IS-GPS-200F (811524), страница 9
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Each Gi(t) sequence is a 1023-bit Gold-code which is itself the modulo-2 sum of two1023-bit linear patterns, G1 and G2i. The G2i sequence is formed by effectively delaying the G2 sequence by aninteger number of chips. The G1 and G2 sequences are generated by 10-stage shift registers having the followingpolynomials as referred to in the shift register input (see Figures 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 shift registers are initializedat the P-coder X1 epoch. The G1 and G2 registers are clocked at 1.023 MHz derived from the 10.23 MHz P-coderclock. The initialization by the X1 epoch phases the 1.023 MHz clock to insure that the first chip of the C/A codebegins 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 oftwo stages of the G2 shift register by modulo-2 addition (see Figure 3-10).However, this two-tap coderimplementation generates only a limited set of valid C/A codes.
Table 3-I contains a tabulation of the G2 shiftregister taps selected and their corresponding P-code X2i and PRN signal numbers together with the first severalchips of each resultant PRN code. Timing relationships related to the C/A code are shown in Figure 3-11.32IS-GPS-200F21 Sep 2011POLYNOMIAL G1:1 + X 3 + X 10STAGENUMBERSINPUT012345678910111111111112345678INITIALCONDITIONS9OUTPUT10TAPNUMBERSSHIFT DIRECTIONFigure 3-8. G1 Shift Register Generator Configuration33IS-GPS-200F21 Sep 2011POLYNOMIAL G2:1 + X 2 + X 3 +X 6 + X 8 + X 9 + X 10STAGENUMBERSINPUT012345678910111111111112345678INITIALCONDITIONS9OUTPUT10TAPNUMBERSSHIFT DIRECTIONFigure 3-9.
G2 Shift Register Generator Configuration34IS-GPS-200F21 Sep 2011310X1 EPOCHISC10.23 MHzSYNCH102G EPOCH1 Kbps3ICSSYNCH20G1G1REGISTER68910G21023DECODEGiG2iPHASE 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.35IS-GPS-200F21 Sep 2011X1 Epoch @ 2/3 bps102310231023102310231023 BIT Gold Code @ 1023 Kbps01etc.1 msec218190Gold Code Epochs @ 1000/secData @ 50 cps20 msecFigure 3-11. C/A-Code Timing Relationships36IS-GPS-200F21 Sep 20113.3.2.4 L2 CM-/L2 CL-Code Generation. Each CM,i(t) pattern (L2 CM-code) and CL,i(t) pattern (L2 CL-code) aregenerated using the same code generator polynomial each clocked at 511.5 Kbps.
Each pattern is initiated and resetwith a specified initial state (defined in Table 3-II). CM,i(t) pattern is reset after 10230 chips resulting in a codeperiod 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 startssynchronously with the end/start of week epoch. Timing relationships related to the L2 CM-/L2 CL-codes areshown in Figure 3-12.The maximal polynomial used for L2 CM- and L2 CL-codes is 1112225171 (octal) of degree 27. The L2 CM andL2 CL code generator is conceptually described in Figure 3-13 using modular-type shift register generator.37IS-GPS-200F21 Sep 201101End/start ofweek1.5 secondX1 Epoch @ 2/3bps767250767250 BIT L2 CL-Code @ 511.5 ChipsKbps412310230102301023010230etc.7310230741023010230 BIT L2 CM-Code @ 511.5Kbps7510230etc.20msecData @ 50cpsL2 CM @ 511.5KbpsL2 CL @ 511.5KbpsL2 C @ 1023KbpsFigure 3-12. L2 CM-/L2 CL-Code Timing Relationships38IS-GPS-200F21 Sep 2011Figure 3-13.
L2 CM/L2 CL Shift Register Generator Configuration39IS-GPS-200F21 Sep 201132332234569113SHIFT DIRECTION11 + X + X +X + X + X + XPOLYNOMIAL:INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG)3NUMBERSDELAY+X13116+X19121+X +X24+X327+XOUTPUT3.3.3 Navigation Data. The content and format of the LNAV data, D(t) are given in Appendices II/IV of thisdocument.
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, theCNAV bit train, DC(t), is rate ½ encoded and, thus, clocked at 50 sps. The resultant symbol sequence is thenmodulo-2 added to the L2 CM-code. During the initial period of Block IIR-M SVs operation, prior to InitialOperational 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 beconvolutionally 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 theoutput 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)-codeX1 epochs. However, the navigation message is FEC encoded in a continuous process independent of messageboundaries (i.e. at the beginning of each new message, the encoder registers illustrated in Figure 3-14 contains thelast six bits of the previous message).Because the FEC encoding convolves successive messages, it is necessary to define which transmitted symbol issynchronized to SV time, as follows. The beginning of the first symbol that contains any information about the firstbit 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 constraintlengths, or 35 bits), for which they must compensate to determine system time from the received signal. Thisconvolutional decoding delay and the various relationships with the start of the data block transmission and SV timeare illustrated in Figure 3-15.40IS-GPS-200F21 Sep 2011G2 (133 OCTAL)OUTPUT SYMBOLS(50 SPS)DATA INPUT(25 BPS)(ALTERNATING G1/G2)G1 (171 OCTAL)SYMBOLCLOCKFigure 3-14. Convolutional EncoderENCODED DATA BLOCKENCODEDDATA BLOCKDATA BLOCKDECODED BYUSER’S DECODING DELAYDOWNLINK DELAYLATEREARLYSV 12 SECOND EPOCHSFigure 3-15. Convolutional transmit/Decoding Timing Relationships41IS-GPS-200F21 Sep 20113.3.4 GPS Time and SV Z-Count.
GPS time is established by the Control Segment and is referenced to CoordinatedUniversal Time (UTC) as maintained by the U.S. Naval Observatory (UTC (USNO)) zero time-point defined asmidnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time isone week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be a continuous timescale, while UTC is corrected periodically with an integer number of leap seconds. There also is an inherent butbounded drift rate between the UTC and GPS time scales.
The OCS shall control the GPS time scale to be withinone 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 thetransmission 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 relationshipmay degrade if for some reason the CS is unable to upload data to a SV. At this point, it is assumed that alternatesources of UTC are no longer 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 transfer error, and undernormal operating circumstances (two frequency time transfers from SV(s) whose navigation message indicates aURA 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 asfollows:a.The binary number represented by the 19 least significant bits of the Z-count is referred to as thetime of week (TOW) count and is defined as being equal to the number of X1 epochs that have occurred since thetransition from the previous week.
The count is short-cycled such that the range of the TOW-count is from 0 to403,199 X1 epochs (equaling one week) and is reset to zero at the end of each week. The TOW-count's zero state isdefined 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 UTCscale which is nominally referenced to the Greenwich Meridian. Over the years the occurrence of the "zero stateepoch" may differ by a few seconds from 0000 hours on the UTC scale since UTC is periodically corrected withleap seconds while the TOW-count is continuous without such correction.
To aid rapid ground lock-on to the Pcode signal, a truncated version of the TOW-count, consisting of its 17 most significant bits, is contained in thehand-over word (HOW) of the L1 and L2 NAV data (D(t)) stream; the relationship between the actual TOW-countand its truncated HOW version is illustrated by Figure 3-16.b.The most significant bits of the Z-count are a binary representation of the sequential numberassigned to the current GPS week (see paragraph 6.2.4).42IS-GPS-200F21 Sep 2011P(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.
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