IS-GPS-800A - US Coast Guard Navigation Center

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3.4 Interface CriteriaThe following specifies additional criteria for the SS/US interface for the GPS L1C signal.3.4.1 GPS TimeGPS time is established by the GPS Control Segment and is referenced to Coordinated Universal Time (UTC) asrealized by the U.S. Naval Observatory (UTC (USNO)) with zero time-point defined as midnight on the night ofJanuary 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time is one week defined as604,800 seconds, concatenated with the GPS week number. GPS time may differ from UTC because GPS time is acontinuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is aninherent but bounded drift rate between the UTC and GPS time scales. The GPS CS shall control the GPS timescale to be within 50 nanoseconds (95% probability) of UTC (USNO) (modulo one second).The L1C message (henceforth referred to as CNAV-2) 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 to UTC (USNO) towithin 1.5 nanoseconds (RMS over 30 days). This data is generated by the GPS CS; therefore, the accuracy of thisrelationship may degrade if for some reason the GPS CS is unable to upload data to a SV. Propagation delay errorsand receiver equipment biases unique to the user add to this time transfer uncertainty.The CNAV-2 data contains thirteen bits representing the sequential number assigned to the current GPS week (seeparagraph 6.2.2). The range of this count is from 0 to 8191 with its zero state being defined as the GPS weeknumber zero and every integer multiple of 8192 weeks thereafter (i.e. 0, 8192, 16384, etc.).3.4.2 SV Time vs. GPS TimeIn controlling the SVs and uploading of data, the CS shall allow for the following timing relationships:Each SV operates on its own SV time;All TOI and Interval Time of Week (ITOW) in the CNAV-2 messages shall be in SV-time;All other data in the CNAV-2 messages shall be relative to GPS-time;The acts of transmitting the CNAV-2 messages shall be executed by the SV on SV time.3.4.3 Speed of LightThe speed of light used by the CS for generating the data described in the above paragraphs isc = 2.99792458 x 10 8meters per secondwhich is the official WGS 84 speed of light. The user shall use the same value for the speed of light in allcomputations.28 IS-GPS-800A8 June 2010
3.5 Message DefinitionAs shown in Figure 3.2-3, the L1C message, CNAV-2, structure utilizes three different subframe formats. Thissection defines and specifies the content of each subframe.3.5.1 Message ContentSubframe 1 provides TOI count defined as being equal to the number of 18-second epochs that have occurred sincethe start of current Interval Time of Week (ITOW) epoch (reference Section 3.5.3).Subframe 2 provides clock and ephemeris data which is nominally invariant over multiple frames. Subframe 2 alsoprovides the ITOW count defined as being equal to the number of two-hour epochs that have occurred since thetransition from the previous week.Subframe 3 provides other navigation data which is commutated over multiple pages. Each page of subframe 3provides different data as shown in Figures 3.5-2 through 3.5-8. Additional subframe 3 pages may be defined in thefuture. It shall be noted that the broadcast sequence of subframe 3 pages is variable and, as such, users must notexpect a fixed pattern of page sequence. Subframe 3 provides an 8-bit PRN number of the transmitting SV with arange of 0 (00000000) to 255 (11111111).In the event of message generation failure, the SV shall replace each affected subframe 2 and/or 3 with the defaultmessage. The data content of the default message shall be alternating ones and zeros beginning with one and themessage shall contain a proper CRC block. In addition, in the event of message generation failure of eithersubframe 2 or 3, the SV shall replace the content of subframe 1 with the default TOI count (111111111). The SVshall also broadcast the default TOI count in the event of a subframe 1 message generation failure.3.5.2 Subframe 1The TOI count utilizes a 9-bit data word that represents SV time at the start of the next 18-second frame. The countrepresents the number of 18-second epochs that have occurred since the start of the two-hour period represented byITOW count in the subframe 2 of the next 18-second frame. The TOI count range is from 0 (000000000) to 399(110001111). The beginning epoch of a two-hour period shall correspond to a start of subframe 1 and TOI count one(000000001) shall correspond to the start of the next 18-second frame following the beginning of a two-hour period.The TOI data is nominally the same on all SVs (for those SVs that broadcast TOI data).The 9-bit subframe 1 data is channel encoded into 52 symbols for broadcast as specified in paragraph 3.2.3.2.29 IS-GPS-800A8 June 2010
Page 1 and 2: IS-GPS-800A8 June 2010GLOBAL POSITI
Page 3 and 4: REVISION RECORDLTR DESCRIPTION DATE
Page 5 and 6: 3.2.3.3 Cyclic Redundancy Check ...
Page 7 and 8: LIST OF FIGURESFigure 3.2-1. Genera
Page 9 and 10: 1. INTRODUCTION1.1 ScopeThis Interf
Page 11 and 12: 3. SIGNAL REQUIREMENTSThe requireme
Page 13 and 14: Carriers of the two L1C components
Page 15 and 16: 3.2.2 PRN Code CharacteristicsThe c
Page 17 and 18: The overlay codes are derived from
Page 19 and 20: 3.2-2 L1C Ranging Codes Parameter A
Page 21 and 22: GPS PRNSignal No.3.2-3 L1C O Overla
Page 23 and 24: FIXED LENGTH-10223 LEGENDRE SEQUENC
Page 25 and 26: 3.2.2.2 Non-Standard CodesThe non-s
Page 27 and 28: corresponding to MSB = 0. (A SV tra
Page 29 and 30: This code has the following charact
Page 31 and 32: 3.2.3.5 InterleavingThe 1748 encode
Page 33 and 34: 3.3 Signal ModulationThe signals mo
Page 35: Bit valueL1C DbitstreamSubcarrier s
Page 40 and 41: DIRECTION OF DATA FLOW FROM SV100 B
Page 46 and 47: 3.5.3 Subframe 2Subframe 2 provides
Page 48 and 49: Table 3.5-1. Subframe 2 Parameters
Page 50 and 51: 3.5.3.1 Transmission Week NumberBit
Page 52 and 53: 3.5.3.6 Ephemeris Parameter Charact
Page 54 and 55: Table 3.5-2. Elements of Coordinate
Page 56 and 57: The user shall use the broadcast UR
Page 58 and 59: where, ISC L1CD is provided to the
Page 60 and 61: 3.5.3.10.1 Integrity Status Flag (I
Page 62 and 63: The ionospheric data shall be updat
Page 64 and 65: 3.5.4.2.3 User Algorithm for Applic
Page 66 and 67: transmitted by the SVs will degrade
Page 68 and 69: Table 3.5-7.Midi Almanac Parameters
Page 70 and 71: MSBCDC = Clock Differential Correct
Page 72 and 73: 3.5.4.5 Subframe 3, Page 6 - TextSu
Page 74 and 75: 5. RESERVED66 IS-GPS-800A8 June 201
Page 76 and 77: LSF - Leap Seconds FutureL1C - Comm
Page 78 and 79: Table 6.2-1.Legendre Sequence (Octa
Page 80 and 81: Table 6.2-2 LDPC Submatrix A for Su
Page 86 and 87:
Table 6.2-2 LDPC Submatrix A for Su
Page 88 and 89:
Table 6.2-2 LDPC Submatrix A for Su
Page 90 and 91:
Table 6.2-3. LDPC Submatrix B for S
Page 92 and 93:
Table 6.2-7. LDPC Submatrix T for s
Page 94 and 95:
Table 6.2-7. LDPC Submatrix T for s
Page 96 and 97:
Table 6.2-8. LDPC Submatrix A for s
Page 98 and 99:
Table 6.2-8. LDPC Submatrix A for s
Page 100 and 101:
Table 6.2-9. LDPC Submatrix B for S
Page 102 and 103:
Table 6.2-13. LDPC Submatrix T for
Page 104 and 105:
Table 6.2-16. Number of 1’s in LD
Page 106 and 107:
6.3 Supporting Material6.3.1 Additi
Page 108 and 109:
Table 6.3-1 Additional L1C Ranging
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
PRNSignalNo.Table 6.3-2. Additional
Page 116 and 117:
PRNSignalNo.Table 6.3-2 Additional
Page 118 and 119:
Page 120 and 121:
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IS-GPS-800A - US Coast Guard Navigation Center

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