IS-GPS-200F (811524), страница 16
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For those parameters whose characteristics are not fully defined inSection 20.3.3.3.1, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range,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 induced effects),and (c) allow the "two frequency" (L1 and L2) user to correct for the group propagation delay due to ionosphericeffects (the single frequency user may correct for ionospheric effects as described in paragraph 20.3.3.5.2.5).95IS-GPS-200F21 Sep 2011Table 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*Units(see text)(see text)604,784secondsParameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB;*****EffectiveRange***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.20.3.3.3.3.1 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)96IS-GPS-200F21 Sep 2011t=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 is−2 µF =c2sec= - 4.442807633 (10)-10meter ,wheremeters 3second 2=value of Earth's universal gravitational parametersmetersc = 2.99792458 x 10 second=speed of light.µ = 3.986005 x 10148Note that equations (1) and (2), as written, are coupled.
While the coefficients af0, af1 and af2 are generated by usingGPS time as indicated in equation (2), sensitivity of tsv to t is negligible. This negligible sensitivity will allow the97IS-GPS-200F21 Sep 2011user to approximate t by tSV in equation (2). The value of t must account for beginning or end of week crossovers.That is, if the quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If the quantity t - tocis 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.20.3.3.3.3.2 L1 - L2 Correction.
The L1 and L2 correction term, TGD, is initially calculated by the CS to account forthe 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 - γTGD98IS-GPS-200F21 Sep 2011where, 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,1TGD =1-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.20.3.3.3.3.3 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.99IS-GPS-200F21 Sep 201120.3.3.3.3.4 Example Application of Correction Parameters.A typical system application of the correctionparameters for a user receiver is shown in Figure 20-3. The ionospheric model referred to in Figure 20-3 isdiscussed in paragraph 20.3.3.5.2.5 in conjunction with the related data contained in page 18 of subframe 4.
TheERDcterm referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related datacontained in page 13 of subframe 4.Figure 20-3.Sample Application of Correction Parameters100IS-GPS-200F21 Sep 201120.3.3.4 Subframes 2 and 3. The contents of words three through ten of subframes 2 and 3 are defined below,followed by material pertinent to the use of the data.20.3.3.4.1 Content of Subframes 2 and 3.
The third through tenth words of subframes 2 and 3 shall each contain sixparity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of wordten of each subframe for parity computation purposes. Bits 288 through 292 of subframe 2 shall contain the Age ofData Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4 (referenceparagraph 20.3.3.5.1.9). The remaining 375 bits of those two subframes shall contain the ephemeris representationparameters of the transmitting SV.The ephemeris parameters describe the orbit during the curve fit intervals described in section 20.3.4.
Table 20-IIgives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it shall benoted, however, that the transmitted parameter values are such that they provide the best trajectory fit in EarthCentered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediatecoordinate values as pertaining to any conventional coordinate system.The issue of ephemeris data (IODE) term shall provide the user with a convenient means for detecting any change inthe ephemeris representation parameters.
The IODE is provided in both subframes 2 and 3 for the purpose ofcomparison with the 8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, a data setcutover has occurred and new data must be collected. The timing of the IODE and constraints on the IODC andIODE are defined in paragraph 20.3.4.4.Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change in both IODE words.The CS (Block II/Block IIA/IIR/IIR-M/IIF) and SS (GPS III) shall assure that the toe value, for at least the first dataset transmitted by an SV after an upload, is different from that transmitted prior to the cutover (reference paragraph20.3.4.5).A "fit interval" flag is provided in subframe 2 to indicate whether the ephemerides are based on a four-hour fitinterval or a fit interval greater than four hours (reference paragraph 20.3.3.4.3.1).The AODO word is provided in subframe 2 to enable the user to determine the validity time for the NMCT dataprovided in subframe 4 of the transmitting SV.
The related algorithm is given in paragraph 20.3.3.4.4.101IS-GPS-200F21 Sep 2011Table 20-II.Ephemeris Data DefinitionsM0Mean Anomaly at Reference Time∆nMean Motion Difference From Computed ValueeEccentricityASquare Root of the Semi-Major AxisΩ0Longitude of Ascending Node of Orbit Plane at Weekly Epochi0Inclination Angle at Reference TimeωArgument of Perigee•ΩRate of Right AscensionIDOTRate of Inclination AngleCucAmplitude of the Cosine Harmonic Correction Term to the Argument of LatitudeCusAmplitude of the Sine Harmonic Correction Term to the Argument of LatitudeCrcAmplitude of the Cosine Harmonic Correction Term to the Orbit RadiusCrsAmplitude of the Sine Harmonic Correction Term to the Orbit RadiusCicAmplitude of the Cosine Harmonic Correction Term to the Angle of InclinationCisAmplitude of the Sine Harmonic Correction Term to the Angle of InclinationtoeReference Time Ephemeris (reference paragraph 20.3.4.5)IODEIssue of Data (Ephemeris)20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics.
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