IS-GPS-200H (797934), страница 26
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The parameters are definedbelow, followed by material pertinent to the use of the data.177IS-GPS-200H24 Sep 201330.3.3.5.1 EOP Content.Message type 32, Figure 30-5, provides SV clock correction parameters (ref. Section 30.3.3.2)and earth orientation parameters. The EOP message provides users with parameters to constructthe ECEF and ECI coordinate transformation (a simple transformation method is defined inSection 20.3.3.4.3.3.2). The number of bits, scale factors (LSBs), the range, and the units of allEOP fields of message type 32 are given in Table 30-VII.30.3.3.5.1.1 User Algorithm for Application of the EOP.The EOP fields in the message type 32 contain the EOP data needed to construct the ECEF-toECI coordinate transformation.
The user computes the ECEF position of the SV antenna phasecenter using the equations shown in Table 30-II.The full coordinate transformation fortranslating to the corresponding ECI SV antenna phase center position may be accomplished inaccordance with the computations detailed in Chapter 5 of IERS Technical Note 36: IERSConventions (2010) and equations for UT1, xp and yp as documented in Table 30-VIII. Figure5.1 on page 73 of that document depicts the computational flow starting from GCRS (GeocentricCelestial Reference System) to ITRS (International Terrestrial Reference System). OngoingWGS 84 re-adjustment at NGA and incorporating the 2010 IERS Conventions, are expected tobring Earth based coordinate agreement to within 2 cm.
In the context of the Conventions, theuser may as a matter of convenience choose to implement the transformation computations viaeither the Celestial Intermediate Origin (CIO) based approach” or the “Equinox basedapproach”. The EOP parameters for ΔUT1 are to be applied within the “Rotation to terrestrialsystem” process, and the parameters for xp and yp are applied in the “Rotation for polar motion”process. Users are advised that the broadcast message type 32 EOP parameters already accountfor zonal, diurnal and semidiurnal effects (described in Chapter 8 of the IERS Conventions(2010)), so these effects should not be further applied by the user.The relevant computations utilize elementary rotation matrices Ri(α), where α is a positiverotation about the ith-axis ordinate, as follows:1=R1 ( α ) 000cos(α )− sin (α )sin (α ) cos(α )0 cos (α )R3 ( α ) = − sin(α ) 0cos(α ), =R2 ( α ) 0 sin(α )sin(α )cos(α )0010− sin(α )0 cos(α ) 001The Inertial-to-Geodetic rotation matrix shall be calculated in accordance with the computationsdetailed in Chapter 5 of IERS Technical Note 36: IERS Conventions (2010) and equations forUT1, xp and yp as documented in Table 30-VIII.178IS-GPS-200H24 Sep 2013Table 30-VII.Earth Orientation ParametersParameterNo.
ofBits**ScaleFactor(LSB)EffectiveRange***UnitstEOPEOP Data Reference Time1624604,784secondsPM_X †X-Axis Polar Motion Valueat Reference Time.21*2-201arc-secondsPM_XX-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3arc-seconds/dayPM_Y ††Y-Axis Polar Motion Valueat Reference Time.21*2-201arc-secondsPM_YY-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3arc-seconds/day∆UT1 †††UT1-UTC Difference atReference Time.31*2-2464secondsRate of UT1-UTCDifference at ReferenceTime19*2-257.8125 x 10-3seconds/day•••∆UT1 †††****Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB;**See Figure 30-5 for complete bit allocation in Message type 32;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.†Represents the predicted angular displacement of instantaneous Celestial Ephemeris Pole with respect tosemi-minor axis of the reference ellipsoid along Greenwich meridian.††Represents the predicted angular displacement of instantaneous Celestial Ephemeris Pole with respect tosemi-minor axis of the reference ellipsoid on a line directed 90° west of Greenwich meridian.†††With zonal tides restored.179IS-GPS-200H24 Sep 2013Table 30-VIII.Element/Equation•UT 1 = UTC + ∆UT 1 + ∆ UT 1 (t − t EOP ) *Application of EOP ParametersDescriptionCompute Universal Time at time t•x p = PM _ X + PM X (t − t EOP ) *Polar Motion in the x-axis•y p = PM _ Y + PM Y (t − t EOP ) *Polar Motion in the y-axis*t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light).Furthermore, the quantity (t-tEOP) shall be the actual total time difference between the time t and the epoch timetEOP, and must account for beginning or end of week crossovers.
That is, if (t-tEOP) is greater than 302,400seconds, subtract 604,800 seconds from (t-tEOP). If (t-tEOP) is less than -302,400 seconds, add 604,800 secondsto (t-tEOP).30.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters.Message type 33, Figure 30-6 contains the UTC parameters. The contents of message type 33are defined below, followed by material pertinent to the use of the UTC data.30.3.3.6.1 UTC Parameter Content.Message type 33 provides SV clock correction parameters (ref. Section 30.3.3.2) and also, shallcontain 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 Figure30-6 for complete bit allocation in message type 33.The parameters relating GPS time to UTC (USNO) shall be updated by the CS at least onceevery three days while the CS is able to upload the SVs. If the CS is unable to upload the SVs,the accuracy of the UTC parameters transmitted 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 messageupload) 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 to calculate tUTC is in paragraph 20.3.3.5.2.4 exceptthe 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)) seconds2180IS-GPS-200H24 Sep 2013Table 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 UTCtime scale7*2-68sec/sec2ΔtLSCurrent or past leap second count8*1secondstotTime data reference Time of Week1624WNotTime data reference Week Number131weeksWNLSFLeap second reference WeekNumber131weeksDNLeap second reference Day Number4****1daysΔtLSFCurrent or future leap second count8*1seconds604,784seconds****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.30.3.3.7 Message Types 34, 13, and 14 Differential Correction Parameters.Differential Correction (DC) parameters are provided either in message types 34 or in types 13and 14.
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.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, shallcontain DC parameters that apply to the clock and ephemeris data transmitted by another SV.One message type 34, Figure 30-7, shall contain 34 bits of clock differential correction (CDC)181IS-GPS-200H24 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 CNAV data, Dc(t), and one (1) signifies that the corrections apply to NAV data, D(t).Message types 13 and 14 together also provide DC parameters. Message type 13, Figure 30-12,shall contain CDC parameters applicable to 6 SVs and message type 14, Figure 30-13, 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 30-16. The number of bits, scale factors (LSB), the range, and the units of all fields in theDC 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 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; and User Differential Range Accuracy•(UDRA) and UDRA indices that enable users to estimate the accuracy obtained after correctionsare applied.
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