IS-GPS-800D (797938), страница 10
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The bit lengths, scale factors, ranges, and units of these parameters are given inTable 3.5-3. See Figure 3.5-2 for complete bit allocation in subframe 3, page 1.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.3.5.4.1.1.1 UTC and GPS TimeSubframe 3, page 1 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 navigationmessage upload) value of the delta time due to leap seconds (∆tLSF), together with the weeknumber (WNLSF) and the day number (DN) at the end of which the leap second becomeseffective. Information required to use these parameters to calculate 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 + 604800 (WN - WNot)) + A2-n (tE - tot + 604800 (WN WNot))2 seconds.57IS-GPS-800D24 Sep 2013Table 3.5-3.
UTC ParametersParameterNo. ofBits**Scale Factor(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 of GPStime scale relative to UTC time scale7*2-68sec/sec2∆tLSCurrent or past leap second count8*1secondstotTime data reference Time of Week1624WNotTime data reference Week Number131weeksLeap second reference Week Number131weeksWNLSF604,784secondsDNLeap second reference Day Number4****1days∆tLSFCurrent or future leap second count8*1seconds**********Parameters so indicated shall be in two's complement notation;See Figure 3.5-2 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.3.5.4.1.2 Ionospheric DataThe ionospheric parameters which allow the “L1 only” user to utilize the ionospheric model forcomputation of the ionospheric delay are contained in subframe 3, page 1.
The “one frequency”user should use the model given in paragraph 20.3.3.5.2.5 of IS-GPS-200 to make this correctionfor the ionospheric effects. The bit lengths, scale factors, ranges, and units of these parametersare given in Table 20-X of IS-GPS-200.The ionospheric data shall be updated by the CS at least once every six days while the CS is ableto upload the SVs.
If the CS is unable to upload the SVs, the ionospheric data transmitted by theSVs may not be accurate.58IS-GPS-800D24 Sep 20133.5.4.2 Subframe 3, Page 2 - GGTO & EOPSubframe 3, page 2, as depicted in Figure 3.5-3, contains the GPS/Global Navigation SatelliteSystem (GNSS) Time Offset (GGTO) parameters and Earth Orientation Parameters (EOP). Thecontents of subframe 3, page 2, is defined below, and followed by material pertinent to the use ofthe data.3.5.4.2.1 GGTO Parameter ContentSubframe 3, page 2 shall contain the parameters related to correlating GPS time with otherGNSS time.
Bits 15 through 17 of subframe 3, page 2 shall identify the other GPS-likenavigation system to which the offset data applies. The three bits are defined as follows:000 = no data available,001 = Galileo,010 = GLONASS,011 through 111 = reserved for other systems.The bit lengths, scale factors, ranges, and units of the GGTO parameters are given in Table 3.54. See Figure 3.5-3 for complete bit allocation in subframe 3, page 2.The validity period of the GGTO shall be 1 day as a minimum.3.5.4.2.1.1 GPS and GNSS TimeThe GPS/GNSS-time relationship is given by,tGNSS = tE - (A0GGTO + A1GGTO (tE - tGGTO + 604,800 (WN - WNGGTO)) + A2GGTO (tE - tGGTO +604,800 (WN - WNGGTO))2)where tGNSS is in seconds, tE and WN are as defined in Section 20.3.3.5.2.4 of IS-GPS-200, andthe remaining parameters are as defined in Table 3.5-4.The GGTO parameters provide a global average of the time offset between GPS time and theother GNSS time scales modulo one second.
Users must also apply any integer secondsdifference between the systems using definitions of each system time scale as defined inrespective signal interface documents.59IS-GPS-800D24 Sep 2013Table 3.5-4. GPS/GNSS Time Offset ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***UnitsA0GGTOBias coefficient of GPS time scalerelative to GNSS time scale16*2-35secondsA1GGTODrift coefficient of GPS time scalerelative to GNSS time scale13*2-51sec/secA2GGTODrift rate correction coefficient ofGPS time scale relative to GNSStime scale7*2-68sec/sec2tGGTOTime data reference Time of Week1624WNGGTOTime data reference WeekNumber1320GNSS IDGNSS Type ID3******604,784secondsweekssee textParameters so indicated shall be in two's complement notation;See Figure 3.5-3 for complete bit allocation;Unless otherwise indicated in this column, effective range is the maximum rangeattainable with indicated bit allocation and scale factor.3.5.4.2.2 EOP Parameter ContentSubframe 3, page 2 shall contain earth orientation parameters.
The EOP message provides userswith parameters to construct the ECEF and ECI coordinate transformation (a simpletransformation method that does not account for EOP, is defined in Section 20.3.3.4.3.3.2 of ISGPS-200). The bit lengths, scale factors, ranges, and units of all EOP fields of subframe 3, page2, are given in Table 3.5-5.3.5.4.2.3 User Algorithm for Application of the EOPThe EOP fields in subframe 3, page 2 contain the EOP needed to construct the ECEF-to-ECIcoordinate transformation.
The user computes the ECEF position of the SV antenna phase centerusing the equations shown in Table 3.5-2. The coordinate transformation, for translating to thecorresponding ECI SV antenna phase center position, is derived using the equations shown inIERS Technical Note 36 and Table 30-VIII of IS-GPS-200. The coordinate systems are definedin Section 20.3.3.4.3.3 of IS-GPS-200.60IS-GPS-800D24 Sep 2013Table 3.5-5.
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-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_Y∆UT1 †††•∆UT1 †††******Parameters so indicated are in two’s complement notation;See Figure 3.5-3 for complete bit allocation in subframe 3, page 2;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.3.5.4.3 Subframe 3, Page 3 & Page 4 - AlmanacThe almanac parameters are provided in any one of subframe 3, pages 3 and 4.
Page 3 providesthe reduced almanac parameters and Midi almanac parameters are provided in page 4. Thereduced almanac parameters (i.e. subframe 3, page 3) for the complete set of SVs in theconstellation will be broadcast by a SV using shorter duration of time compared to the broadcastof the complete set of Midi almanac parameters (i.e.
subframe 3, page 4). The parameters aredefined below, followed by material pertinent to the use of the data.61IS-GPS-800D24 Sep 20133.5.4.3.1 Almanac Reference WeekBits 15 through 27 of subframe 3, pages 3 and 4 shall indicate the number of the week (WNa-n) towhich the almanac reference time (toa) is referenced (see paragraph 3.5.4.3.2). The WNa-n termconsists of 13 bits which shall be a modulo-8192 binary representation of the GPS week number(see paragraph 6.2.2) to which the toa is referenced. Bits 28 through 35 of subframe 3, pages 3and 4 shall contain the value of toa, which is referenced to this WNa-n.3.5.4.3.2 Almanac Reference TimeSee paragraph 20.3.3.5.2.2 of IS-GPS-200.3.5.4.3.3 SV PRN NumberBits 36 through 43 of subframe 3, page 4 and bits 1 through 8 in each packet of reduced almanacshall specify PRN number of the SV whose almanac or reduced almanac, respectively, isprovided in the message or in the packet.3.5.4.3.4 Signal Health (L1/L2/L5)The three, one-bit, health indication in bits 44, 45 and 46 of subframe 3, page 4 and bits 31, 32and 33 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 all signals on the associated frequency are okay and “1” signifies that some or allsignals on the associated frequency are bad. The predicted health data will be updated at thetime of upload when a new reduced almanac has been built by the CS. The transmitted healthdata may not correspond to the actual health of the transmitting SV or other SVs in theconstellation.3.5.4.3.5 Reduced Almanac DataSubframe 3, page 3, Figure 3.5-4, shall contain reduced almanac data packets for 6 SVs.
Thereduced almanac data of a SV is broadcast in a packet of 33 bits long, as described in Figure 3.59. The reduced almanac data are a subset of the almanac data which provide an ephemeris withless precision than that derived from parameters in subframe 2. The reduced almanac data valuesare provided relative to pre-specified reference values. The bit lengths, scale factors, ranges, andunits of the reduced almanac parameters are given in Table 3.5-6.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 then the accuracy of thereduced almanac parameters transmitted by the SVs will degrade over time.3.5.4.3.5.1 Reduced Almanac PacketThe following shall apply when interpreting the data provided in each packet of reduced almanac(see Figure 3.5-9).62IS-GPS-800D24 Sep 20133.5.4.3.5.1.1 Reduced AlmanacThe reduced almanac data is provided in bits 9 through 30 of each packet.
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