Интерфейсный документ GPS, страница 22

Message type 37 provides Midi almanac parameters and the reduced almanacparameters are provided in either message type 31 or type 12. The SV shall broadcast both message types 31(and/or 12) and 37. However, the reduced almanac parameters (i.e. message types 31 and/or 12) for the complete setof SVs in the constellation will be broadcast by a SV using shorter duration of time compared to the broadcast of thecomplete set of Midi almanac parameters (i.e. message type 37). The parameters are defined below, followed bymaterial pertinent to the use of the data.30.3.3.4.1 Almanac Reference Week.

Bits 39 through 51 of message type 12, and bits 128 through 140 of messagetypes 31 and 37 shall indicate the number of the week (WNa-n) to which the almanac reference time (toa) isreferenced (see paragraph 20.3.3.5.2.2). The WNa-n term consists of 13 bits which shall be a modulo-8192 binaryrepresentation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 52 through 59 ofmessage type 12, and bits 141 to 148 of message types 31 and 37 shall contain the value of toa, which is referencedto this WNa-n.30.3.3.4.2 Almanac Reference Time. See paragraph 20.3.3.5.2.2.30.3.3.4.3 SV PRN Number.

Bits 149 through 154 of message type 37 and bits 1 through 6 in each packet ofreduced almanac shall specify PRN number of the SV whose almanac or reduced almanac, respectively, is providedin the message or in the packet.30.3.3.4.4 Signal Health (L1/L2/L5). The three, one-bit, health indication in bits 155, 156, and 157 of message type37 and bits 29,30 and 31 of each packet of reduced almanac refers to the L1, L2, and L5 signals of the SV whosePRN number is specified in the message or in the packet.

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.IS-GPS-200D7 Dec 200417130.3.3.4.5 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 theunits of the almanac parameters are given in Table 30-V. The user algorithm is essentially the same as the useralgorithm used for computing the precise ephemeris as specified in Table 20-IV. Other parameters appearing in theequations of Table 20-IV, but not provided by the Midi almanac with the reference values, are set to zero for SVposition determination. 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.30.3.3.4.6.1 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).30.3.3.4.6.2.1 Reduced Almanac.

The reduced almanac data is provided in bits 7 through 28 of each packet. Thedata from a packet along with the reference values (see Table 30-VI) provide ephemeris with further reducedprecision. The user algorithm is essentially the same as the user algorithm used for computing the precise ephemerisfrom the parameters of the message types 10 and 11 (see paragraph 30.3.3.1.3 and Table 30-II). Other parametersappearing in the equations of Table 30-II, but not provided by the reduced almanac with the reference values, are setto zero for SV position determination.IS-GPS-200D7 Dec 2004172Table 30-V.ParameterNo. ofBits**Midi Almanac ParametersScaleFactor(LSB)EffectiveRange***12toa82e112-16dimensionlessδi****11*2-14semi-circles11*2-33semi-circles/secA172-4Ω016*2-15semi-circlesω16*2-15semi-circlesM016*2-15semi-circlesaf011*2-20secondsaf110*2-37sec/sec•Ω602,112Unitssecondsmeters* 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.IS-GPS-200D7 Dec 200417331 BITS17152229 30PRNaδAΩ0Φ06 BITS8 BITS7 BITS7 BITS31L1 HEALTHL2 HEALTH*See Figures 30-4 and 30-11 for complete bit allocation in therespective messages.L5 HEALTHFigure 30-15.

Reduced Almanac Packet ContentTable 30-VI. Reduced Almanac Parameters *****ParameterNo. of BitsScale 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.IS-GPS-200D7 Dec 200417430.3.3.5 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.30.3.3.5.1 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 EOPneeded to construct the ECEF-to-ECI coordinate transformation. The user computes the ECEF position of the SVantenna phase center using the equations shown in Table 30-II. The coordinate transformation, for translating to thecorresponding ECI SV antenna phase center position, is derived using the equations shown in Table 30-VIII.

Thecoordinate systems are defined in Section 20.3.3.4.3.3An ECI postion, Reci , is related to an ECEF position, Recef , by a series of rotation matrices as following:Recef = [A][B][C][D] Reciwhere the rotation matrices, A, B, C, and D, represent the effects of Polar Motion, Earth Rotation, Nutation andPrecession, respectively. The message type 32 specifies the EOP parameters used in the construction of the PolarMotion, A, and Earth Rotation, B, matrices.The rotation matrices, A, B, C and D are specified in terms of elementary rotation matrices, Ri(α), where α is apositive rotation about the ith-axis ordinate, as follows:1R1 ( α ) = 00sin (α ) cos(α )00cos(α )− sin (α ) cos (α )R3 ( α ) =  − sin(α ) 0sin(α )cos(α )0cos(α ), R2 ( α ) =  0 sin(α )010− sin(α )0 cos(α ) 001The user shall compute the Inertial-to-Geodetic rotation matrix, ABCD using the equations shown in Table 30-VIII.IS-GPS-200D7 Dec 2004175Table 30-VII.

Earth Orientation ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***tEOPEOP Data Reference Time1624604,784PM_X †X-Axis Polar Motion Valueat Reference Time.21*2-201X-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3Y-Axis Polar Motion Valueat Reference Time.21*2-201Y-Axis Polar Motion Drift atReference Time.15*2-217.8125 x 10-3UT1-UTC Difference atReference Time.31*2-2464Rate of UT1-UTCDifference at ReferenceTime19*2-257.8125 x 10-3•PM_XPM_Y ††•PM_Y∆UT1 †††•∆UT1 †††******Unitssecondsarc-secondsarc-seconds/dayarc-secondsarc-seconds/daysecondsseconds/dayParameters 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.IS-GPS-200D7 Dec 2004176Table 30-VIII.

Application of EOP Parameters (Part 1 of 2)Element/EquationDescriptionTDT = t + 51 s.184Compute Terrestrial Dynamical Time relative to GPSTime tJ.E.D. = TDT expressed in days of 86400 secCompute Julian Ephemeris Dateg=J.E.D. − 2451545 π °357.528+ 35999.° 05180° 36525J.B.D. = J.E.D. +T=Compute Mean Anomaly of Earth in its orbit, g0.s001658 sin ( g + 0.0167 sin g)Compute Julian Date in Barycentric Dynamical Time86400sJ.B.D.