IS-GPS-200H (797934), страница 8
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The G2i sequence is formed by effectively delaying the G2sequence by an integer number of chips. The G1 and G2 sequences are generated by 10-stageshift registers having the following polynomials as referred to in the shift register input (seeFigures 3-8 and 3-9).G1 = X10 + X3 + 1, andG2 = X10 + X9 + X8 + X6 + X3 + X2 + 1.The initialization vector for the G1 and G2 sequences is 1111111111. The G1 and G2 shiftregisters are initialized at the P-coder X1 epoch. The G1 and G2 registers are clocked at 1.023MHz derived from the 10.23 MHz P-coder clock. The initialization by the X1 epoch phases the1.023 MHz clock to insure that the first chip of the C/A code begins at the same time as the firstchip of the P-code.The effective delay of the G2 sequence to form the G2i sequence may be accomplished bycombining the output of two stages of the G2 shift register by modulo-2 addition (see Figure 310).
However, this two-tap coder implementation generates only a limited set of valid C/Acodes. Table 3-I contains a tabulation of the G2 shift register taps selected and theircorresponding P-code X2i and PRN signal numbers together with the first several chips of eachresultant PRN code. Timing relationships related to the C/A code are shown in Figure 3-11.31IS-GPS-200H24 Sep 2013POLYNOMIAL G1:1 + X 3 + X 10STAGENUMBERSINPUT012345678910111111111112345INITIALCONDITIONS6789OUTPUT10TAPNUMBERSSHIFT DIRECTIONFigure 3-8.G1 Shift Register Generator Configuration32IS-GPS-200H24 Sep 2013POLYNOMIAL G2:1 + X 2 + X 3 +X 6 + X 8 + X 9 + X 10STAGENUMBERSINPUT012345678910111111111112345INITIALCONDITIONS6789OUTPUT10TAPNUMBERSSHIFT DIRECTIONFigure 3-9.G2 Shift Register Generator Configuration33IS-GPS-200H24 Sep 2013310X1 EPOCHISC10.23 MHzG1G1REGISTERSYNCH102ICSSYNCH203G EPOCH1 Kbps68910G21023DECODEGiG2iPHASE SELECTLOGIC50 bps TO DATA ENCODERREGISTER INPUTSCIS-CLOCKINPUTSET ALL ONESFigure 3-10: Example C/A-Code GenerationValid for C/A PRNs 1-37.
For PRNs 38-63, the G1 Register should be XOR-ed directly to the G2 Register in orderto make Gi. These PRNs do not use the Phase Select Logic box for G2i generation.34IS-GPS-200H24 Sep 2013X1 Epoch @ 2/3 bps102310231023102310231023 BIT Gold Code @ 1023 Kbps01etc.1 msec218190Gold Code Epochs @ 1000/secData @ 50 cps20 msecFigure 3-11.C/A-Code Timing Relationships35IS-GPS-200H24 Sep 20133.3.2.4 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 samecode generator polynomial each clocked at 511.5 Kbps. Each pattern is initiated and reset with aspecified initial state (defined in Table 3-II). CM,i(t) pattern is reset after 10230 chips resulting ina code period of 20 milliseconds, and CL,i(t) pattern is reset after 767250 chips resulting in a codeperiod of 1.5 seconds. The L2 CM and L2 CL shift registers are initialized at the P-coder X1epoch.
The first L2 CM-code chip starts synchronously with the end/start of week epoch.Timing relationships related to the L2 CM-/L2 CL-codes 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 modulartype shift register generator.36IS-GPS-200H24 Sep 201301End/start ofweek1.5 secondX1 Epoch @ 2/3bps767250767250 BIT L2 CL- Code @ 511.5 ChipsKbps47312374102301023010230 10230 10230 1023010230 BIT L2 CM-Code@ 511.5Kbpsetc.7510230etc.20msecData @ 50cpsL2 CM @ 511.5KbpsL2 CL @ 511.5KbpsL2 C @ 1023KbpsFigure 3-12.L2 CM-/L2 CL-Code Timing Relationships37IS-GPS-200H24 Sep 2013Figure 3-13.L2 CM/L2 CL Shift Register Generator Configuration38IS-GPS-200H24 Sep 20133233245691113161921242723SHIFT DIRECTION11131 + X + X +X + X + X + X + X + X + X + X + X + X3POLYNOMIAL:INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG)3DELAYNUMBERSOUTPUT3.3.3 Navigation Data.The content and format of the LNAV data, D(t) are given in Appendices II/IV of this document.The content and format of the CNAV data, Dc(t) are given in Appendix III 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 ½encoded and, thus, clocked at 50 sps.
The resultant symbol sequence is then modulo-2 added tothe L2 CM-code.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 1/2convolutional code. For Block IIR-M, the NAV bit train, D(t), can be selected to beconvolutionally encoded. The resulting symbol rate is 50 sps. The convolutional coding will beconstraint 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 ofthe SV's P(Y)-code X1 epochs.
However, the navigation message is FEC encoded in acontinuous process independent of message boundaries (i.e. at the beginning of each newmessage, the encoder registers illustrated in Figure 3-14 contains the last six bits of the previousmessage).Because the FEC encoding convolves successive messages, it is necessary to define whichtransmitted symbol is synchronized to SV time, as follows. The beginning of the first symbolthat contains any information about the first bit of a message will be synchronized to everyeighth X1 epoch (referenced to end/start of week). The users’ convolutional decoders willintroduce 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 blocktransmission and SV time are illustrated in Figure 3-15.39IS-GPS-200H24 Sep 2013G2 (133 OCTAL)OUTPUT SYMBOLS(50 SPS)DATA INPUT(25 BPS)(ALTERNATING G1/G2)G1 (171 OCTAL)Figure 3-14.SYMBOLCLOCKConvolutional EncoderENCODED DATA BLOCKENCODEDDATA BLOCKDATA BLOCKDECODED BYUSER’S DECODING DELAYDOWNLINK DELAYLATEREARLYSV 12 SECOND EPOCHSFigure 3-15.Convolutional transmit/Decoding Timing Relationships3.3.4 GPS Time and SV Z-Count.GPS time is established by the Control Segment and is referenced to Coordinated UniversalTime (UTC) as maintained by the U.S.
Naval Observatory (UTC (USNO)) zero time-pointdefined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largestunit used in stating GPS time is one week defined as 604,800 seconds. GPS time may differfrom UTC because GPS time shall be a continuous time scale, while UTC is correctedperiodically with an integer number of leap seconds.
There also is an inherent but bounded drift40IS-GPS-200H24 Sep 2013rate between the UTC and GPS time scales. The OCS shall control the GPS time scale to bewithin one microsecond of UTC (modulo one second).The NAV data contains the requisite data for relating GPS time to UTC.
The accuracy of thisdata during the transmission interval shall be such that it relates GPS time (maintained by theMCS of the CS) to UTC (USNO) within 90 nanoseconds (one sigma). This data is generated bythe CS; therefore, the accuracy of this relationship may degrade if for some reason the CS isunable to upload data to a SV. At this point, it is assumed that alternate sources of UTC are nolonger available, and the relative accuracy of the GPS/UTC relationship will be sufficient forusers. Range error components (e.g. SV clock and position) contribute to the GPS time transfererror, 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 97nanosecond (one sigma) apparent uncertainty at the SV. Propagation delay errors and receiverequipment 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 andcommunicating time.
Time stated in this manner is referred to as Z-count, which is given as abinary number consisting of two parts as follows:a.The binary number represented by the 19 least significant bits of the Z-count isreferred to as the time of week (TOW) count and is defined as being equal to the number of X1epochs that have occurred since the transition from the previous week. The count is short-cycledsuch that the range of the TOW-count is from 0 to 403,199 X1 epochs (equaling one week) andis reset to zero at the end of each week. The TOW-count's zero state is defined as that X1 epochwhich 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 UTCscale which is nominally referenced to the Greenwich Meridian.
Over the years the occurrenceof the "zero state epoch" may differ by a few seconds from 0000 hours on the UTC scale sinceUTC is periodically corrected with leap seconds while the TOW-count is continuous withoutsuch correction. To aid rapid ground lock-on to the P-code signal, a truncated version of theTOW-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 itstruncated HOW version is illustrated by Figure 3-16.b.The most significant bits of the Z-count are a binary representation of thesequential number assigned to the current GPS week (see paragraph 6.2.4).41IS-GPS-200H24 Sep 2013P(Y)-CODE EPOCH(END/START OF WEEK)X1 EPOCHS1.5 sec0403,192403,19612345678403,199DECIMAL EQUIVALENTSOF ACTUAL TOW COUNTSSUBFRAME EPOCHS6 sec100,7990123DECIMAL EQUIVALENTS OF HOW-MESSAGE TOW COUNTSNOTES:1.TO AID IN RAPID GROUND LOCK-ON THE HAND-OVER WORD (HOW ) OF EACHSUBFRAME CONTAINS A TRUNCATED TIME-OF-WEEK (TOW) COUNT2.THE HOW IS THE SECOND WORD IN EACH SUBFRAME (REFERENCEPARAGRAPH 20.3.3.2).3.THE HOW-MESSAGE TOW COUNT CONSISTS OF THE 17 MSBs OF THEACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME.4.TO CONVERT FROM THE HOW-MESSAGE TOW COUNT TO THE ACTUAL TOWCOUNT AT THE START OF THE NEXT SUBFRAME, MULTIPLY BY FOUR.5.THE FIRST SUBFRAME STARTS SYNCHRONOUSLY WITH THE END/START OFWEEK EPOCH.Figure 3-16.Time Line Relationship of HOW Message42IS-GPS-200H24 Sep 20134 NOT APPLICABLE43IS-GPS-200H24 Sep 20135 NOT APPLICABLE44IS-GPS-200H24 Sep 20136 NOTES6.1AcronymsAI-Availability IndicatorAODO-Age of Data OffsetA-S-Anti-SpoofingAutonav-Autonomous NavigationBPSK-Bi-Phase Shift KeyCDC-Clock Differential CorrectionCNAV-Civil Navigationcps-cycles per secondCRC-Cyclic Redundancy CheckCS-Control SegmentDC-Differential CorrectiondBc-Power ratio of a signal to a (unmodulated) carrier signal, expressed in decibelsdBi-Decibel with respect to isotropic antennadBW-Decibel with respect to 1 WDN-Day NumberEAROM-Electrically Alterable Read-Only MemoryECEF-Earth-Centered, Earth-FixedECI-Earth-Centered, InertialEDC-Ephemeris Differential CorrectionEOE-Edge-of-EarthEOL-End of LifeERD-Estimated Range DeviationFEC-Forward Error CorrectionGGTO-GPS/GNSS Time Offset45IS-GPS-200H24 Sep 2013GNSS-Global Navigation Satellite SystemGPS-Global Positioning SystemGPSW-Global Positioning System WingHOW-Hand-Over WordICC-Interface Control ContractorID-IdentificationIERS-International Earth Rotation and Reference Systems ServiceIODC-Issue of Data, ClockIODE-Issue of Data, EphemerisIRM-IERS Reference MeridianIRP-IERS Reference PoleIS-Interface SpecificationISC-Inter-Signal CorrectionLSB-Least Significant BitLSF-Leap Seconds FutureL2 C-L2 Civil SignalL2 CL-L2 Civil-Long CodeL2 CM-L2 Civil-Moderate CodeMCS-Master Control StationMSB-Most Significant BitNAV-NavigationNDUS-Nudet Detection User SegmentNMCT-Navigation Message Correction TableNSC-Non-Standard C/A-CodeNSCL-Non-Standard L2 CL-CodeNSCM-Non-Standard L2 CM-CodeNSY-Non-Standard Y-code46IS-GPS-200H24 Sep 2013OBCP-On-Board Computer ProgramOCS-Operational Control SystemPPS-Precise Positioning ServicePRN-Pseudo-Random NoiseRF-Radio FrequencyRMS-Root Mean SquareSA-Selective AvailabilitySEP-Spherical Error ProbableSPS-Standard Positioning Servicesps-symbols per secondSS-Space SegmentSSV-Space Service VolumeSV-Space VehicleSVN-Space Vehicle NumberTBD-To Be DeterminedTBS-To Be SuppliedTLM-TelemetryTOW-Time Of WeekUE-User EquipmentURA-User Range AccuracyURE-User Range ErrorUS-User SegmentUSNO-U.S.
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