Интерфейсный документ GPS, страница 13

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.

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.IS-GPS-200D7 Dec 200491af0, af1, af2, tocTGD*CLOCKCORRECTIONPOLYNOMIAL∆ trESTIMATE OF SVTRANSMISSION TIME∆ tSVTtropoTionoCODE PHASE OFFSET- TRUE SV CLOCK EFFECTS- EQUIPMENT GROUP DELAYDIFFERENTIAL EFFECTS- RELATIVISTIC EFFECTSTROPOSPHERICMODELIONOSPHERICMODEL*αn, βnFILTER ANDCOORDINATECONVERTERPSEUDORANGEDIVIDED BY THESPEED OF LIGHTGPS TIMEERD **cPATH DELAY- GEOMETRIC- TROPOSHERIC- IONOSPHERIC*USER CLOCK BIASGPS TIMEUSER POSITION,VELOCITY, andTIME (CLOCK BIAS)- RANGE DATA FROMOTHER SATELLITES- CALIBRATION DATA- AUXILIARY SENSOR* SINGLE FREQUENCY USER ONLY** OPTIONALFigure 20-3.

Sample Application of Correction ParametersIS-GPS-200D7 Dec 20049220.3.3.4 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 containsix parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 ofword ten of each subframe for parity computation purposes.

Bits 288 through 292 of subframe 2 shall contain theAge of Data Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4(reference paragraph 20.3.3.5.1.9). The remaining 375 bits of those two subframes shall contain the ephemerisrepresentation parameters 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 shall assure that the toe value, for at least the first data set transmitted by an SV after an upload, is differentfrom that transmitted prior to the cutover (reference paragraph 20.3.4.5).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.IS-GPS-200D7 Dec 200493Table 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)IS-GPS-200D7 Dec 20049420.3.3.4.2 Subframe 2 and 3 Parameter Characteristics.

For each ephemeris parameter contained in subframes 2and 3, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the unitsshall be as specified in Table 20-III.The AODO word (which is not an ephemeris parameter) is a five-bit unsigned term with an LSB scale factor of900, a range from 0 to 31, and units of seconds.20.3.3.4.3 User Algorithm for Ephemeris Determination. The user shall compute the ECEF coordinates of positionfor the phase center of the SVs’ antennas utilizing a variation of the equations shown in Table 20-IV.