IS-GPS-705D (797936), страница 14
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These parameters provide users with sets of correction terms that apply to the clock andephemeris data transmitted by other SVs. DC parameters are grouped in packets, as described inthe next sections. The availability of these message types is subject to the control anddetermination of the CS.20.3.3.7.1 Differential Correction Parameters Content.Message type 34 provides SV clock correction parameters (ref. Section 20.3.3.2) and also, shallcontain DC parameters that apply to the clock and ephemeris data transmitted by another SV.One message type 34, Figure 20-7, shall contain 34 bits of clock differential correction (CDC)83IS-GPS-705D24 Sep 2013parameters and 92 bits of ephemeris differential correction (EDC) parameters for one SV otherthan the transmitting SV.
Bit 150 of message type 34 shall be a DC Data Type indicator thatindicates the data type for which the DC parameters apply. Zero (0) signifies that the correctionsapply to L5 CNAV data, D5(t), and one (1) signifies that the corrections apply to NAV data, D(t),described in Appendix II of IS-GPS-200.Message types 13 and 14 together also provide DC parameters. Message type 13, Figure 20-12,shall contain CDC parameters applicable to 6 SVs and message type 14, Figure 20-13, shallcontain EDC parameters applicable to 2 SVs. There shall be a DC Data Type indicatorpreceding each CDC or EDC packet. The content of an individual data packet is depicted inFigure 20-17. The number of bits, scale factors (LSB), the ranges, and the units of all fields inthe DC packet are given in Table 20-X.20.3.3.7.2 DC Data Packet.Each DC data packet contains: corrections to SV clock polynomial coefficients provided in anyone of the message types 30 to 37 of the corresponding SV; corrections to quasi-Keplerianelements referenced to tOD of the corresponding SV; User Differential Range Accuracy (UDRA)•and UDRA indices that enable users to estimate the accuracy obtained after corrections areapplied.
Each DC packet is made up of two different segments. The first segment contains 34bits for the CDC parameters and the second segment contains 92 bits of EDC parameters totaling126 bits. The CDC and EDC parameters form an indivisible pair and users must utilize CDC andEDC as a pair. Users must utilize CDC and EDC data pairs of the same top-D (top-D =DC datapredict time of week) and of the same tOD.20.3.3.7.2.1 Differential Correction Data Predict Time of Week.The DC data predict time of week (top-D) provides the epoch time of week, in increments of 300seconds (i.e.
five minutes), at which the prediction for the associated DC data was performed.20.3.3.7.2.2 Time of Differential Correction Data.The time of DC data, tOD, specifies the reference time of week, in increments of 300 seconds(i.e., five minutes) relative to the GPS week, for the associated CDC and EDC data.20.3.3.7.2.3 SV PRN Identification.The PRN ID of both CDC and EDC of Figure 20-17 identifies the satellite to which the subject126-bit differential correction packet data applies (by PRN code assignment). A value of allones “11111111” in any PRN ID field shall indicate that no DC data is contained in theremainder of the data block. In this event, the remainder of the data block shall be filler bits, i.e.,alternating ones and zeros beginning with one.84IS-GPS-705D24 Sep 2013CDC = Clock Differential CorrectionMSB19LSB223034PRN IDδaf0δaf1UDRA.8 BITS.13 BITS.8 BITS.5 BITSEDC = Ephemeris Differential CorrectionMSB1LSB23936PRN IDΔαΔβ.8 BITS.14 BITS.14 BITSLSBMSB523763ΔiΔγ.12 BITS.15 BITS.LSBMSB88766492ΔΩΔA•UDRA12 BITS12 BITS5 BITSFigure 20-17.
Differential Correction Data Packet85IS-GPS-705D24 Sep 2013Table 20-X. Differential Correction ParametersScaleFactor(LSB)ParameterSymbolPRN IDParameter DescriptionNo. ofBits**8δaf0SV Clock Bias Correction13*2-35secondsδaf1SV Clock Drift Correction8*2-51seconds/secondUDRAUser Differential RangeAccuracy Index.5*Alpha Correction to EphemerisParameters14*Beta Correction to EphemerisParameters14*∆α∆β∆γGamma Correction to EphemerisParameters∆iAngle of Inclination Correction2-34dimensionless15*2-32semi-circles12*2-3212*-32semi-circles12*•UDRA******Change Rate of User DifferentialRange Accuracy Indexsee textdimensionlessSemi-Major Correction∆AUnitssee text2-34Angle of Right AscensionCorrection∆ΩEffectiveRange***22-95*semi-circlesmeterssee textParameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB;See Figures 20-7, 11 and 12 for complete bit allocation in message types 34, 13 and 14.Unless otherwise indicated in this column, effective range is the maximum range attainable with indicatedbit allocation and scale factor.20.3.3.7.3 Application of Clock-Related Differential Correction Data.The SV PRN code phase offset, uncorrected by clock correction coefficient updates, is given byequation 2 in 20.3.3.3.3.1 of IS-GPS-200 (see paragraph 20.3.3.2.3).
If the matched pair of DCdata for the subject SV is available, the user may apply clock correction coefficient update valuesby;∆tsv = (af0 + δaf0) + (af1 + δaf1)(t − toc) + af2(t − toc )2 + ∆tr86IS-GPS-705D24 Sep 2013where δaf0 and δaf1, (see Table 20-X), are given in message types 34 or 13, and all other termsare as stated in 20.3.3.3.3.1 of IS-GPS-200.
Clock-related DC data shall not be applied to anySV transmitting clock correction parameters message(s) containing a top value greater than thetop-D value of messages types 34 or 13 containing the clock-related DC data.20.3.3.7.4 Application of Orbit-Related Differential Correction Data.The DC data packet includes corrections to parameters that correct the state estimates forephemeris parameters transmitted in the message types 10 and 11 (broadcast by the SV to whichthe DC data packet applies).
The user will update the ephemeris parameters utilizing a variationof the algorithm expressed in the following equations. The user will then incorporate theupdated quasi-Keplerian element set in all further calculations of SV position, as represented bythe equations in Table 20-II (see para. 20.3.3.1.3). Ephemeris-related DC data shall not beapplied to any SV transmitting message types 10 and 11 containing a top value greater than thetop-D value of message types 34 or 14 containing the ephemeris-related DC data.The user will construct a set of initial (uncorrected) elements by:Ai=A0ei=enii=i0-nΩi=Ω0-nαi=en • cos(ωn)βi=en • sin(ωn)γi=M0-n + ωnwhere A0, en, i0-n, Ω0-n, ωn and M0-n are obtained from the applicable SV’s message types 10 and11 data. The terms αi, βi, and γi form a subset of stabilized ephemeris elements which aresubsequently corrected by ∆α, ∆β and ∆γthe values of which are supplied in the message types34 or 14-as follows:αc=αi + ∆αβc=βi + ∆βγc=γi + ∆γThe quasi-Keplerian elements are then corrected by87IS-GPS-705D24 Sep 2013Ac=Ai + ∆Aec=(αc2 + βc2)1/2ic=ii + ∆iΩc=Ω i + ∆Ωωc=tan-1 (βc/αc)M0_c=γc − ωc + ∆M0where ∆A, ∆i and ∆Ω are provided in the EDC data packet of the message type 34 or 14 and∆M0 is obtained from1− 3 µ 2 ∆A0∆M 0 =2 A0 3 A0[(t oe + WN oe * 604,800 ) − (t OD + WN * 604,800 )]The corrected quasi-Keplerian elements above are applied to the user algorithm fordetermination of antenna phase center position in Section 20.3.3.1.3, Table 20-II.20.3.3.7.5 SV Differential Range Accuracy Estimates.•The UDRAop-D and UDRA shall give the differential user range accuracy for the SV.
It must benoted that the two parameters provide estimated accuracy after both clock and ephemeris DC are•applied. The UDRAop-D and UDRA indices are signed, two’s complement integers in the rangeof +15 to -16 and have the following relationship:•Index ValueUDRA (10-6 m/sec)UDRAop-D (meters)156144.00 <UDRAop-D6144.00143072.00 <UDRAop-D≤ 6144.003072.00131536.00 <UDRAop-D≤ 3072.001536.0012768.00<UDRAop-D≤ 1536.00768.0011384.00<UDRAop-D≤ 768.00384.0088IS-GPS-705D24 Sep 201310192.00 <UDRAop-D≤ 384.00192.00996.00 <UDRAop-D≤ 192.0096.00848.00 <UDRAop-D≤ 96.0048.00724.00 <UDRAop-D≤ 48.0024.00613.65 <UDRAop-D≤ 24.0013.6559.65<UDRAop-D≤ 13.659.6546.85<UDRAop-D≤ 9.656.8534.85<UDRAop-D≤ 6.854.8523.40<UDRAop-D≤ 4.853.4012.40<UDRAop-D≤ 3.402.4001.70<UDRAop-D≤ 2.401.70-11.20<UDRAop-D≤ 1.701.20-20.85<UDRAop-D≤ 1.200.85-30.60<UDRAop-D≤ 0.850.60-40.43<UDRAop-D≤ 0.600.43-50.30<UDRAop-D≤ 0.430.30-60.21<UDRAop-D≤ 0.300.21-70.15<UDRAop-D≤ 0.210.15-80.11<UDRAop-D≤ 0.150.11-90.08<UDRAop-D≤ 0.110.08-100.06<UDRAop-D≤ 0.080.06-110.04<UDRAop-D≤ 0.060.04-120.03<UDRAop-D≤ 0.040.03-130.02<UDRAop-D≤ 0.030.0289IS-GPS-705D24 Sep 2013-140.01-15-16<UDRAop-D≤0.020.01UDRAop-D≤0.010.005No accuracy prediction available-use at own riskFor any time, tk, other than top-D, UDRA is found by,•UDRA = UDRAop-D + UDRA (tk - top-D)20.3.3.8 Message Type 35 GPS/GNSS Time Offset.Message type 35, Figure 20-8, contains the GPS/Global Navigation Satellite System (GNSS)Time Offset (GGTO) parameters.
The contents of message type 35 are defined below. Thevalidity period of the GGTO shall be 1 day as a minimum.20.3.3.8.1 GPS/GNSS Time Offset Parameter Content.Message Type 35 provides SV clock correction parameters (ref. Section 20.3.3.2) and also, shallcontain the parameters related to correlating GPS time with other GNSS time. Bits 157 through159 of message type 35 shall identify the other GPS-like navigation system to which the offsetdata applies.
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