Интерфейсный документ GPS, страница 12

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.

While the coefficients af0, af1 and af2 are generated byusing GPS time as indicated in equation (2), sensitivity of tsv to t is negligible. This negligible sensitivity will allowthe user to approximate t by tSV in equation (2). The value of t must account for beginning or end of weekcrossovers. That is, if the quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If thequantity t - toc is less than -302,400 seconds, add 604,800 seconds to t.The control segment will utilize the following alternative but equivalent expression for the relativistic effect whenestimating the NAV parameters:→∆tr = −→2 R •Vc2where→R is the instantaneous position vector of the SV,→V is the instantaneous velocity vector of the SV, andc 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.IS-GPS-200D7 Dec 20048920.3.3.3.3.2 L1 - L2 Correction.

The L1 and L2 correction term, TGD, is initially calculated by the CS to accountfor the effect of SV group delay differential between L1 P(Y) and L2 P(Y) based on measurements made by the SVcontractor during SV manufacture. The value of TGD for each SV may be subsequently updated to reflect the actualon-orbit group delay differential. This correction term is only for the benefit of "single-frequency" (L1 P(Y) or L2P(Y)) users; it is necessitated by the fact that the SV clock offset estimates reflected in the af0 clock correctioncoefficient (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 thecode phase offset in accordance with paragraph 20.3.3.3.3.1 with the equation(∆tSV)L1P(Y) = ∆tSV - TGDwhere TGD is provided to the user as subframe 1 data.

For the user who utilizes L2 P(Y) only, the code phasemodification is given by(∆tSV)L2P(Y) = ∆tSV - γTGDwhere, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively,γ = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2.The value of TGD is not equal to the mean SV group delay differential, but is a measured value that represents themean group delay differential multiplied by 1/(1- γ). That is,TGD =11-γ(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 SVantenna phase center.IS-GPS-200D7 Dec 20049020.3.3.3.3.3 Ionospheric Correction.

The two frequency (L1 P(Y) and L2 P(Y)) user shall correct for the groupdelay due to ionospheric effects by applying the relationship:PR =PR L2P(Y) - γ PR L1P(Y)1- γwherePR=pseudorange corrected for ionospheric effects,PRi=pseudorange measured on the channel indicated by the subscript.and γ 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.20.3.3.3.3.4Example Application of Correction Parameters.A typical system application of the correctionparameters for a user receiver is shown in Figure 20-3.