IS-GPS-200H (797934), страница 15
Текст из файла (страница 15)
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 defined in Section 20.3.3.3.1, thenumber of bits, the scale factor of the LSB (which shall be the last bit received), the range, andthe 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 the code phase time received from theSV with respect to both SV code phase offset and relativistic effects, (b) permit the "singlefrequency" (L1 or L2) user to compensate for the effects of SV group delay differential (the user93IS-GPS-200H24 Sep 2013who utilizes both frequencies does not require this correction, since the clock parameters accountfor the induced effects), and (c) allow the "two frequency" (L1 and L2) user to correct for thegroup propagation delay due to ionospheric effects (the single frequency user may correct forionospheric effects as described in paragraph 20.3.3.5.2.5).94IS-GPS-200H24 Sep 2013Table 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 to determine the effective SV PRN codephase offset referenced to the phase center of the antennas (∆tsv) with respect to 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 aretransmitted. This estimated correction accounts for the deterministic SV clock errorcharacteristics of bias, drift and aging, as well as for the SV implementation characteristics ofgroup delay bias and mean differential group delay. Since these coefficients do not includecorrections for relativistic effects, the user's equipment must determine the requisite relativisticcorrection. 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)95IS-GPS-200H24 Sep 2013t=tsv - ∆tsv(1)wheret=GPS system time (seconds),tsv=effective SV PRN code phase time at message transmissiontime (seconds),∆tsv=SV PRN code phase time offset (seconds).The 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 datareference time in seconds (reference paragraph 20.3.4.5), and ∆tr is the relativisticcorrection term (seconds) which is given by∆tr = F e A sin Ek.The orbit parameters (e, A , Ek) used here are described in discussions of data contained insubframes 2 and 3, while F is a constant whose value is−2 µc 2 = - 4.442807633 (10)-10F =secmeter ,whereµ = 3.986005 x 1014meters 3second 2 =metersc = 2.99792458 x 10 second =8value of Earth's universal gravitational parametersspeed of light.96IS-GPS-200H24 Sep 2013Note that equations (1) and (2), as written, are coupled.
While the coefficients af0, af1 and af2 aregenerated by using GPS time as indicated in equation (2), sensitivity of tsv to t is negligible. Thisnegligible sensitivity will allow the user to approximate t by tSV in equation (2). The value of tmust account for beginning or end of week crossovers. That is, if the quantity t - toc is greaterthan 302,400 seconds, subtract 604,800 seconds from t. If the quantity 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 therelativistic effect when estimating the NAV parameters:→−∆tr =→2 R •Vc2where→Ris the instantaneous position vector of the SV,→Vis the instantaneous velocity vector of the SV, andc is the speed of light. (Reference paragraph 20.3.4.3).→→RVIt is immaterial whether the vectors andin earth-centered, inertial coordinates.are expressed in earth-fixed, rotating coordinates or20.3.3.3.3.2 L1 - L2 Correction.The L1 and L2 correction term, TGD, is initially calculated by the CS to account for the effect ofSV group delay differential between L1 P(Y) and L2 P(Y) based on measurements made by theSV contractor during SV manufacture.
The value of TGD for each SV may be subsequentlyupdated to reflect the actual on-orbit group delay differential. This correction term is only forthe benefit of "single-frequency" (L1 P(Y) or L2 P(Y)) users; it is necessitated by the fact thatthe SV clock offset estimates reflected in the af0 clock correction coefficient (see paragraph20.3.3.3.3.1) are based on the effective PRN code phase as apparent with two frequency (L1P(Y) and L2 P(Y)) ionospheric corrections.
Thus, the user who utilizes the L1 P(Y) signal onlyshall modify the code phase offset in accordance with paragraph 20.3.3.3.3.1 with the equation(∆tSV)L1P(Y) = ∆tSV - TGD97IS-GPS-200H24 Sep 2013where TGD is provided to the user as subframe 1 data.
For the user who utilizes L2 P(Y) only,the code phase modification 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 valuethat represents the mean group delay differential multiplied by 1/(1- γ). That is,11-γTGD =(tL1P(Y) - tL2P(Y))where tLiP(Y) is the GPS time the ith frequency P(Y) signal (a specific epoch of the signal) istransmitted from the SV antenna 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 group delay due toionospheric 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,98IS-GPS-200H24 Sep 2013and γ is as defined in paragraph 20.3.3.3.3.2.
The clock correction coefficients are based on "twofrequency" measurements and therefore account for the effects of mean differential delay in SVinstrumentation.20.3.3.3.3.4 Example Application of Correction Parameters.A typical system application of the correction parameters for a user receiver is shown in Figure20-3. The ionospheric model referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.5 inERDconjunction with the related data contained in page 18 of subframe 4. The c term referred toin Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related datacontained in page 13 of subframe 4.99IS-GPS-200H24 Sep 2013Figure 20-3.Sample Application of Correction Parameters20.3.3.4 Subframes 2 and 3.The contents of words three through ten of subframes 2 and 3 are defined below, followed bymaterial 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 six parity bits as theirLSBs; 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 shallcontain the Age 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 of100IS-GPS-200H24 Sep 2013those two subframes shall contain the ephemeris representation parameters of the transmittingSV.The ephemeris parameters describe the orbit during the curve fit intervals described in section20.3.4. Table 20-II gives the definition of the orbital parameters using terminology typical ofKeplerian orbital parameters; it shall be noted, however, that the transmitted parameter valuesare such that they provide the best trajectory fit in Earth-Centered, Earth-Fixed (ECEF)coordinates for each specific fit interval.
The user shall not interpret intermediate coordinatevalues as pertaining to any conventional coordinate system.The issue of ephemeris data (IODE) term shall provide the user with a convenient means fordetecting any change in the ephemeris representation parameters. The IODE is provided in bothsubframes 2 and 3 for the purpose of comparison with the 8 LSBs of the IODC term in subframe1. Whenever these three terms do not match, a data set cutover has occurred and new data mustbe collected.
The timing of the IODE and constraints on the IODC and IODE are defined inparagraph 20.3.4.4.Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change inboth IODE words. The CS (Block II/Block IIA/IIR/IIR-M/IIF) and SS (GPS III) shall assurethat 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 afour-hour fit interval 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 forthe NMCT data provided in subframe 4 of the transmitting SV.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
