IS-GPS-200F (811524), страница 27
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For each health indicator, a “0” signifies that allnavigation data are okay and “1” signifies that some or all navigation data are bad. The predicted health data will beupdated at the time of upload when a new reduced almanac has been built by the CS. The transmitted health datamay not correspond to the actual health of the transmitting SV or other SVs in the constellation.30.3.3.4.5 Midi Almanac Parameter Content.
Message type 37, Figure 30-10, provides Midi almanac data for a SVwhose PRN number is specified in the message. The number of bits, the scale factor (LSB), the range, and the unitsof the almanac parameters are given in Table 30-V. The user algorithm is essentially the same as the user algorithmused for computing the precise ephemeris as specified in Table 20-IV. Other parameters appearing in the equationsof Table 20-IV, but not provided by the Midi almanac with the reference values, are set to zero for SV positiondetermination. See paragraph 20.3.3.5.2.3 for almanac time parameters.30.3.3.4.6 Reduced Almanac Parameter Content. Message type 31, Figure 30-4, provides SV clock correctionparameters (ref.
Section 30.3.3.2) and reduced almanac data packets for 4 SVs. Message type 12, Figure 30-11,contains reduced almanac data packets for 7 SVs.172IS-GPS-200F21 Sep 201130.3.3.4.6.1 Reduced Almanac Data. Message type 31 or 12 contains reduced almanac data and SV health wordsfor SVs in the constellation. The reduced almanac data of a SV is broadcast in a packet of 31 bits long, as describedin Figure 30-15. The reduced almanac data are a subset of the almanac data which provide less precise ephemeris.The reduced almanac data values are provided relative to pre-specified reference values.
The number of bits, thescale factor (LSB), the range, and the units of the reduced almanac parameters are given in Table 30-VI. Thealgorithms and other material related to the use of the reduced almanac data are given in Section 30.3.3.4.6.2.The reduced almanac parameters shall be updated by the CS at least once every 3 days while the CS is able toupload the SVs. If the CS is unable to upload the SVs, the accuracy of the reduced almanac parameters transmittedby the SVs will degrade over time.30.3.3.4.6.2 Reduced Almanac Packet. The following shall apply when interpreting the data provided in eachpacket of reduced almanac (see Figure 30-15).The reduced almanac data is provided in bits 7 through 28 of each packet.
The data from a packet along with thereference values (see Table 30-VI) provide ephemeris with further reduced precision. The user algorithm isessentially the same as the user algorithm used for computing the precise ephemeris from the parameters of themessage types 10 and 11 (see paragraph 30.3.3.1.3 and Table 30-II). Other parameters appearing in the equations ofTable 30-II, but not provided by the reduced almanac with the reference values, are set to zero for SV positiondetermination.173IS-GPS-200F21 Sep 2011Table 30-V.Midi Almanac ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***Unitstoa8212602,112secondse112-16dimensionlessδi****11*2-14semi-circles11*2-33semi-circles/secA172-4metersΩ016*2-15semi-circlesω16*2-15semi-circlesM016*2-15semi-circlesaf011*2-20secondsaf110*2-37sec/sec•Ω* Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;** See Figure 30-10 for complete bit allocation in message type 37;*** Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;**** Relative to i0 = 0.30 semi-circles.174IS-GPS-200F21 Sep 201131 BITS171522δAPRNa29 30Ω031Φ0L1 HEALTHL2 HEALTH*See Figures 30-4 and 30-11 for complete bit allocation in therespective messages.Figure 30-15.Table 30-VI.ParameterNo.
of BitsL5 HEALTHReduced Almanac Packet ContentReduced Almanac Parameters *****Scale Factor (LSB)Effective Range **Units+9**metersδA ***8*2Ω07*2-6**semi-circles7*-6**semi-circlesΦ0 ****2*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/second.30.3.3.5 Message Type 32 Earth Orientation Parameters (EOP). The earth orientation parameters are provided inmessage type 32. The parameters are defined below, followed by material pertinent to the use of the data.175IS-GPS-200F21 Sep 201130.3.3.5.1 EOP Content. Message type 32, Figure 30-5, provides SV clock correction parameters (ref.
Section30.3.3.2) and earth orientation parameters. The EOP message provides users with parameters to construct the ECEFand ECI coordinate transformation (a simple transformation method is defined in Section 20.3.3.4.3.3.2). Thenumber of bits, scale factors (LSBs), the range, and the units of all EOP fields of message type 32 are given in Table30-VII.30.3.3.5.1.1 User Algorithm for Application of the EOP. The EOP fields in the message type 32 contain the EOPdata needed to construct the ECEF-to-ECI coordinate transformation. The user computes the ECEF position of theSV antenna phase center 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 in accordance with thecomputations detailed in Chapter 5 of IERS Technical Note 36: IERS Conventions (2010) and equations for UT1, xpand yp as documented in Table 30-VIII.Figure 5.1 on page 73 of that document depicts the computational flowstarting from GCRS (Geocentric Celestial Reference System) to ITRS (International Terrestrial Reference System).Ongoing WGS 84 re-adjustment at NGA and incorporating the 2010 IERS Conventions, are expected to bring Earthbased coordinate agreement to within 2 cm.
In the context of the Conventions, the user may as a matter ofconvenience choose to implement the transformation computations via either the Celestial Intermediate Origin(CIO) based approach” or the “Equinox based approach”. The EOP parameters for ∆UT1 are to be applied withinthe “Rotation to terrestrial system” process, and the parameters for xp and yp are applied in the “Rotation for polarmotion” process. Users are advised that the broadcast message type 32 EOP parameters already account for zonal,diurnal and semidiurnal effects (described in Chapter 8 of the IERS Conventions (2010)), so these effects should notbe further applied by the user.The relevant computations utilize elementary rotation matrices Ri(α), where α is a positive rotation about the ith-axisordinate, as follows:1R1 ( α ) = 000sin(α ) cos(α )0cos(α )− sin(α )cos(α ), R2 ( α ) = 0 sin(α ) cos (α )R3 ( α ) = − sin(α ) 0sin(α )cos(α )0010− sin(α )0 cos(α ) 001176IS-GPS-200F21 Sep 2011The Inertial-to-Geodetic rotation matrix shall be calculated in accordance with the computations detailed in Chapter5 of IERS Technical Note 36: IERS Conventions (2010) and equations for UT1, xp and yp as documented in Table30-VIII.Table 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-secondsX-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3arc-seconds/dayY-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 Difference atReference Time.31*2-2464secondsRate of UT1-UTCDifference at ReferenceTime19*2-257.8125 x 10-3seconds/day•PM_XPM_Y ††•PM_Y∆UT1 †††•∆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.177IS-GPS-200F21 Sep 2011Table 30-VIII.Application of EOP ParametersElement/EquationDescription•UT 1 = UTC + ∆UT 1 + ∆ UT 1 (t − t EOP )Compute 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).30.3.3.6 Message Type 33 Coordinated Universal Time (UTC) Parameters.
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