IS-GPS-705D (797936), страница 13
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Section 20.3.3.2)and reduced almanac data packets for 4 SVs. Message type 12, Figure 20-11 contains reducedalmanac data packets for 7 SVs.20.3.3.4.6.1 Reduced Almanac Data.Message types 31 or 12 contain reduced almanac data and SV health words for SVs in theconstellation. The reduced almanac data of a SV is broadcast in a packet 31 bits long, asdescribed in Figure 20-16.
The reduced almanac data are a subset of the almanac data whichprovide less precise ephemeris. The reduced almanac data values are provided relative to prespecified reference values. The number of bits, the scale factor (LSB), the ranges, and the units77IS-GPS-705D24 Sep 2013of the reduced almanac parameters are given in Table 20-VI.
The algorithms and other materialrelated to the use of the reduced almanac data are given in Section 20.3.3.4.6.2.The reduced almanac parameters shall be updated by the CS at least once every 3 days while theCS is able to upload the SVs.
If the CS is unable to upload the SVs, the accuracy of the reducedalmanac parameters transmitted by the SVs will degrade over time.A 6-bit value of “000000” in the PRNa field shall indicate that no further Status Words arecontained in the remainder of the data block.
In this event, all subsequent bits in the data blockfield shall be filler bits, i.e., alternating ones and zeros beginning with one.20.3.3.4.6.2 Reduced Almanac Packet.The following shall apply when interpreting the data provided in each packet of reduced almanac(see Figure 20-16).20.3.3.4.6.2.1 Reduced Almanac.The reduced almanac data is provided in bits 7 through 28 of each packet. The data from apacket along with the reference values (see Table 20-VI) provide ephemeris with further reducedprecision.
The user algorithm is essentially the same as the user algorithm used for computingthe precise ephemeris from the parameters of the message types 10 and 11 (see paragraph20.3.3.1.3 and Table 20-II). Other parameters appearing in the equations of Table 20-II, but notprovided by the reduced almanac with the reference values, are set to zero for SV positiondetermination.78IS-GPS-705D24 Sep 201331 BITS1715δAPRNa2229 30 31Ω0Φ0L1 HEALTHL2 HEALTH*See Figures 20-4 and 20-11 for complete bit allocation in therespective messages.L5 HEALTHFigure 20-16.
Reduced Almanac Packet ContentParameter*****Table 20-VI. Reduced Almanac ParametersNo. of BitsScale Factor (LSB)Effective Range **UnitsδA ***8*2Ω07*2-6**semi-circles7*-6**semi-circlesΦ0 ****+9**meters2*Parameters so indicated shall be two’s complement with the sign bit (+ or -) occupying the MSB;**Effective range is the maximum range attainable with indicated bit allocation and scale factor;***Relative to Aref = 26,559,710 meters;****Φ0 = Argument of Latitude at Reference Time = M0 + ω;*****Relative to following reference values:e=0δi = +0.0056 semi-circles (i = 55 degrees)•Ω =-2.6 x 10-9 semi-circles/second20.3.3.5 Message Type 32 Earth Orientation Parameters (EOP).The earth orientation parameters are provided in message type 32.
The parameters are definedbelow, followed by material pertinent to the use of the data.79IS-GPS-705D24 Sep 201320.3.3.5.1 EOP Content.Message type 32, Figure 20-5, provides SV clock correction parameters (ref. Section 20.3.3.2)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 of IS-GPS-200). The number of bits, scale factors (LSBs), the ranges, andthe units of all EOP fields of message type 32 are given in Table 20-VII.20.3.3.5.1.1 User Algorithm for Application of the EOPThe 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 20-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 20-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 ) 000sin (α ) cos(α )0cos(α )− sin (α )cos(α ), =R2 ( α ) 0 sin(α ) cos (α )R3 ( α ) = − sin(α ) 080sin(α )cos(α )0010− sin(α )0 cos(α ) 001IS-GPS-705D24 Sep 2013The 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 20-VIII.Table 20-VII.
Earth Orientation ParametersParameter SymboltEOPParameter DescriptionNo. ofScaleFactorEffectiveBits**(LSB)Range***Units604,784seconds4EOP Data Reference Time162X-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-secondsY-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3arc-seconds/dayUT1-UTC DifferenceReference Time.at31*2-2464secondsUT1-UTCReference19*2-257.8125 x 10-3seconds/dayPM_X †••PM_Y∆UT1 †††•∆UT1 †††RateofDifference atTime******Parameters so indicated are two’s complement, with the sign bit (+ or -) occupying the MSB;See Figure 20-5 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.81IS-GPS-705D24 Sep 2013Table 20-VIII.
Application of EOP ParametersElement/Equation•UT 1 = UTC + ∆UT 1 + ∆ UT 1 (t − t EOP )DescriptionCompute 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-axist is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light).20.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters.Message type 33, Figure 20-6, contains the UTC parameters. The contents of message type 33are defined below, followed by material pertinent to the use of the UTC data.20.3.3.6.1 UTC Parameter Content.Message type 33 provides SV clock correction parameters (ref. Section 20.3.3.2) 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 20-IX. See Figure20-6 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.20.3.3.6.2 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 (and define) tUTC is in paragraph20.3.3.5.2.4 of IS-GPS-200 except the following definition of ∆tUTC shall be used.∆tUTC = ∆tLS + A0-n + A1-n (tE - tot + 604,800 (WN - WNot)) + A2-n (tE - tot + 604,800 (WN WNot))2 seconds82IS-GPS-705D24 Sep 2013Table 20-IX.
UTC ParametersParameterSymbolParameter DescriptionNo. ofScaleFactorEffectiveBits**(LSB)Range***-35UnitsA0-nBias coefficient of GPS time scalerelative to UTC time scale16*2A1-nDrift coefficient of GPS time scalerelative to UTC time scale13*2-51sec/secA2-nDrift rate correction coefficient ofGPS time scale relative of UTCtime scale7*2-68sec/sec2∆tLSCurrent or past leap second count8*1secondstotTime data reference Time of Week1624WNotTime data reference Week Number131weeksLeap secondNumber81weeks4****1days8*1secondsWNLSFreferenceWeekDNLeap second reference Day Number∆tLSFCurrent or future leap second count**********Seconds604,784secondsParameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;See Figure 20-6 for complete bit allocationUnless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;Right justified.20.3.3.7 Message Types 34, 13, and 14 Differential Correction Parameters.Differential Correction (DC) parameters are provided either in message type 34, or in types 13and 14.
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