IS-GPS-705D (797936), страница 12
Текст из файла (страница 12)
The bit string of “1000000000000” shall indicate that the group delay value is notavailable. The related algorithms are given in paragraphs 20.3.3.3.1.2.1, 20.3.3.3.1.2.2, and20.3.3.3.1.2.3.70IS-GPS-705D24 Sep 2013Table 20-IV. Group Delay Differential Parameters ****Parameter**********No. ofBits**Scale Factor(LSB)EffectiveRange***Units-35secondsTGD13*2ISCL1C/A13*2-35secondsISCL2C13*2-35secondsISCL5I513*2-35secondsISCL5Q513*2-35secondsParameters so indicated are two’s complement with the sign bit (+ or -) occupying the MSB;See Figure 20-3 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.20.3.3.3.1.2.1 L1/L5 Inter-Signal Correction.The L5 correction terms, TGD, ISCL5I5 and ISCL5Q5 are provided by the CS to account for theeffect of inter-signal biases between L1 P(Y) and L2 P(Y), L1 P(Y) and L5 I5, and between L1P(Y) and L5 Q5, respectively.
These values are initially based on measurements made by the SVcontractor during SV manufacture. The values of TGD and ISC’s for each SV may besubsequently updated to reflect the actual on-orbit group delay differential. For maximumaccuracy, the single frequency L5 I5 user must use the correction terms to make furthermodifications to the code phase offset in paragraph 20.3.3.3.3.1 of IS-GPS-200 with theequation:(∆tSV)L5I5 = ∆tSV − TGD + ISCL5I5where TGD (see paragraph 20.3.3.3.3.2 of IS-GPS-200) and ISCL5I5 (described in paragraph20.3.3.3.1.2) are provided to the user as message type 30 data.
For maximum accuracy, thesingle frequency L5 Q5 user must use the correction terms to make further modifications to thecode phase offset given by:(∆tSV)L5Q5 = ∆tSV − TGD + ISCL5Q5where ISCL5Q5 (described in paragraph 20.3.3.3.1.2) is provided to the user as message type 30data.71IS-GPS-705D24 Sep 2013The values of ISCL5I5 and ISCL5Q5 are measured values that represent the mean SV group delaydifferential between the L1 P(Y)-code and the L5 I5-code or L5 Q5-code respectively as follows.ISCL5I5 = tL1P(Y) − tL5I5ISCL5Q5 = tL1P(Y) − tL5Q5where tLix is the GPS time the ith frequency x signal (a specific epoch of the signal) is transmittedfrom the SV antenna phase center.20.3.3.3.1.2.2 L1/L5 Ionospheric Correction.The two frequency (L1 C/A and L5 I5) user shall correct for the group delay and ionosphericeffects by applying the relationship:PR =(PR L5I5 − γ 15 PR L1C / A ) + c(ISC L5I5 - γ 15 ISC L1C / A )− cTGD1 − γ 15The two frequency (L1 C/A and L5 Q5) user shall correct for the group delay and ionosphericeffects by applying the relationship:PR =(PR L5Q5 − γ 15 PR L1C / A ) + c(ISC L5Q5 - γ 15 ISC L1C / A )1 − γ 15− cTGDwherePR= 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 (seeparagraph 20.3.3.3.1.2),TGD= see paragraph 20.3.3.3.3.2 of IS-GPS-200,c= speed of light (see paragraph 20.3.4.3),and where, denoting the nominal center frequencies of L1 and L5 as fL1 and fL5 respectively,72IS-GPS-705D24 Sep 2013γ15 = (fL1/fL5)2 = (1575.42/1176.45)2 = (154/115)2.20.3.3.3.1.2.3 L2/L5 Ionospheric Correction.The two frequency (L2 C and L5 I5) user shall correct for the group delay and ionosphericeffects by applying the relationship:PR =(PR L5I5 − γ 25 PR L 2C ) + c(ISC L5I5 - γ 25 ISC L 2C )− cTGD1 − γ 25The two frequency (L2 C and L5 Q5) user shall correct for the group delay and ionosphericeffects by applying the relationship:PR =(PR L5Q5 − γ 25 PR L 2C ) + c(ISC L5Q5 - γ 25 ISC L 2C )1 − γ 25− cTGDwherePR= 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 (seeparagraph 20.3.3.3.1.2),TGD= see paragraph 20.3.3.3.3.2 of IS-GPS-200,c= speed of light (see paragraph 20.3.4.3).and where, denoting the nominal center frequencies of L2 and L5 as fL2 and fL5 respectively.γ25 = (fL2/fL5)2 = (1227.6/1176.45)2 = (24/23)220.3.3.3.1.3 Ionospheric Data.The ionospheric parameters which allow the “L5 only” user to utilize the ionospheric model forcomputation of the ionospheric delay are contained in message type 30.
The “one frequency”user should use the model given in Figure 20-4 of IS-GPS-200 to make this correction. Thecalculated value of Tiono (Tiono = ionospheric correction parameter) in the model is referred to the73IS-GPS-705D24 Sep 2013L1 frequency; if the user is operating on the L5 frequency, the correction term must bemultiplied by γ15 (reference paragraph 20.3.3.3.1.2.2). It is estimated that the use of this modelwill provide at least a 50 percent reduction in the single-frequency user’s RMS error due toionospheric propagation effects. The bit lengths, scale factors, ranges, and units of theseparameters are given in Table 20-X of IS-GPS-200 (See Figure 20-3 for complete ionospheric bitallocation).The ionospheric data shall be updated by the CS at least once every six days while the CS is ableto upload the SVs.
If the CS is unable to upload the SVs, the ionospheric data transmitted by theSVs may not be accurate. During extended operations or in the Autonav mode, if the CS isunable to upload the SVs, the use of this model will yield unpredictable results.20.3.3.3.1.4 Example Application of Correction Parameters.A typical system application of the correction parameters for a user receiver is shown in Figure20-15. The ionospheric model referred to in Figure 20-15 is discussed in paragraph 20.3.3.3.1.3.74IS-GPS-705D24 Sep 2013TGD*, ISCL1C/A**, ISCL2C**, ISCL5I5* , ISCL5Q5*ESTIMATE OF SVTRANSMISSION TIMEaf0, af1, af2, tocCLOCKCORRECTIONPOLYNOMIAL∆tr∆tSVTROPOSPHERICMODELCODE PHASE OFFSET- TRUE SV CLOCK EFFECTS- EQUIPMENT GROUP DELAYDIFFERENTIAL EFFECTS- RELATIVISTIC EFFECTSTtropoIONOSPHERICMODELTionoαn, β nFILTER ANDCOORDINATECONVERTERGPS TIME PATH DELAY- GEOMETRIC- TROPOSPHERIC- IONOSPHERIC*PSEUDORANGEDIVIDED BY THESPEED OF LIGHTUSER CLOCK BIASUSER POSITION,VELOCITY andTIME (CLOCK BIAS)- RANGE DATA FROMOTHER SATELLITES- CALIBRATION DATA- AUXILIARY SENSOR* SINGLE FREQUENCY AND DUALFREQUENCY (L1/L5 AND L2/L5)USERSGPS TIME** DUAL FREQUENCY USERS ONLYFigure 20-15: Sample Application of Correction Parameters20.3.3.3.1.5 Data Predict Week Number.Bits 257-264 of Message Type 30 shall indicate the Data Predict Week Number (WNOP) towhich the Data Predict Time of Week (top) is referenced (see 20.3.3.1.1.3 and 20.3.3.2.1.2).
TheWNOP term consists of eight bits which shall be a modulo 256 binary representation of the GPSweek number to which the top is referenced. The user must account for the truncated nature ofWNop in all calculations in which WNop is used.75IS-GPS-705D24 Sep 201320.3.3.4 Message Types 31, 12 and 37 Almanac Parameters.The almanac parameters are provided in any one of message types 31, 37, and 12.
Message type37 provides Midi almanac parameters and the reduced almanac parameters are provided in eithermessage type 31 or type 12. The SV shall broadcast both message types 31 (and/or 12) and 37.However, the reduced almanac parameters (i.e. message types 31 and/or 12) for the complete setof SVs in the constellation will be broadcast by a SV using shorter duration of time compared tothe broadcast of the complete set of Midi almanac parameters (i.e. message type 37). Theparameters are defined below, followed by material pertinent to the use of the data.20.3.3.4.1 Almanac Reference Week.Bits 39 through 51 of message types 12, and bits 128 through 140 of message type 31 and 37shall indicate the number of the week (WNa-n) to which the almanac reference time (toa) isreferenced (see paragraph 20.3.3.4.2).
The WNa-n term consists of 13 bits which shall be amodulo-8192 binary representation of the GPS week number (see paragraph 6.2.4) to which thetoa is referenced. Bits 52 through 59 of message types 12, and bits 141 to 148 of message types31 and 37 shall contain the value of toa, which is referenced to this WNa-n.20.3.3.4.2 Almanac Reference Time. See paragraph 20.3.3.5.2.2 of IS-GPS-200.20.3.3.4.3 SV PRN Number.Bits 149 through 154 of message type 37 and bits 1 through 6 in each packet of reduced almanacshall specify PRN number of the SV whose almanac or reduced almanac, respectively, isprovided in the message or in the packet.20.3.3.4.4 Signal Health (L1/L2/L5).The three, one-bit, health indication in bits 155, 156 and 157 of message type 37 and bits 29,30and 31 of each packet of reduced almanac refers to the L1, L2, and L5 signals of the SV whosePRN number is specified in the message or in the packet.
For each health indicator, a “0”signifies that all signals on the associated frequency are okay and “1” signifies that some or allsignals on the associated frequency are bad. The predicted health data will be updated at thetime of upload when a new reduced almanac has been built by the CS. The transmitted healthdata may not correspond to the actual health of the transmitting SV or other SVs in theconstellation.20.3.3.4.5 Midi Almanac Parameter Content.Message type 37, Figure 20-10, provides Midi almanac data for a SV whose PRN number isspecified in the message. The number of bits, the scale factor (LSB), the range, and the units ofthe almanac parameters are given in Table 20-V.
The user algorithm is essentially the same asthe user algorithm used for computing the precise ephemeris as specified in Table 20-IV of ISGPS-200. Other parameters appearing in the equations of Table 20-IV of IS-GPS-200, but notprovided by the Midi almanac with the reference values, are set to zero for SV positiondetermination.
See paragraph 20.3.3.5.2.3 of IS-GPS-200 for almanac time parameters.76IS-GPS-705D24 Sep 2013Table 20-V. Midi Almanac ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***toa8212602,112e112-16dimensionlessδi****11*2-14semi-circles11*2-33semi-circles/secA172-4Ω016*2-15semi-circlesω16*2-15semi-circlesM016*2-15semi-circlesaf011*2-20secondsaf110*2-37sec/sec•ΩUnitssecondsmeters* Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;** See Figure 20-10 for complete bit allocation in message type 37;*** Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;**** Relative to i0 = 0.30 semi-circles.20.3.3.4.6 Reduced Almanac Parameter Content.Message type 31, Figure 20-4, provides SV clock correction parameters (ref.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
