IS-GPS-800D (797938), страница 11
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The data from apacket along with the reference values (see Table 3.5-6) provide ephemeris with further reducedprecision. The user algorithm is essentially the same as the user algorithm employed forcomputing the ephemeris from the parameters in subframe 2 (see Section 3.5.3.6.1 and Table3.5-2). Other parameters appearing in the equations of Table 3.5-2, but not provided by thereduced almanac with the reference values, are set to zero for SV position determination.An 8-bit value of “00000000” 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.Figure 3.5-9. Reduced Almanac Packet Content63IS-GPS-800D24 Sep 2013Table 3.5-6. Reduced Almanac Parameters *****Parameter*****No. of BitsScale Factor (LSB)Effective Range **UnitsδA ***8*2+9**MetersΩ07*2-6**semi-circles7*-6**semi-circlesΦ0 ****2*Parameters so indicated shall be in two’s complement notation;**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/second3.5.4.3.6 Midi Almanac Parameter ContentSubframe 3, page 4 shall contain Midi almanac data for a SV whose PRN number is specified inthe message.
The bit lengths, scale factors, ranges, and units of the almanac parameters aregiven in Table 3.5-7. The user algorithm is essentially the same as the user algorithm employedfor computing the ephemeris as specified in Table 20-IV of IS-GPS-200. Other parametersappearing in the equations of Table 20-IV of IS-GPS-200, but not provided by the Midi almanacwith the reference values, are set to zero for SV position determination. See paragraph20.3.3.5.2.3 of IS-GPS-200 for almanac time parameters.64IS-GPS-800D24 Sep 2013Table 3.5-7. Midi Almanac ParametersParameterNo.
ofBits**ScaleFactor(LSB)EffectiveRange***Unitstoa8212602,112secondse112-16dimensionlessδi****11*2-14semi-circles11*2-33semi-circles/secA172-4Ω016*2-15semi-circlesω16*2-15semi-circlesM016*2-15semi-circlesaf011*2-20secondsaf110*2-37sec/sec•Ωmeters* Parameters so indicated shall be in two's complement notation;** See Figure 3.5-5 for complete bit allocation in subframe 3, page 4;*** 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.3.5.4.4 Subframe 3, Page 5 - Differential CorrectionSubframe 3, page 5, Figure 3.5-6, contains the Differential Correction (DC) parameters. Theseparameters provide users with sets of correction terms that apply to the clock and ephemeris datatransmitted by other SVs.
DC parameters are grouped in packets, as described in this section.The availability of this data is subject to the control and determination of the CS. The contentsof subframe 3, page 5, are defined below, and followed by material pertinent to the use of thedata.65IS-GPS-800D24 Sep 20133.5.4.4.1 Differential Correction Parameter ContentSubframe 3, page 5 shall contain DC parameters that apply to the clock and ephemeris datatransmitted by another SV.
One subframe 3, page 5, as depicted in Figure 3.5-6, shall contain 34bits of clock differential correction (CDC) parameters and 92 bits of ephemeris differentialcorrection (EDC) parameters for one SV other than the transmitting SV. Bit 37 of subframe 3,page 5 shall be a DC Data Type indicator that indicates the data type for which the DCparameters apply. Zero (0) signifies that the corrections apply to CNAV-2 data, DL1C(t), and one(1) signifies that the corrections apply to NAV (legacy) data, D(t), defined in Appendix II of ISGPS-200.The content of an individual data packet is depicted in Figure 3.5-10.
The bit lengths, scalefactors, ranges, and units of all fields in the DC packet are given in Table 3.5-8.3.5.4.4.2 Differential Correction Data Predict Time of WeekThe DC data predict time of week (top-D) provides the epoch time of week, in increments of 300seconds (i.e.
five minutes), at which the prediction for the associated DC data was performed.3.5.4.4.3 Time of Differential Correction DataThe time of DC data, tOD, specifies the reference time of week, in increments of 300 seconds(i.e., five minutes) relative to the GPS week, for the associated CDC and EDC data.3.5.4.4.4 DC Data PacketEach DC data packet contains: corrections to SV clock polynomial coefficients provided insubframe 2 of the corresponding SV; corrections to quasi-Keplerian elements referenced to tODof the corresponding SV; and User Differential Range Accuracy (UDRA) and UDRA indicesthat enable users to estimate the accuracy obtained after corrections are applied.
Each DC packetis made up of two different segments. The first segment contains 34 bits for the CDC parametersand the second segment contains 92 bits of EDC parameters totaling 126 bits. The CDC andEDC parameters form an indivisible pair and users must utilize CDC and EDC as a pair.3.5.4.4.4.1 SV PRN IdentificationThe PRN ID of both CDC and EDC as depicted in Figure 3.5-10 identifies the satellite to whichthe subject 126-bit differential correction packet data applies (by PRN code assignment). Avalue of all ones “11111111” in any PRN ID field shall indicate that no DC data is contained inthe remainder of the data block. In this event, the remainder of the data block shall be filler bits,i.e., alternating ones and zeros beginning with one.3.5.4.4.4.2 Application of DC DataThe application of CDC data and EDC data is defined in paragraphs 30.3.3.7.3, 30.3.3.7.4, and30.3.3.7.5 of IS-GPS-200.66IS-GPS-800D24 Sep 2013CDC = Clock Differential CorrectionMSB19LSB223034PRN IDδaf0δaf1UDRA.8 BITS.13 BITS.8 BITS.5 BITSEDC = Ephemeris Differential CorrectionMSB1LSB23936PRN IDΔαΔβ.8 BITS.14 BITS.14 BITSLSBMSB523763ΔiΔγ.12 BITS.15 BITS.LSBMSB88766492ΔΩΔA•UDRA12 BITS12 BITS5 BITSFigure 3.5-10.
Differential Correction Data Packet67IS-GPS-800D24 Sep 2013Table 3.5-8. Differential Correction ParametersParameterPRN IDNo. ofBits**ScaleFactor(LSB)8EffectiveRange***Unitssee textδaf0SV Clock Bias Correction13*2-35secondsδaf1SV Clock Drift Correction8*2-51seconds/secondUser Differential RangeAccuracy Index5*∆αAlpha Correction to EphemerisParameters14*2-34dimensionless∆βBeta Correction to EphemerisParameters14*2-34dimensionless∆γGamma Correction to EphemerisParameters15*2-32semi-circles∆iAngle of Inclination Correction12*2-32semi-circles∆ΩAngle of Right AscensionCorrection12*2-32semi-circles∆ASemi-Major Correction12*2-9metersChange Rate of User DifferentialRange Accuracy Index.5*UDRA•UDRA******see textsee textParameters so indicated are in two’s complement notation;See Figure 3.5-6 for complete bit allocation in subframe 3, page 5;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.3.5.4.5 Subframe 3, Page 6 - TextSubframe 3, page 6, as depicted in Figure 3.5-7, contains the Text message.
The specificcontents of text message will be at the discretion of the Operating Command. Subframe 3, page6 can accommodate the transmission of 29 eight-bit American Standard Code for InformationInterchange (ASCII) characters. The requisite bits shall occupy bits 19 through 250 of subframe3, page 6. The eight-bit ASCII characters shall be limited to the set described in paragraph20.3.3.5.1.8 of IS-GPS-200.68IS-GPS-800D24 Sep 20133.5.4.6 Subframe 3, Page 7 - (Reserved)(Reserved)3.5.5 Timing RelationshipsThe following conventions shall apply.3.5.5.1 Paging and CutoversBroadcast sequence of subframe 3 pages is completely arbitrary and, as such, users must notexpect a fixed pattern of page sequence.Cutovers of subframe 2 data to new data sets will nominally occur on hour boundaries except forthe first data set of a new upload.
The first data set of newly uploaded data will cutover on 15minute boundaries.3.5.5.2 Data SetsThe toe shall be equal to the toc of the same CNAV data set. The following rules govern thetransmission of toe and toc values in different data sets: (1) The transmitted toc will be differentfrom any value transmitted by the SV during the preceding seven days; (2) The transmitted toewill be different from any value transmitted by the SV during the preceding six hours.Cutovers to new data sets will occur only on hour boundaries except for the first data set of anew upload. The first data set may be cut-in (reference paragraph 3.5.5.1) at any time during thehour and therefore may be transmitted by the SV for less than one hour.The start of the transmission interval for each data set corresponds to the beginning of the curvefit interval for the data set.
Each data set remains valid for the duration of its transmissioninterval, and nominally also remains valid for the duration of its curve fit interval. A data set isrendered invalid before the end of its curve fit interval when it is superseded by the SV cuttingover to the first data set of a new upload.Normal Operations. The subframe 2 data sets are transmitted by the SV for periods of two hours.The corresponding curve fit interval is three hours.3.5.5.3 Reference TimesThe CNAV reference time information may be found in paragraph 30.3.4.5 in IS-GPS-200.69IS-GPS-800D24 Sep 20134 NOT APPLICABLE70IS-GPS-800D24 Sep 20135 RESERVED71IS-GPS-800D24 Sep 20136 NOTES6.1AcronymsAPC-antenna phase centerASCII-American Standard Code for Information InterchangeBCH-Bose, Chaudhuri, and HocquenghemBOC-Binary Offset CarrierBPSK-Bi-Phase Shift KeyCCB-Configuration Control BoardCDC-clock differential correctionCNAV-2--L1C Navigation MessageCRC-Cyclic Redundancy CheckCS-Control SegmentdBc-Power ratio of a signal to a (unmodulated) carrier signal,expressed in decibelsDC-differential correctionDN-Day NumberECEF-Earth-Centered, Earth-FixedECI-Earth-Centered, InertialEDC-ephemeris differential correctionEOE-Edge-of-EarthEOL-End-of-LifeEOP-Earth Orientation ParametersFEC-Forward Error CorrectionGBAS-Ground Based Augmentation SystemGGTO-GPS/GNSS Time OffsetGNSS-Global Navigation Satellite System72IS-GPS-800D24 Sep 2013GPS-Global Positioning SystemGPSW-GPS WingICC-Interface Control ContractorICWG-Interface Control Working GroupIRN-Interface Revision NoticeIS-Interface SpecificationISC-Inter-Signal CorrectionITOW-Interval Time of WeekLDPC-Low Density Parity CheckLFSR-Linear Feedback Shift RegisterLSB-Least Significant BitLSF-Leap Seconds FutureL1C-Common L1 SignalMCS-Master Control StationMHz-MegahertzMSB-Most Significant BitNAV-Legacy Navigation Message, D(t)NSCD-non-standard L1CDNSCP-non-standard L1CPPIRN-Proposed Interface Revision NoticePRN-Pseudo-Random NoiseRF-Radio FrequencyRHCP-Right-Hand Circularly PolarizedRMS-Root Mean SquareSBAS-Satellite Based Augmentation Systemsps-symbols per secondSS-Space Segment73IS-GPS-800D24 Sep 2013SSV-Space Service VolumeSV-Space VehicleTBD-To Be DeterminedTBR-To Be ResolvedTBS-To Be SuppliedTMBOC-Time-Multiplexed BOCTOI-Time of IntervalTOW-Time of WeekUDRA-User Differential Range AccuracyUE-User EquipmentURA-User Range AccuracyUS-User SegmentUSNO-U.S.
Naval ObservatoryUTC-Coordinated Universal TimeWGS 84-World Geodetic System 198474IS-GPS-800D24 Sep 20136.2Definitions6.2.1 User Range AccuracyUser Range Accuracy (URA) is a statistical indicator of the GPS ranging accuracy obtainablewith a specific signal and SV. URA provides a conservative RMS estimate of the user rangeerror (URE) in the associated navigation data for the transmitting SV. It includes all errors forwhich the Space and Control Segments are responsible. Whether the integrity status flag is 'off'or 'on', 4.42 times URA bounds the instantaneous URE under all conditions with 1-(1e-5) perhour probability (‘legacy’ level of integrity assurance). When the integrity status flag is 'on',5.73 times URA bounds the instantaneous URE under all conditions with 1-(1e-8) per hourprobability (‘enhanced’ level of integrity assurance).
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