Интерфейсный документ GPS, страница 12

The ten MSBs of word three shall contain the ten MSBs of the 29-bit Zcount as qualified herein. These ten bits shall be a modulo 1024 binary representation of the current GPS weeknumber at the start of the data set transmission interval (see paragraph 3.3.4(b)). The GPS week number incrementsat each end/start of week epoch. For Block II SVs in long-term extended operations, beginning approximately 28days after upload, the transmission week number may not correspond to the actual GPS week number due to curvefit 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) commandedON for the L2 channel, as follows:00 =Reserved,01 =P code ON,10 =C/A code ON.IS-GPS-200D7 Dec 20048320.3.3.3.1.3 SV Accuracy. Bits 13 through 16 of word three shall give the URA index of the SV (referenceparagraph 6.2.1) for the unauthorized 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 INDEX0123456789101112131415URA (meters)0.002.403.404.856.859.6513.6524.0048.0096.00192.00384.00768.001536.003072.006144.00< URA ≤2.40< URA ≤3.40< URA ≤4.85< URA ≤6.85< URA ≤9.65< URA ≤13.65< URA ≤24.00< URA ≤48.00< URA ≤96.00< URA ≤192.00< URA ≤384.00< URA ≤768.00< URA ≤ 1536.00< URA ≤ 3072.00< URA ≤ 6144.00< URA (or no accuracy prediction is available - unauthorized users areadvised to use the SV at their own risk.)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 unauthorizeduser 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 indexIS-GPS-200D7 Dec 20048420.3.3.3.1.4 SV Health.

The six-bit health indication given by bits 17 through 22 of word three refers to thetransmitting SV. The MSB shall indicate a summary of the health of the NAV data, where0 = 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 paragraph20.3.3.5.1.3.The health indication shall be given relative to the "as designed" capabilities of each SV (asdesignated by the configuration code - see paragraph 20.3.3.5.1.4). Accordingly, any SV which does not have acertain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or if it hasbeen 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 thatshown in subframes 4 and/or 5 of other SVs since the latter may be updated at a different time.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 theten-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 ofdetecting any change in the correction parameters.

Constraints on the IODC as well as the relationship between theIODC 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 than4 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 streamwas commanded OFF on the P-code of the L2 channel.IS-GPS-200D7 Dec 20048520.3.3.3.1.7 Estimated Group Delay Differential. Bits 17 through 24 of word seven contain the L1-L2 correctionterm, TGD, for the benefit of "L1 only" or "L2 only" users; the related user algorithm is given in paragraph20.3.3.3.3.20.3.3.3.1.8 SV Clock Correction.

Bits nine through 24 of word eight, bits one through 24 of word nine, and bitsone 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 definedin Section 20.3.3.3.1, the number of bits, the scale factor of the LSB (which shall be the last bit received), therange, 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 thecode phase time received from the SV with respect to both SV code phase offset and relativistic effects, (b) permitthe "single frequency" (L1 or L2) user to compensate for the effects of SV group delay differential (the user whoutilizes both frequencies does not require this correction, since the clock parameters account for the inducedeffects), and (c) allow the "two frequency" (L1 and L2) user to correct for the group propagation delay due toionospheric effects (the single frequency user may correct for ionospheric effects as described in paragraph20.3.3.5.2.5).IS-GPS-200D7 Dec 200486Table 20-I.Subframe 1 ParametersNo.

ofBits**ScaleFactor(LSB)Code on L221discretesWeek No.101weekL2 P data flag11discreteSV accuracy4SV health61discretesTGD8*2-31secondsIODC10toc1624af28*2-55sec/sec2af116*2-43sec/secaf022*2-31secondsParameter******EffectiveRange***Units(see text)(see text)604,784secondsParameters 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 withindicated bit allocation and scale factor.IS-GPS-200D7 Dec 20048720.3.3.3.3.1 User Algorithm for SV Clock Correction. The polynomial defined in the following allows the user todetermine the effective SV PRN code phase offset referenced to the phase center of the antennas (∆tsv) with respectto GPS system time (t) at the time of data transmission.

The coefficients transmitted in subframe 1 describe theoffset apparent to the two-frequency user for the interval of time in which the parameters are transmitted. Thisestimated correction accounts for the deterministic SV clock error characteristics of bias, drift and aging, as well asfor the SV implementation characteristics of group delay bias and mean differential group delay. Since thesecoefficients do not include corrections for relativistic effects, the user's equipment must determine the requisiterelativistic correction.

Accordingly, the offset given below includes a term to perform this function.The user shall correct the time received from the SV with the equation (in seconds)t=tsv - ∆tsvt=GPS system time (seconds),tsv=effective SV PRN code phase time at message transmission time (seconds),∆tsv=SV PRN code phase time offset (seconds).(1)whereThe SV PRN code phase offset is given by∆tsv= af0 + af1(t - toc) + af2(t - toc)2 + ∆tr(2)whereaf0, af1 and af2 are the polynomial coefficients given in subframe 1, toc is the clock data reference time inseconds (reference paragraph 20.3.4.5), and ∆tr is the relativistic correction term (seconds) which is givenby∆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 isF =−2 µc2= - 4.442807633 (10)-10secmeter,IS-GPS-200D7 Dec 200488whereµ = 3.986005 x 1014meters 3second 2c = 2.99792458 x 108meterssecond=value of Earth's universal gravitational parameters=speed of light.Note that equations (1) and (2), as written, are coupled.