IS-GPS-200F (811524), страница 28
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Message type 33, Figure 30-6 containsthe UTC parameters. The contents of message type 33 are defined below, followed by material pertinent to the useof the UTC data.30.3.3.6.1 UTC Parameter Content. Message type 33 provides SV clock correction parameters (ref. Section30.3.3.2) and also, shall contain the parameters related to correlating UTC (USNO) time with GPS Time.
The bitlengths, scale factors, ranges, and units of these parameters are given in Table 30-IX. See Figure 30-6 for completebit allocation in message type 33.The parameters relating GPS time to UTC (USNO) shall be updated by the CS at least once every three days whilethe CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameterstransmitted by the SVs will degrade over time.30.3.3.6.2 UTC and GPS Time. Message type 33 includes: (1) the parameters needed to relate GPS Time to UTC(USNO), and (2) notice to the user regarding the scheduled future or recent past (relative to Nav message upload)value of the delta time due to leap seconds (∆tLSF), together with the week number (WNLSF) and the day number(DN) at the end of which the leap second becomes effective. Information required to use these parameters tocalculate tUTC is in paragraph 20.3.3.5.2.4 except the following definition of ∆tUTC shall be used.∆tUTC = ∆tLS + A0-n + A1-n (tE - tot + 604800 (WN - WNot)) + A2-n (tE - tot + 604800 (WN - WNot))2 seconds178IS-GPS-200F21 Sep 2011Table 30-IX.ParameterUTC ParametersNo.
ofBits**ScaleFactor(LSB)EffectiveRange***UnitsA0-nBias coefficient of GPS time scalerelative to UTC time scale16*2-35SecondsA1-nDrift coefficient of GPS time scalerelative to UTC time scale13*2-51sec/secA2-nDrift rate correction coefficient of GPStime scale relative to UTC time scale7*2-68sec/sec28*1seconds1624131weeks81weeks4***1days8*1secondsCurrent or past leap second count∆tLSTime data reference Time of Weektot604,784secondsTime data reference Week NumberWNotLeap second reference Week NumberWNLSFLeap second reference Day NumberDNCurrent or future leap second count∆tLSF****Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;**See Figure 30-6 for complete bit allocation;Unless otherwise indicated in this column, effective range is the maximum range attainable with indicatedbit allocation and scale factor;****Right justified.179IS-GPS-200F21 Sep 201130.3.3.7 Message Types 34, 13, and 14 Differential Correction Parameters.
Differential Correction (DC) parametersare provided either in message types 34 or in types 13 and 14. These parameters provide users with sets ofcorrection terms that apply to the clock and ephemeris data transmitted by other SVs. DC parameters are grouped inpackets, as described in the next sections. The availability of these message types is subject to the control anddetermination of the CS.30.3.3.7.1 Differential Correction Parameters Content. Message type 34 provides SV clock correction parameters(ref. Section 30.3.3.2) and also, shall contain DC parameters that apply to the clock and ephemeris data transmittedby another SV.
One message type 34, Figure 30-7, shall contain 34 bits of clock differential correction (CDC)parameters and 92 bits of ephemeris differential correction (EDC) parameters for one SV other than the transmittingSV. Bit 150 of message type 34 shall be a DC Data Type indicator that indicates the data type for which the DCparameters apply.
Zero (0) signifies that the corrections apply to CNAV data, Dc(t), and one (1) signifies that thecorrections apply to NAV data, D(t).Message types 13 and 14 together also provide DC parameters. Message type 13, Figure 30-12, shall contain CDCparameters applicable to 6 SVs and message type 14, Figure 30-13, shall contain EDC parameters applicable to 2SVs. There shall be a DC Data Type indicator preceding each CDC or EDC packet. The content of an individualdata packet is depicted in Figure 30-16. The number of bits, scale factors (LSB), the range, and the units of all fieldsin the DC packet are given in Table 30-X.180IS-GPS-200F21 Sep 201130.3.3.7.2 DC Data Packet.
Each DC data packet contains: corrections to SV clock polynomial coefficientsprovided in any one of the message types 30 to 37 of the corresponding SV; corrections to quasi-Keplerian elementsreferenced to tOD of the corresponding SV; and User Differential Range Accuracy (UDRA) and UDRA indices thatenable users to estimate the accuracy obtained after corrections are applied.
Each DC packet is made up of twodifferent segments. The first segment contains 34 bits for the CDC parameters and the second segment contains 92bits of EDC parameters totaling 126 bits. The CDC and EDC parameters form an indivisible pair and users mustutilize CDC and EDC as a pair. Users must utilize CDC and EDC data pair of same top-D and of same tOD.30.3.3.7.2.1 Differential Correction Data Predict Time of the Week. The DC data predict time of week (top-D)provides the epoch time of week, in increments of 300 seconds (i.e. five minutes), at which the prediction for theassociated DC data was performed.30.3.3.7.2.2 Time of Differential Correction Data.
The time of DC data, tOD, specifies the reference time of week, inincrements of 300 seconds (i.e., five minutes) relative to the GPS week, for the associated CDC and EDC data.30.3.3.7.2.3 SV PRN Identification. The PRN ID of both CDC and EDC of Figure 30-16 identifies the satellite towhich the subject 126-bit differential correction packet data applies (by PRN code assignment).
A value of all ones“11111111” in any PRN ID field shall indicate that no DC data is contained in the remainder of the data block. Inthis event, the remainder of the data block shall be filler bits, i.e., alternating ones and zeros beginning with one.181IS-GPS-200F21 Sep 2011CDC = Clock Differential CorrectionMSB19LSB223034PRN IDδaf0δa f1UDRA.8 BITS.13 BITS.8 BITS.5 BITSEDC = Ephemeris Differential CorrectionMSBLSB123936PRN ID∆α∆β.8 BITS.14 BITS.14 BITSMSBLSB523763∆i∆γ.12 BITS.15 BITS.LSBMSB88766492∆Ω∆A•UDRA12 BITS12 BITS5 BITSFigure 30-16.Differential Correction Data Packet182IS-GPS-200F21 Sep 2011Table 30-X.ParameterPRN IDDifferential Correction ParametersNo.
ofBits**ScaleFactor(LSB)EffectiveRange***Unitssee text8δaf0SV Clock Bias Correction13*2-35secondsδaf1SV Clock Drift Correction8*2-51seconds/secondUDRAUser Differential RangeAccuracy Index5*∆αAlpha Correction to EphemerisParameters14*2-34dimensionless∆βBeta Correction to EphemerisParameters14*2-34dimensionless∆γGamma Correction to EphemerisParameters15*2-32semi-circles∆iAngle of Inclination Correction12*2-32semi-circles∆ΩAngle of Right AscensionCorrection12*2-32semi-circles∆ASemi-Major Correction12*2-9metersChange Rate of User DifferentialRange Accuracy Index.5*•UDRAsee textsee text*Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB;**See Figure 30-7 , 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 withindicated bit allocation and scale factor.30.3.3.7.3 Application of Clock Related DC Data.
The SV PRN code phase offset, uncorrected by clock correctioncoefficient updates, is given by equation 2 in paragraph 20.3.3.3.3.1 (see para. 30.3.3.2.3). If the matched pair ofDC data for the subject SV is available, the user may apply clock correction coefficient update values by;∆tsv = (af0 + δaf0) + (af1 + δaf1)(t − toc) + af2(t − toc )2 + ∆tr,183IS-GPS-200F21 Sep 2011where δaf0 and δaf1, (see Table 30-X), are given in message types 34 or 13, and all other terms are as stated inparagraph 20.3.3.3.3.1.Clock-related DC data shall not be applied to any SV transmitting clock correctionparameters message(s) containing a top value greater than the top-D value of messages types 34 or 13 containing theclock-related DC data.30.3.3.7.4 Application of Orbit-Related DC Data. The DC data packet includes corrections to parameters thatcorrect the state estimates for ephemeris parameters transmitted in the message types 10 and 11 (broadcast by theSV to which the DC data packet applies).
The user will update the ephemeris parameters utilizing a variation of thealgorithm expressed in the following equations. The user will then incorporate the updated quasi-Keplerian elementset in all further calculations of SV position, as represented by the equations in Table 30-II (see para. 30.3.3.1.3).Ephemeris-related DC data shall not be applied to any SV transmitting message types 10 and 11 containing a topvalue greater than the top-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 and 11 data. The termsαi, β i, and γi form a subset of stabilized ephemeris elements which are subsequently corrected by ∆α, ∆β and∆γthe values of which are supplied in the message types 34 or 14 - as follows:αc=αi + ∆αβc=βi + ∆βγc=γi + ∆γThe quasi-Keplerian elements are then corrected by184IS-GPS-200F21 Sep 2011Ac=Ai + ∆Aec=(αc2 + β c2)1/2ic=ii + ∆iΩc=Ωi + ∆Ωωc=tan-1 (β c/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 fromµ∆M0=−3Ac 2[(toe) − (tOD )].The corrected quasi-Keplerian elements above are applied to the user algorithm for determination of antenna phasecenter position in Section 30.3.3.1.3, Table 30-II.30.3.3.7.5 SV Differential Range Accuracy Estimates.
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