Интерфейсный документ GPS, страница 21

In general, any message type 30’s (i.e. 30-39) willprovide SV clock correction parameters as described in this section.30.3.3.2.1.1 SV Clock Correction. Any one of message types 30 through 37, Figure 30-3 through Figure 30-10,contains the parameters needed by the users for apparent SV clock correction. Bits 61 to 71 contain toc , clock datareference time of week. Bits 72 to 127 contain SV clock correction coefficients.

The related algorithm is given inparagraph 20.3.3.3.3.1.30.3.3.2.1.2 Data Predict Time of Week. Bits 39 through 49 of message types 30 through 37 shall contain the datapredict time of week (top). The top term provides the epoch time of week of the state estimate utilized for theprediction of SV clock correction coefficients.30.3.3.2.2 Clock Parameter Characteristics. The number of bits, the scale factor of the LSB (which is the last bitreceived), the range, and the units of clock correction parameters shall be as specified in Table 30-III.30.3.3.2.3 User Algorithms for SV Clock Correction Data.

The algorithms defined in paragraph 20.3.3.3.3.1 allowall users to correct the code phase time received from the SV with respect to both SV code phase offset andrelativistic effects. However, since the SV clock corrections of equations in paragraph 20.3.3.3.3.1 are estimated bythe CS using dual frequency L1 and L2 P(Y) code measurements, the single-frequency L1 or L2 user and the dualfrequency L1 C/A – L2 C users must apply additional terms to the SV clock correction equations. These terms aredescribed in paragraph 30.3.3.3.1.IS-GPS-200D7 Dec 2004163Table 30-III.Clock Correction and Accuracy ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***Units300604,500secondstocClock Data Reference Time of Week11URAoc IndexSV Clock Accuracy Index5*(see text)URAoc1 IndexSV Clock Accuracy Change Index3(see text)URAoc2 IndexSV Clock Accuracy Change Rate Index3(see text)af2-nSV Clock Drift Rate Correction Coefficient10*2-60sec/sec2af1-nSV Clock Drift Correction Coefficient20*2-48sec/secaf0-nSV Clock Bias Correction Coefficient26*2-35seconds******Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB;See Figure 30-3 through 30-10 for complete bit allocation in Message types 30 to 37;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.IS-GPS-200D7 Dec 200416430.3.3.2.4 SV Clock Accuracy Estimates.

Bits 50 through 54, and 55 through 57, and 58 through 60 of messagetypes 30 through 37 shall contain the URAoc Index,URAoc1 Index, and URAoc2 Index, respectively, of the SV(reference paragraph 6.2.1) for the unauthorized user. The URAoc Index together with URAoc1 Index and URAoc2Index shall give the clock-related user range accuracy of the SV as a function of time since the prediction (top) usedto generate the uploaded clock correction polynomial terms.The user shall calculate the clock-related URA with the equation (in meters);URAoc = URAocb + URAoc1 (t – top)for t-top < 93,600 secondsURAoc = URAocb + URAoc1 (t – top) + URAoc2 (t – top – 93,600)2for t-top > 93,600 secondswheret = GPS time (must account for beginning or end of week crossovers),top = time of week of the state estimate utilized for the prediction of satellite clock correction parameters.The CS shall derive URAocb at time top which, when used together with URAoc1 and URAoc2 in the above equations,results in the minimum URAoc that is greater than the predicted URAoc during the entire duration up to 14 days aftertop.IS-GPS-200D7 Dec 2004165The user shall use the broadcast URAoc Index to derive URAocb.

The index is a signed, two’s complement integer inthe range of +15 to –16 and has the following relationship to the clock-related user derived URAocb:URAoc Index1514131211109876543210-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16URAocb (meters)6144.003072.001536.00768.00384.00192.0096.0048.0024.0013.659.656.854.853.402.401.701.200.850.600.430.300.210.150.110.080.060.040.030.020.01<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<URAocbURAocb ≤6144.00URAocb ≤3072.00URAocb ≤1536.00URAocb ≤768.00URAocb ≤384.00URAocb ≤192.00URAocb ≤96.00URAocb ≤48.00URAocb ≤24.00URAocb ≤13.65URAocb ≤9.65URAocb ≤6.85URAocb ≤4.85URAocb ≤3.40URAocb ≤2.40URAocb ≤1.70URAocb ≤1.20URAocb ≤0.85URAocb ≤0.60URAocb ≤0.43URAocb ≤0.30URAocb ≤0.21URAocb ≤0.15URAocb ≤0.11URAocb ≤0.08URAocb ≤0.06URAocb ≤0.04URAocb ≤0.03URAocb ≤0.02URAocb ≤0.01No accuracy prediction available—use at own riskThe user may use the upper bound value in the URAocb range corresponding to the broadcast index, therebycalculating the maximum URAoc that is equal to or greater than the CS predicted URAoc, or the user may use thelower bound value in the range which will provide the minimum URAoc that is equal to or less than the CS predictedURAoc.IS-GPS-200D7 Dec 2004166The transmitted URAoc1 Index is an integer value in the range 0 to 7.

URAoc1 Index has the following relationship tothe URAoc1:URAoc1 =1(meters/second)2NwhereN = 4 + URAoc1 IndexThe transmitted URAoc2 Index is an integer value in the range 0 to 7. URAoc2 Index has the following relationship tothe URAoc2:URAoc2 =1(meters/second2)N2whereN = 25 + URAoc2 IndexIS-GPS-200D7 Dec 200416730.3.3.3 Message Type 30 Ionospheric and Group Delay Correction Parameters.30.3.3.3.1 Message Type 30 Ionospheric and Group Delay Correction Parameter Content. Message type 30provides SV clock correction parameters (ref. Section 30.3.3.2) and ionospheric and group delay correctionparameters. Bits 128 through 192 of message type 30 provide the group delay differential correction terms for L1,L2, and L5 signal users.

Bits 193 through 256 provide the ionospheric correction parameters for single frequencyuser. The following algorithms shall apply when interpreting the correction parameters in the message.30.3.3.3.1.1 Estimated L1-L2 Group Delay Differential. The group delay differential correction terms, TGD,ISCL1C/A, ISCL2C for the benefit of single frequency L1 P, L1 C/A, L2 P, L2 C users and dual frequency L1/L2 usersare contained in bits 128 through 166 of message type 30 (see Figure 30-3 for complete bit allocation). The bitlength, scale factors, ranges, and units of these parameters are given in Table 30-IV.The bit string of“1000000000000” shall indicate that the group delay value is not available.

The related algorithm is given inparagraphs 30.3.3.3.1.1.1 and 30.3.3.3.1.1.2.Table 30-IV. Group Delay Differential Parameters **************ParameterNo. ofBits**Scale Factor(LSB)EffectiveRange***TGD13*2-35secondsISCL1C/A13*2-35secondsISCL2C13*2-35secondsUnitsParameters so indicated are two’s complement with the sign bit (+ or -) occupying the MSB;See Figure 30-3 for complete bit allocation in Message type 30;Effective range is the maximum range attainable with indicated bit allocation and scale factor;The bit string of “1000000000000” will indicate that the group delay value is not available.IS-GPS-200D7 Dec 200416830.3.3.3.1.1.1 Inter-Signal Group Delay Differential Correction.

The correction terms, TGD, ISCL1C/A and ISCL2C,are initially provided by the CS to account for the effect of SV group delay differential between L1 P(Y) and L2P(Y), L1 P(Y) and L1 C/A, and between L1 P(Y) and L2 C, respectively, based on measurements made by the SVcontractor during SV manufacture. The values of TGD and ISCs for each SV may be subsequently updated to reflectthe actual on-orbit group delay differential. For maximum accuracy, the single frequency L1 C/A user must use thecorrection terms to make further modifications to the code phase offset in paragraph 20.3.3.3.3.1 with the equation:(∆tSV)L1C/A = ∆tSV - TGD + ISCL1C/Awhere TGD (see paragraph 20.3.3.3.3.2) and ISCL1C/A are provided to the user as Message Type 30 data, described inparagraph 30.3.3.3.1.1.

For the single frequency L2 C user, the code phase offset modification is given by:(∆tSV)L2C = ∆tSV - TGD + ISCL2Cwhere, ISCL2C is provided to the user as Message Type 30 data.The values of ISCL1C/A and ISCL2C are measured values that represent the mean SV group delay differential betweenthe L1 P(Y)-code and the L1 C/A- or L2 C-codes respectively as follows,ISCL1C/A = tL1P(Y) - tL1C/AISCL2C = tL1P(Y) - tL2C.where, tLix is the GPS time the ith frequency x signal (a specific epoch of the signal) is transmitted from the SVantenna phase center.IS-GPS-200D7 Dec 200416930.3.3.3.1.1.2 L1 /L2 Ionospheric Correction. The two frequency (L1 C/A and L2 C) user shall correct for thegroup delay and ionospheric effects by applying the relationship:PR =(PR L 2C − γ12 PR L1C / A ) + c (ISC L 2C − γ12 ISC L1C / A )− c TGD1 − γ12where,PR = pseudorange corrected for ionospheric effects,PRi = pseudorange measured on the channel indicated by the subscript,ISCi = inter-signal correction for the channel indicated by the subscript (see paragraph 30.3.3.3.1.1),TGD = see paragraph 20.3.3.3.3.2,c = speed of light,and where, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively,γ12 = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2.30.3.3.3.1.2 Ionospheric Data.

The ionospheric parameters which allow the “L1 only” or “L2 only” user to utilizethe ionospheric model for computation of the ionospheric delay are contained in Message Type 30. The “onefrequency” user should use the model given in paragraph 20.3.3.5.2.5 to make this correction for the ionosphericeffects. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X.The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload theSVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate.IS-GPS-200D7 Dec 200417030.3.3.4 Message Types 31, 12, and 37 Almanac Parameters. The almanac parameters are provided in any one ofmessage types 31, 37, and 12.