IS-GPS-800D (797938), страница 7
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Subframe 2 - Clock, Ephemeris, ITOW32IS-GPS-800D24 Sep 2013Figure 3.5-2. Subframe 3, Page 1 - UTC & IONO33IS-GPS-800D24 Sep 2013DIRECTION OF DATA FLOW FROM SVMSB FIRST100 BITS9115 18PRN PageNo8 BITS4734tGGTO16 BITSWNGGTO13 BITS63A0-GGTO16 BITS998376A1-GGTO A2-GGTO13 BITS 7 BITStEOP16 BITS6 BITSPM -X – 2 MSBsGNSS ID– 3 BITSDIRECTION OF DATA FLOW FROM SVMSB FIRST100 BITS101120PM-X19 LSBs•135PM -X15 BITS156PM-Y21 BITSDIRECTION OF DATA FLOW FROM SV•171∆UT130 MSBsPM -Y15 BITSMSB FIRST74 BITS202•∆UT119 BITS251221RESERVED30 BITS274CRC24 BITS∆UT1– 1 LSBFigure 3.5-3 Subframe 3, Page 234IS-GPS-800D24 Sep 2013Figure 3.5-4.
Subframe 3, Page 3 - Reduced Almanac35IS-GPS-800D24 Sep 2013Figure 3.5-5. Subframe 3, Page 4 - Midi Almanac36IS-GPS-800D24 Sep 2013Figure 3.5-6. Subframe 3, Page 5 - Differential Correction37IS-GPS-800D24 Sep 2013Figure 3.5-7. Subframe 3, Page 6 – Text38IS-GPS-800D24 Sep 2013(Reserved)Figure 3.5-8. Subframe 3, Page 7 - (Reserved)3.5.3 Subframe 2Subframe 2 provides users with the requisite data to correct SV time and to calculate SVposition. Nominally, the subframe 2 data is invariant for the nominal transmission interval oftwo hours. The contents of the SV ephemeris representation, clock correction, and accuracyparameters are defined below, followed by material pertinent to the use of the data.The general format of ephemeris data of subframe 2 consists of data fields for reference timetags, a set of gravitational harmonic correction terms, rates and rate corrections to quasiKeplerian elements, and an accuracy indicator for ephemeris-related data.The ephemeris parameters describe the orbit of the transmitting SV during the curve fit intervalof three hours.
The nominal transmission interval is two hours, and shall coincide with the firsttwo hours of the curve fit interval. The period of applicability for ephemeris data coincides withthe entire three-hour curve fit interval. Table 3.5-1 gives the definition of the orbital parametersusing terminology typical of Keplerian orbital parameters; it is noted, however, that thetransmitted parameter values are expressed such that they provide the best trajectory fit in EarthCentered, Earth-Fixed (ECEF) coordinates for each specific fit interval.The user shall not interpret intermediate coordinate values as pertaining to any conventionalcoordinate system.Any change in the subframe 2 ephemeris and clock data shall be accomplished with asimultaneous change in the toe value. The SV shall assure that the toe value, for at least the firstdata set transmitted by an SV after an upload, is different from that transmitted prior to thecutover.
The eight LSBs of toe for each data set shall be different from the eight LSBs of toetransmitted during the previous six hours by the SV.The general format of clock data in subframe 2 consists of data fields for SV clock correctioncoefficients. The clock parameters of subframe 2 describe the SV time scale during the period ofvalidity. The parameters are applicable during the time in which they are transmitted. Beyondthat time, they are still applicable, however, the most recent data set should be used since theaccuracy degrades over time.39IS-GPS-800D24 Sep 2013Table 3.5-1. Subframe 2 Parameters (1 of 3)No. ofBits**ScaleFactor(LSB)Week No.131Interval time of week8Data predict time of week11ParameterWNITOWtopL1C healthURAED Indextoe∆A ****•A∆n0•**********300EffectiveRange***Unitsweeks83(see text)604,500seconds1(see text)ED accuracy index5*(see text)Ephemeris/clock datareference time of week11300Semi-major axis difference atreference time26*2-9metersChange rate in semi-majoraxis25*2-21meters/sec17*2-44semi-circles/secMean Motion difference fromcomputed value at referencetime604,500seconds∆n0Rate of mean motiondifference from computedvalue23*2-57semi-circles/sec2M0-nMean anomaly at referencetime33*2-32semi-circlesenEccentricity332-34dimensionlessωnArgument of perigee33*2-32semi-circlesParameters so indicated are in two’s complement notation;See Figure 3.5-1 for complete bit allocation in Subframe 2;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.Relative to AREF = 26,559,710 meters.40IS-GPS-800D24 Sep 2013Table 3.5-1.
Subframe 2 Parameters (2 of 3)No. ofBits**ScaleFactor(LSB)Longitude of Ascending Node of OrbitPlane at Weekly Epoch33*2-32semi-circlesRate of right ascension difference17*2-44semi-circles/secInclination angle at reference time33*2-32semi-circlesRate of inclination angle15*2-44semi-circles/secCis-nAmplitude of the sine harmonic correctionterm to the angle of inclination16*2-30radiansCic-nAmplitude of the cosine harmoniccorrection term to the angle of inclination16*2-30radiansCrs-nAmplitude of the sine correction term tothe orbit radius24*2-8metersCrc-nAmplitude of the cosine correction term tothe orbit radius24*2-8metersCus-nAmplitude of the sine harmonic correctionterm to the argument of latitude21*2-30radiansCuc-nAmplitude of the cosine harmoniccorrection term to the argument of latitude21*2-30radiansParameterΩ0-n•∆Ω****i0-ni0-n –DOT**********EffectiveRange***UnitsParameters so indicated are in two’s complement notation;See Figure 3.5-1 for complete bit allocation in Subframe 2;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.•Relative to ΩREF = -2.6 x 10-9 semi-circles/second.41IS-GPS-800D24 Sep 2013Table 3.5-1.
Subframe 2 Parameters (3 of 3)No. ofBits**ParameterScaleFactor(LSB)EffectiveRange***UnitsURANED0 IndexNED Accuracy Index5*(see text)URANED1 IndexNED Accuracy Change Index3(see text)URANED2 IndexNED Accuracy Change Rate Index3(see text)af2-nSV Clock Drift Rate Correction10*2-60sec/sec2Coefficientaf1-nSV Clock Drift Correction Coefficient20*2-48sec/secaf0-nSV Clock Bias Correction Coefficient26*2-35secondsInter-Signal Correction for L1 or L213*2-35secondsTGD****P(Y)ISCL1CP****Inter-Signal Correction for L1CP13*2-35secondsISCL1CD****Inter-Signal Correction for L1CD13*2-35seconds81weeksWNOPData Predict Week Number*Parameters so indicated are in two’s complement notation;**See Figure 3.5-1 for complete bit allocation in Subframe 2;***Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.****The bit string of “1000000000000” will indicate that the group delay value is not available.42IS-GPS-800D24 Sep 20133.5.3.1 Transmission Week NumberBits 1 through 13 of subframe 2 shall contain 13 bits that are a modulo-8192 binaryrepresentation of the current GPS week number at the start of the data set transmission interval(see paragraph 6.2.2).3.5.3.2 ITOWBits 14 through 21 of subframe 2 shall contain 8 bits representing ITOW count defined as beingequal to the number of two-hour epochs that have occurred since the transition from the previousweek.
The count is short-cycled such that the range of the ITOW-count is from 0 to 83 2-hourepochs (equaling one week) and is reset to zero at the end of each week. The ITOW-count's zerostate is defined as that 2-hour epoch which is coincident with the start of the present week. Thisepoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight isdefined as 0000 hours on the UTC scale that is nominally referenced to the Greenwich Meridian.The occurrence of the “zero state epoch” may differ by a few seconds from 0000 hours on theUTC scale since UTC is periodically corrected with leap seconds while GPS time is continuouswithout such correction.3.5.3.3 Data Predict Time of WeekBits 22 through 32 of subframe 2 shall contain the data predict time of week (top). The top termprovides the epoch time of week of the state estimate utilized for the prediction of satellite quasiKeplerian ephemeris parameters.3.5.3.4 L1C Signal HealthThe one-bit health indication in bit 33 of subframe 2 refers to the L1C signal of the transmittingSV.
The health of the signal is indicated by:0 = Signal OK,1 = Signal bad or unavailable.The predicted health data will be updated at the time of upload when a new data set has beenbuilt by the CS. The transmitted health data may not correspond to the actual health of thetransmitting SV. In real time, if the L1C signal becomes unhealthy, the status change willnormally be indicated by the broadcast of non-standard code or be indicated by the health bits asdescribed in subframe 2.Additional SV health data are given in the almanac in subframe 3, pages 3 and 4. The data givenin subframe 2 may differ from that shown in other messages of the transmitting SV and/or other43IS-GPS-800D24 Sep 2013SVs since the latter may be updated at a different time. Subframe 2 data is the most reliable;subframe 3 data is intended only to aid acquisition.3.5.3.5 SV AccuracyBits 34 through 38 of subframe 2 shall contain the elevation-dependent (ED) component UserRange Accuracy (URAED) index for the unauthorized user.
The URAED index shall provide theED-related URA index for the current ephemeris curve fit interval. While the ED-related URAmay vary over the ephemeris curve fit interval and over the satellite footprint, the URAED index(N) in subframe 2 shall correspond to the maximum URAED expected over the entire ephemeriscurve fit interval for the worst-case location within the SV footprint (i.e., nominally two points atthe edge of the SV footprint).
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