Интерфейсный документ GPS, страница 23

− 245154536525Compute time from J2000 Julian Epoch in JulianCenturiesζ = 2306 ′′. 2181 T + 0 ′′. 30188 T 2 + 0 ′′. 017998 T3Compute Precession Fundamental Angles at time tz = 2306 ′′. 2181 T + 1′′. 09468 T 2 + 0 ′′. 018203 T3θ = 2004 ′′. 3109 T − 0 ′′. 42665 T2 − 0 ′′. 041833 T3()(D = R3 −90o − z R1 ( θ ) R3 90o − ζ)Calculate Precession Matrix at time, tε = 23° 26 ′21′′.

448 − 46 ′′. 815 T − 0 ′′.00059 T2Compute Mean Obliquity, ε , at time t+ 0 ′′.001813 T3C = R1 ( −( ε + ∆ε ) ) R3 ( −∆ψ ) R1 ( ε )Compute Nutation Matrix at time, tIS-GPS-200D7 Dec 2004177Table 30-VIII. Application of EOP Parameters (Part 2 of 2)Element/Equation106 5∆ψ = ∑ ai sin  ∑ e j E j i =1 j =164 5∆ε = ∑ bi cos  ∑ e j E j i =1 j =1Description††Nutation in LongitudeNutation in Obliquity††•UT1 = UTC + ∆UT1 + ∆UT1 (t – tEOP)Compute Universal Time at time tJ.D. − 245154536525where J.D. = UT1 exp ressed in days of 86400 secCompute Universal Time from J2000 Julian Epochin Julian Centuries UT1 + 6h 41m 50 s.548412π sα=+ 8640184 .812866TUh24 + 0 s.093104T 2 − 6 s. 2 × 10−6 T3UUCompute Mean Greenwich Hour AngleTU =α = α + ∆ψ cos( ε + ∆ε )Compute True Greenwich Hour AngleB = R3 ( α )Compute Rotation Matrix at time, tA = R2 (-xp) R1 (-yp)Compute Polar Motion Matrix at time, t•where xp = PM_X + PM_X (t – tEOP)•yp = PM_Y + PM_Y (t – tEOP)Compute Inertial-to-Geodetic Rotation matrix, ABCDABCD =  A  B  C  Dt is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light).††The Nutation in Longitude and the Nutation in Obliquity are as described in The Astronomical Almanac (1983),pp.

S23-S26, evaluated at time T.IS-GPS-200D7 Dec 200417830.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters. 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 toUTC(USNO), and (2) notice to the user regarding the scheduled future or recent past (relative to Nav messageupload) value of the delta time due to leap seconds (∆tLSF), together with the week number (WNLSF) and the daynumber (DN) at the end of which the leap second becomes effective.

Information required to use these parametersto calculate 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 secondsIS-GPS-200D7 Dec 2004179Table 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 ofGPS time scale relative to UTC timescale7*2-68sec/sec2∆tLSCurrent or past leap second count8*1secondstotTime data reference Time of Week1624WNotTime data reference Week Number131weeksWNLSFLeap second reference Week Number81weeksDNLeap second reference Day Number4****1days∆tLSFCurrent or future leap second count8*1seconds**********604,784secondsParameters 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 withindicated bit allocation and scale factor;Right justified.IS-GPS-200D7 Dec 200418030.3.3.7Message Types 34, 13, and 14 Differential Correction Parameters.Differential Correction (DC)parameters are provided either in message types 34 or in types 13 and 14.

These parameters provide users with setsof correction terms that apply to the clock and ephemeris data transmitted by other SVs. DC parameters are groupedin packets, 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.30.3.3.7.2 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 elements•referenced 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 Week. The DC data predict time of week (top-D) providesthe epoch time of week, in increments of 300 seconds (i.e. five minutes), at which the prediction for the associatedDC data was performed.IS-GPS-200D7 Dec 200418130.3.3.7.2.2 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.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.CDC = Clock Differential CorrectionMSB1LSB2293430PRN IDδaf0δaf1UDRA.8 BITS.13 BITS.8 BITS.5 BITSEDC = Ephemeris Differential CorrectionMSB1LSB36239PRN ID∆α∆β.8 BITS.14 BITS.14 BITSLSBMSB523763∆i∆γ.12 BITS.15 BITS.LSBMSB88766492∆Ω∆A•UDRA12 BITS12 BITS5 BITSFigure 30-16. Differential Correction Data PacketIS-GPS-200D7 Dec 2004182Table 30-X.ParameterDifferential Correction ParametersNo.

ofBits**ScaleFactor(LSB)8PRN IDEffectiveRange***Unitssee textδaf0SV Clock Bias Correction13*2-35secondsδaf1SV Clock Drift Correction8*2-51seconds/secondUser 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*UDRA•UDRA******see textsee textParameters 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.IS-GPS-200D7 Dec 200418330.3.3.7.3 Application of Clock-Related DC Data.