Интерфейсный документ GPS (1014597), страница 15
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The six-bit words provide a one-bit summary of the NAV data's health status in the MSB position inaccordance with paragraph 20.3.3.3.1.4. The five LSBs of both the eight-bit and the six-bit words provide thehealth status of the SV's signal components in accordance with the code given in Table 20-VIII. A specialmeaning is assigned, however, to the "6 ones" combination of the six-bit health words in the 25th page ofsubframes 4 and 5: it indicates that "the SV which has that ID is not available and there may be no data regardingthat SV in that page of subframes 4 and 5 that is assigned to normally contain the almanac data of that SV"(NOTE: this special meaning applies to the 25th page of subframes 4 and 5 only). The health indication shall begiven relative to the "as designed" capabilities of each SV (as designated by the configuration code -- see paragraph20.3.3.5.1.4).
Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if thelack of this capability is inherent in its design or it has been configured into a mode which is normal from a userstandpoint and does not require that capability.Additional SV health data are given in subframe 1. The data given in subframes 1, 4, and 5 of the other SVs maydiffer from that shown in subframes 4 and/or 5 since the latter may be updated at a different time.The eight-bit health status words shall occupy bits 17 through 24 of word five in those 32 pages which containalmanac data for individual SVs.
The six-bit health status words shall occupy the 24 MSBs of words four throughnine in page 25 of subframe 5 plus bits 19 through 24 of word 8, the 24 MSBs of word 9, and the 18 MSBs of word10 in page 25 of subframe 4.The predicted health data will be updated at the time of upload when a new almanac has been built by the CS. Thetransmitted health data may not correspond to the actual health of the transmitting SV or other SVs in theconstellation.IS-GPS-200D7 Dec 2004108Table 20-VII. NAV Data Health IndicationsBit Position in PageIndication137138139000ALL DATA OK001PARITY FAILURE -- some or all parity bad010TLM/HOW FORMAT PROBLEM -- any departure from standard format (e.g.,preamble misplaced and/or incorrect, etc.), except for incorrect Z-count, asreported in HOW011Z-COUNT IN HOW BAD -- any problem with Z-count value not reflectingactual code phase100SUBFRAMES 1, 2, 3 -- one or more elements in words three through ten ofone or more subframes are bad101SUBFRAMES 4, 5 -- one or more elements in words three through ten of oneor more subframes are bad110ALL UPLOADED DATA BAD -- one or more elements in words threethrough ten of any one (or more) subframes are bad111ALL DATA BAD -- TLM word and/or HOW and one or more elements in anyone (or more) subframes are badIS-GPS-200D7 Dec 2004109Table 20-VIII.
Codes for Health of SV Signal ComponentsMSBLSB0000000001000100001100100001010011000111010000100101010010110110001101011100111110000100011001010011101001010110110101111100011001110101101111100111011111011111***DefinitionAll Signals OKAll Signals Weak*All Signals DeadAll Signals Have No Data ModulationL1 P Signal WeakL1 P Signal DeadL1 P Signal Has No Data ModulationL2 P Signal WeakL2 P Signal DeadL2 P Signal Has No Data ModulationL1 C Signal WeakL1 C Signal DeadL1 C Signal Has No Data ModulationL2 C Signal WeakL2 C Signal DeadL2 C Signal Has No Data ModulationL1 & L2 P Signal WeakL1 & L2 P Signal DeadL1 & L2 P Signal Has No Data ModulationL1 & L2 C Signal WeakL1 & L2 C Signal DeadL1 & L2 C Signal Has No Data ModulationL1 Signal Weak*L1 Signal DeadL1 Signal Has No Data ModulationL2 Signal Weak*L2 Signal DeadL2 Signal Has No Data ModulationSV Is Temporarily Out (Do not use this SV during current pass)**SV Will Be Temporarily Out (Use with caution)**SpareMore Than One Combination Would Be Required To Describe Anomalies (Not including thosemarked with “**”)3 to 6 dB below specified power level due to reduced power output, excess phase noise, SV attitude, etc.See definition above for Health Code 11111.IS-GPS-200D7 Dec 200411020.3.3.5.1.4 Anti-Spoof (A-S) Flags and SV Configurations.
Page 25 of subframe 4 shall contain a four-bit-longterm for each of up to 32 SVs to indicate the A-S status and the configuration code of each SV. The MSB of eachfour-bit term shall be the A-S flag with a "1" indicating that A-S is ON. The three LSBs shall indicate theconfiguration of each SV using the following code:CodeSV Configuration001“Block II/IIA/IIR” SV (A-S capability, plus flags for A-S and "alert" in HOW; memorycapacity as described in paragraph 20.3.2).010“Block IIR-M” SV011“Block IIF” SVAdditional codes will be assigned in the future, should the need arise.These four-bit terms shall occupy bits 9 through 24 of word three, the 24 MSBs of words four through seven, andthe 16 MSBs of word eight, all in page 25 of subframe 4.Since the anti-spoof information is updated by the CS at the time of upload, the anti-spoof data may not correspondto the actual anti-spoof status of the transmitting SV or other SVs in the constellation.IS-GPS-200D7 Dec 200411120.3.3.5.1.5 Almanac Reference Week.
Bits 17 through 24 of word three in page 25 of subframe 5 shall indicatethe number of the week (WNa) to which the almanac reference time (toa) is referenced (see paragraphs 20.3.3.5.1.2and 20.3.3.5.2.2). The WNa term consists of eight bits which shall be a modulo 256 binary representation of theGPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 9 through 16 of word three in page 25 ofsubframe 5 shall contain the value of toa which is referenced to this WNa.20.3.3.5.1.6 Coordinated Universal Time (UTC) Parameters. The 24 MSBs of words six through nine plus theeight MSBs of word ten in page 18 of subframe 4 shall contain the parameters related to correlating UTC time withGPS time.
The bit length, scale factors, ranges, and units of these parameters are given in Table 20-IX. Therelated algorithms are described in paragraph 20.3.3.5.2.4.The UTC parameters shall be updated by the CS at least once every six days while the CS is able to upload theSVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameters transmitted by the SVs willdegrade over time.20.3.3.5.1.7 Ionospheric Data. The ionospheric parameters which allow the "L1 only" or "L2 only" user to utilizethe ionospheric model (reference paragraph 20.3.3.5.2.5) for computation of the ionospheric delay are contained inpage 18 of subframe 4. They occupy bits 9 through 24 of word three plus the 24 MSBs of words four and five.
Thebit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X.The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload theSVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate.IS-GPS-200D7 Dec 2004112Table 20-IX.Parameter*******EffectiveRange***Units32*2-30secondsA124*2-50sec/sec∆ tLS8*1secondstot8212WNt81weeksWNLSF81weeks8****18*1∆ tLSF**ScaleFactor(LSB)A0DN*No.
ofBits**UTC Parameters602,1127secondsdayssecondsParameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;See Figure 20-1 for complete bit allocation in subframe;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;Right justified.IS-GPS-200D7 Dec 2004113Table 20-X.******Ionospheric ParametersParameterNo.
ofBits**ScaleFactor(LSB)α08*2-30secondsα18*2-27sec/semi-circleα28*2-24sec/(semi-circle)2α38*2-24sec/(semi-circle)3β08*211secondsβ18*214sec/semi-circleβ28*216sec/(semi-circle)2β38*216sec/(semi-circle)3EffectiveRange***UnitsParameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;See Figure 20-1 for complete bit allocation in subframe;Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor.IS-GPS-200D7 Dec 200411420.3.3.5.1.8 Special Messages. Page 17 of subframe 4 shall be reserved for special messages with the specificcontents at the discretion of the Operating Command. It shall accommodate the transmission of 22 eight-bit ASCIIcharacters.
The requisite 176 bits shall occupy bits 9 through 24 of word three, the 24 MSBs of words four throughnine, plus the 16 MSBs of word ten. The eight MSBs of word three shall contain the data ID and SV ID, while bits17 through 22 of word ten shall be reserved for system use. The remaining 50 bits of words three through ten areused for parity (six bits/word) and parity computation (two bits in word ten).
The eight-bit ASCII characters shallbe limited to the following set:Alphanumeric CharacterASCII CharacterCode (Octal)A-ZA-Z101 - 1320-90-9060 - 071++053--055. (Decimal point).056' (Minute mark)'047° (Degree sign)°370//057BlankSpace040::072" (Second mark)"042IS-GPS-200D7 Dec 200411520.3.3.5.1.9 NMCT.
Page 13 of subframe 4 shall contain the NMCT data appropriate to the transmitting SV. EachNMCT contains a two-bit availability indicator (AI) followed by 30 slots which may contain up to 30 valid six-bitERD values. The layout of these 31 data items is as shown in Figure 20-1.The two-bit AI in bits 9 and 10 of word three of page 13 of subframe 4 provide the user with the followinginformation.AINavigation Message Correction Table Availability00The correction table is unencrypted and is available to both authorized and unauthorized users.01The correction table is encrypted and is available only to authorized users (normal mode).10No correction table available for either authorized or unauthorized users.11Reserved.Each one of the 30 six-bit ERD slots in bits 11 through 24 of word three, bits 1 through 24 of words four throughnine, and bits 1 through 22 of word ten of page 13 of subframe 4 will correspond to an ERD value for a particularSV ID.
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