1E-8 per hour. The probabilities associated with the nominal and lower bound values of the current broadcast URA index are not defined.
In this context, an "alert" is defined as any indication or characteristic in the conveying signal, as specified elsewhere in this document, which signifies that the conveying signal may be invalid and should not be used, such as, not Operational-Healthy, Non-Standard Code, parity error, etc.
20.3.3.2 Handover Word (HOW). The HOW shall be 30 bits long and shall be the second word in each subframe/page, immediately following the TLM word. A HOW occurs every 6 seconds in the data frame. The format and content of the HOW shall be as shown in Figure 20-2. The MSB is transmitted first. The HOW begins with the 17 MSBs of the time-of-week (TOW) count. (The full TOW count consists of the 19 LSBs of the 29-bit Zcount). These 17 bits correspond to the TOW-count at the X1 epoch which occurs at the start (leading edge) of the next following subframe (reference paragraph 3.3.4).
Bit 18 is an "alert" flag. When this flag is raised (bit 18 = "1"), it shall indicate to the standard positioning service (SPS) user (unauthorized user) that the SV URA may be worse than indicated in subframe 1 and that he shall use that SV at his own risk.
Bit 19 is an anti-spoof (A-S) flag. A "1" in bit-position 19 indicates that the A-S mode is ON in that SV.
Bits 20, 21, and 22 of the HOW provide the ID of the subframe in which that particular HOW is the second word; the ID code shall be as follows:
Subframe 1 2 3 4 5
ID Code 001 010 011 100 101
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TLM Word
1 = Reserved Bits 2 = Integrity Status Flag
MSB
LSB
Parity
Preamble 1 0 0 0 1 0 1 1
TLM Message
21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
HOW
Anti-Spoof Flag “Alert” Flag
Solved for bits to preserve parity check with zeros in bits 29 and 30
MSB
LSB
Parity
TOW-Count Message (Truncated)
Subframe
ID
00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Figure 20-2. TLM and HOW Formats
20.3.3.3 Subframe 1. The content of words three through ten of subframe 1 are defined below, followed by related algorithms and material pertinent to use of the data.
20.3.3.3.1 Subframe 1 Content. The third through tenth words of subframe 1 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten for parity computation purposes. The remaining 190 bits of words three through ten shall contain the clock parameters and other data described in the following.
The clock parameters describe the SV time scale during the period of validity. The parameters are applicable during the time in which they are transmitted. Beyond that time, they are still applicable, however, the most recent data set should be used since the accuracy degrades over time. The timing information for subframes, pages, and data sets is covered in Section 20.3.4.
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20.3.3.3.1.1 Transmission Week Number. The ten MSBs of word three shall contain the ten LSBs of the Week Number as defined in 3.3.4. These ten bits shall be a modulo 1024 binary representation of the current GPS week number at the start of the data set transmission interval (see paragraph 3.3.4(b)). The GPS week number increments at each end/start of week epoch. For Block II SVs in long-term extended operations, beginning approximately 28 days after upload, the transmission week number may not correspond to the actual GPS week number due to curve fit intervals that cross week boundaries.
20.3.3.3.1.2 Code(s) on L2 Channel. Bits 11 and 12 of word three shall indicate which code(s) is (are) commanded ON for the L2 channel, as follows:
00 = Reserved, 01 = P code ON, 10 = C/A code ON.
20.3.3.3.1.3 SV Accuracy. Bits 13 through 16 of word three shall give the URA index of the SV (reference paragraph 6.2.1) for the standard positioning service user. Except for Block IIR/IIR-M SVs in the Autonav mode, the URA index (N) is an integer in the range of 0 through 15 and has the following relationship to the URA of the SV:
URA INDEX 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
URA (meters)
0.00 < URA
2.40
2.40 < URA
3.40
3.40 < URA
4.85
4.85 < URA
6.85
6.85 < URA
9.65
9.65 < URA
13.65
13.65 < URA
24.00
24.00 < URA
48.00
48.00 < URA
96.00
96.00 < URA 192.00
192.00 < URA 384.00
384.00 < URA 768.00
768.00 < URA 1536.00
1536.00 < URA 3072.00
3072.00 < URA 6144.00
6144.00 < URA (or no accuracy prediction is available - standard positioning
service users are advised to use the SV at their own risk.)
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For each URA index (N), users may compute a nominal URA value (X) as given by:
•
If the value of N is 6 or less, X = 2(1 + N/2),
•
If the value of N is 6 or more, but less than 15, X = 2(N - 2),
•
N = 15 shall indicate the absence of an accuracy prediction and shall advise the standard
positioning service user to use that SV at his own risk.
For N = 1, 3, and 5, X should be rounded to 2.8, 5.7, and 11.3 meters, respectively.
For Block IIR/IIR-M SVs in the Autonav mode, the URA shall be defined to mean ―no better than X meters‖, with ―X‖ as defined above for each URA index.
Integrity properties of the URA are specified with respect to the upper bound values of the URA index (see 20.3.3.1).
URA accounts for signal-in-space contributions to user range error that include, but are not limited to, the following: the net effect of clock parameter and code phase error in the transmitted signal for single-frequency L1C/A or single-frequency L2C users who correct the code phase as described in Section 30.3.3.3.1.1.1, as well as the net effect of clock parameter, code phase, and intersignal correction error for dual-frequency L1/L2 and L1/L5 users who correct for group delay and ionospheric effects as described in Section 30.3.3.3.1.1.2.
20.3.3.3.1.4 SV Health. The six-bit health indication given by bits 17 through 22 of word three refers to the transmitting SV. The MSB shall indicate a summary of the health of the NAV data, where
0 = all NAV data are OK, 1 = some or all NAV data are bad.
The five LSBs shall indicate the health of the signal components in accordance with the codes given in paragraph 20.3.3.5.1.3. The health indication shall be given relative to the "as designed" capabilities of each SV (as designated by the configuration code - see paragraph 20.3.3.5.1.4). Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or if it has been configured into a mode which is normal from a user standpoint and does not require that capability.
Additional SV health data are given in subframes 4 and 5. The data given in subframe 1 may differ from that shown in subframes 4 and/or 5 of other SVs since the latter may be updated at a different time.
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20.3.3.3.1.5 Issue of Data, Clock (IODC). Bits 23 and 24 of word three in subframe 1 shall be the two MSBs of the ten-bit IODC term; bits one through eight of word eight in subframe 1 shall contain the eight LSBs of the IODC. The IODC indicates the issue number of the data set and thereby provides the user with a convenient means of detecting any change in the correction parameters. Constraints on the IODC as well as the relationship between the IODC and the IODE (issue of data, ephemeris) terms are defined in paragraph 20.3.4.4.
Short-term and Long-term Extended Operations. Whenever the fit interval flag indicates a fit interval greater than 4 hours, the IODC can be used to determine the actual fit interval of the data set (reference section 20.3.4.4).
20.3.3.3.1.6 Data Flag for L2 P-Code. When bit 1 of word four is a "1", it shall indicate that the NAV data stream was commanded OFF on the P-code of the L2 channel.
20.3.3.3.1.7 Estimated Group Delay Differential. Bits 17 through 24 of word seven contain the L1-L2 correction term, TGD, for the benefit of "L1 only" or "L2 only" users; the related user algorithm is given in paragraph 20.3.3.3.3.
20.3.3.3.1.8 SV Clock Correction. Bits nine through 24 of word eight, bits one through 24 of word nine, and bits one through 22 of word ten contain the parameters needed by the users for apparent SV clock correction (toc, af2, af1, af0). The related algorithm is given in paragraph 20.3.3.3.3.
20.3.3.3.2 Subframe 1 Parameter Characteristics. For those parameters whose characteristics are not fully defined in Section 20.3.3.3.1, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-I.
20.3.3.3.3 User Algorithms for Subframe 1 Data. The algorithms defined below (a) allow all users to correct the code phase time received from the SV with respect to both SV code phase offset and relativistic effects, (b) permit the "single frequency" (L1 or L2) user to compensate for the effects of SV group delay differential (the user who utilizes both frequencies does not require this correction, since the clock parameters account for the induced effects), and (c) allow the "two frequency" (L1 and L2) user to correct for the group propagation delay due to ionospheric effects (the single frequency user may correct for ionospheric effects as described in paragraph 20.3.3.5.2.5).
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Parameter Code on L2 Week No. L2 P data flag SV accuracy SV health
TGD IODC
toc af2 af1 af0
Table 20-I.
No. of Bits** 2 10 1 4 6 8* 10 16 8* 16* 22*
Subframe 1 Parameters
Scale Factor (LSB)
Effective Range***
1
1
1
1 2-31
24
604,784
2-55
2-43
2-31
Units discretes
week discrete (see text) discretes seconds (see text) seconds sec/sec2 sec/sec seconds
* Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB;
** See Figure 20-1 for complete bit allocation in subframe;
*** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor.
20.3.3.3.3.1 User Algorithm for SV Clock Correction. The polynomial defined in the following allows the user to determine the effective SV PRN code phase offset referenced to the phase center of the antennas ( tsv) with respect to GPS system time (t) at the time of data transmission. The coefficients transmitted in subframe 1 describe the offset apparent to the two-frequency user for the interval of time in which the parameters are transmitted. This estimated correction accounts for the deterministic SV clock error characteristics of bias, drift and aging, as well as for the SV implementation characteristics of group delay bias and mean differential group delay. Since these coefficients do not include corrections for relativistic effects, the user's equipment must determine the requisite relativistic correction. Accordingly, the offset given below includes a term to perform this function.
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The user shall correct the time received from the SV with the equation (in seconds)
t
=
tsv - tsv
(1)
where
t
=
GPS system time (seconds),
tsv
=
effective SV PRN code phase time at message transmission time (seconds),
tsv
=
SV PRN code phase time offset (seconds).
The SV PRN code phase offset is given by
tsv
= af0 + af1(t - toc) + af2(t - toc)2 + tr
(2)
where
af0, af1 and af2 are the polynomial coefficients given in subframe 1, toc is the clock data reference time in
seconds (reference paragraph 20.3.4.5), and tr is the relativistic correction term (seconds) which is given
by
tr = F e A sin Ek.
The orbit parameters (e, A , Ek) used here are described in discussions of data contained in subframes 2 and 3, while F is a constant whose value is
2 F =
c2
= - 4.442807633 (10)-10
sec ,
meter
where
=
3.986005 x 1014
meters 3 second 2
=
c = 2.99792458 x 108 meters = second
value of Earth's universal gravitational parameters speed of light.
Note that equations (1) and (2), as written, are coupled. While the coefficients af0, af1 and af2 are generated by using GPS time as indicated in equation (2), sensitivity of tsv to t is negligible. This negligible sensitivity will allow the user to approximate t by tSV in equation (2). The value of t must account for beginning or end of week crossovers. That is, if the quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If the quantity t - toc is less than -302,400 seconds, add 604,800 seconds to t.
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The control segment will utilize the following alternative but equivalent expression for the relativistic effect when estimating the NAV parameters:
tr = where
2R V c 2
R is the instantaneous position vector of the SV,
V is the instantaneous velocity vector of the SV, and c is the speed of light. (Reference paragraph 20.3.4.3).
It is immaterial whether the vectors R and V are expressed in earth-fixed, rotating coordinates or in earth-centered, inertial coordinates.
20.3.3.3.3.2 L1 - L2 Correction. The L1 and L2 correction term, TGD, is initially calculated by the CS to account for the effect of SV group delay differential between L1 P(Y) and L2 P(Y) based on measurements made by the SV contractor during SV manufacture. The value of TGD for each SV may be subsequently updated to reflect the actual on-orbit group delay differential. This correction term is only for the benefit of "single-frequency" (L1 P(Y) or L2 P(Y)) users; it is necessitated by the fact that the SV clock offset estimates reflected in the af0 clock correction coefficient (see paragraph 20.3.3.3.3.1) are based on the effective PRN code phase as apparent with two frequency (L1 P(Y) and L2 P(Y)) ionospheric corrections. Thus, the user who utilizes the L1 P(Y) signal only shall modify the code phase offset in accordance with paragraph 20.3.3.3.3.1 with the equation
( tSV)L1P(Y) = tSV - TGD
where TGD is provided to the user as subframe 1 data. For the user who utilizes L2 P(Y) only, the code phase modification is given by
( tSV)L2P(Y) = tSV - TGD
where, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively, = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2.
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The value of TGD is not equal to the mean SV group delay differential, but is a measured value that represents the mean group delay differential multiplied by 1/(1- ). That is,
1 TGD = 1 -
(tL1P(Y) - tL2P(Y))
where tLiP(Y) is the GPS time the ith frequency P(Y) signal (a specific epoch of the signal) is transmitted from the SV antenna phase center.
20.3.3.3.3.3 Ionospheric Correction. The two frequency (L1 P(Y) and L2 P(Y)) user shall correct for the group delay due to ionospheric effects by applying the relationship:
PR PRL2P(Y) - PRL1P(Y) 1-
where
PR = PRi =
pseudorange corrected for ionospheric effects, pseudorange measured on the channel indicated by the subscript,
and is as defined in paragraph 20.3.3.3.3.2. The clock correction coefficients are based on "two frequency" measurements and therefore account for the effects of mean differential delay in SV instrumentation.
20.3.3.3.3.4 Example Application of Correction Parameters. A typical system application of the correction
parameters for a user receiver is shown in Figure 20-3. The ionospheric model referred to in Figure 20-3 is
discussed in paragraph 20.3.3.5.2.5 in conjunction with the related data contained in page 18 of subframe 4. The
ERD
c term referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related data contained in page 13 of subframe 4.
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TGD*
af0, af1, af2, toc
tr tSV
CLOCK CORRECTION POLYNOMIAL
ESTIMATE OF SV TRANSMISSION TIME
Ttropo
TROPOSPHERIC MODEL
CODE PHASE OFFSET - TRUE SV CLOCK EFFECTS - EQUIPMENT GROUP DELAY
DIFFERENTIAL EFFECTS - RELATIVISTIC EFFECTS
Tiono
IONOSPHERIC MODEL*
n, n
GPS TIME
PATH DELAY - GEOMETRIC - TROPOSHERIC - IONOSPHERIC*
PSEUDORANGE DIVIDED BY THE SPEED OF LIGHT
ERD ** c
USER CLOCK BIAS
FILTER AND COORDINATE CONVERTER
USER POSITION, VELOCITY, and TIME (CLOCK BIAS)
- RANGE DATA FROM OTHER SATELLITES
- CALIBRATION DATA - AUXILIARY SENSOR
GPS TIME
* SINGLE FREQUENCY USER ONLY ** OPTIONAL
Figure 20-3. Sample Application of Correction Parameters
20.3.3.4 Subframes 2 and 3. The contents of words three through ten of subframes 2 and 3 are defined below, followed by material pertinent to the use of the data.
20.3.3.4.1 Content of Subframes 2 and 3. The third through tenth words of subframes 2 and 3 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten of each subframe for parity computation purposes. Bits 288 through 292 of subframe 2 shall contain the Age of Data Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4 (reference paragraph 20.3.3.5.1.9). The remaining 375 bits of those two subframes shall contain the ephemeris representation parameters of the transmitting SV.
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The ephemeris parameters describe the orbit during the curve fit intervals described in section 20.3.4. Table 20-II gives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it shall be noted, however, that the transmitted parameter values are such that they provide the best trajectory fit in EarthCentered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediate coordinate values as pertaining to any conventional coordinate system.
The issue of ephemeris data (IODE) term shall provide the user with a convenient means for detecting any change in the ephemeris representation parameters. The IODE is provided in both subframes 2 and 3 for the purpose of comparison with the 8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, a data set cutover has occurred and new data must be collected. The timing of the IODE and constraints on the IODC and IODE are defined in paragraph 20.3.4.4.
Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change in both IODE words. The CS (Block II/IIA/IIR/IIR-M/IIF) and SS (Block IIIA) shall assure that the toe value, for at least the first data set transmitted by an SV after an upload, is different from that transmitted prior to the cutover (reference paragraph 20.3.4.5).
A "fit interval" flag is provided in subframe 2 to indicate whether the ephemerides are based on a four-hour fit interval or a fit interval greater than four hours (reference paragraph 20.3.3.4.3.1).
The AODO word is provided in subframe 2 to enable the user to determine the validity time for the NMCT data provided in subframe 4 of the transmitting SV. The related algorithm is given in paragraph 20.3.3.4.4.
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M0 n e A
Ω0 i0
IDOT Cuc Cus Crc Crs Cic Cis toe
IODE
Table 20-II.
Ephemeris Data Definitions
Mean Anomaly at Reference Time Mean Motion Difference From Computed Value
Eccentricity Square Root of the Semi-Major Axis Longitude of Ascending Node of Orbit Plane at Weekly Epoch Inclination Angle at Reference Time
Argument of Perigee Rate of Right Ascension Rate of Inclination Angle Amplitude of the Cosine Harmonic Correction Term to the Argument of Latitude Amplitude of the Sine Harmonic Correction Term to the Argument of Latitude Amplitude of the Cosine Harmonic Correction Term to the Orbit Radius Amplitude of the Sine Harmonic Correction Term to the Orbit Radius Amplitude of the Cosine Harmonic Correction Term to the Angle of Inclination Amplitude of the Sine Harmonic Correction Term to the Angle of Inclination Reference Time Ephemeris (reference paragraph 20.3.4.5) Issue of Data (Ephemeris)
20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics. For each ephemeris parameter contained in subframes 2 and 3, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-III.
The AODO word (which is not an ephemeris parameter) is a five-bit unsigned term with an LSB scale factor of 900, a range from 0 to 31, and units of seconds.
20.3.3.4.3 User Algorithm for Ephemeris Determination. The user shall compute the ECEF coordinates of position for the phase center of the SVs’ antennas utilizing a variation of the equations shown in Table 20-IV. Subframes 2 and 3 parameters are Keplerian in appearance; the values of these parameters, however, are produced by the CS (Block II/IIA/IIR/IIR-M/IIF) and SS (Block IIIA) via a least squares curve fit of the predicted ephemeris of the phase center of the SVs’ antennas (time-position quadruples; t, x, y, z expressed in ECEF coordinates). Particulars concerning the periods of the curve fit, the resultant accuracy, and the applicable coordinate system are given in the following subparagraphs.
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20.3.3.4.3.1 Curve Fit Intervals. Bit 17 in word 10 of subframe 2 is a "fit interval" flag which indicates the curvefit interval used by the CS (Block II/IIA/IIR/IIR-M/IIF) and SS (Block IIIA) in determining the ephemeris parameters, as follows:
0 = 4 hours, 1 = greater than 4 hours.
The relationship of the curve-fit interval to transmission time and the timing of the curve-fit intervals is covered in section 20.3.4.
Parameter
Table 20-III.
Ephemeris Parameters
No. of Bits** Scale Factor (LSB) Effective Range***
Units
IODE
8
(see text)
Crs
16*
2-5
n
16*
2-43
meters semi-circles/sec
M0
32*
2-31
Cuc
16*
2-29
e
32
2-33
semi-circles
radians
0.03
dimensionless
Cus
16*
2-29
A
32
2-19
radians
meters
toe
16
24
604,784
seconds
Cic
16*
2-29
radians
Ω0
32*
2-31
semi-circles
Cis
16*
2-29
radians
i0
32*
2-31
semi-circles
Crc
16*
2-5
meters
32*
2-31
semi-circles
24*
2-43
semi-circles/sec
IDOT
14*
2-43
semi-circles/sec
* Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe;
*** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor.
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