IS-GPS-200H (797934), страница 18
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The MSB of each four-bit term shall be theA-S flag with a "1" indicating that A-S is ON. The three LSBs shall indicate the configuration ofeach SV using the following code:115IS-GPS-200H24 Sep 2013Code SV Configuration000Reserved001 A-S capability, plus flags for A-S and "alert" in HOW; memory capacity asdescribed in paragraph 20.3.2 (e.g. Block II/Block IIA/IIR SV).010A-S capability, plus flags for A-S and "alert" in HOW; memory capacity asdescribed in paragraph 20.3.2, M-Code signal capability, L2C signal capability (e.g.,Block IIR-M SV).011 A-S capability, plus flags for A-S and "alert" in HOW; memory capacity asdescribed in paragraph 20.3.2, M-Code capability, L2C signal capability, L5 signalcapability (e.g., Block IIF SV).100A-S capability, plus flags for A-S and "alert" in HOW; memory capacity asdescribed in paragraph 20.3.2, M-Code capability, L1C signal capability, L2C signalcapability, L5 signal capability, no SA capability (e.g., GPS III SVs).Additional 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 fourthrough seven, and the 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 datamay not correspond to the actual anti-spoof status of the transmitting SV or other SVs in theconstellation.20.3.3.5.1.5 Almanac Reference Week.Bits 17 through 24 of word three in page 25 of subframe 5 shall indicate the number of the week(WNa) to which the almanac reference time (toa) is referenced (see paragraphs 20.3.3.5.1.2 and20.3.3.5.2.2).
The WNa term consists of eight bits which shall be a modulo 256 binaryrepresentation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits9 through 16 of word three in page 25 of subframe 5 shall contain the value of toa which isreferenced to this WNa.20.3.3.5.1.6 Coordinated Universal Time (UTC) Parameters.The 24 MSBs of words six through nine plus the eight MSBs of word ten in page 18 of subframe4 shall contain the parameters related to correlating UTC time with GPS time. The bit length,scale factors, ranges, and units of these parameters are given in Table 20-IX.
The relatedalgorithms are described in paragraph 20.3.3.5.2.4.116IS-GPS-200H24 Sep 2013The UTC parameters 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 accuracy of the UTC parameterstransmitted by the SVs will degrade over time.20.3.3.5.1.7 Ionospheric Data.The ionospheric parameters which allow the "L1 only" or "L2 only" user to utilize theionospheric model (reference paragraph 20.3.3.5.2.5) for computation of the ionospheric delayare contained in page 18 of subframe 4. They occupy bits 9 through 24 of word three plus the 24MSBs of words four and five. The bit lengths, scale factors, ranges, and units of theseparameters 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 ableto upload the SVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by theSVs may not be accurate.117IS-GPS-200H24 Sep 2013ParameterNo.
ofBits**Table 20-IX.UTC ParametersScaleFactor(LSB)EffectiveRange***UnitsA032*2-30secondsA124*2-50sec/sec∆ tLS8*1secondstot8212WNt81weeksWNLSF81weeksDN8****1∆ tLSF8*1*7secondsdayssecondsParameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;*****602,112See 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.118IS-GPS-200H24 Sep 2013Table 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)3*UnitsParameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;*****EffectiveRange***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.20.3.3.5.1.8 Special Messages.Page 17 of subframe 4 shall be reserved for special messages with the specific contents at thediscretion of the Operating Command. It shall accommodate the transmission of 22 eight-bitASCII characters. The requisite 176 bits shall occupy bits 9 through 24 of word three, the 24MSBs of words four through nine, plus the 16 MSBs of word ten. The eight MSBs of word threeshall contain the data ID and SV ID, while bits 17 through 22 of word ten shall be reserved forsystem use.
The remaining 50 bits of words three through ten are used for parity (six bits/word)and parity computation (two bits in word ten). The eight-bit ASCII characters shall be limited tothe following set:119IS-GPS-200H24 Sep 2013Alphanumeric 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)"04220.3.3.5.1.9 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 upto 30 valid six-bit ERD 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 thefollowing information.120IS-GPS-200H24 Sep 2013AINavigation Message Correction Table Availability00The correction table is unencrypted and is available to both precise positioning service users and standardpositioning service users.01The correction table is encrypted and is available only to authorized users (normal mode).10No correction table available for either precise positioning service users or standard positioning service users.11ReservedEach one of the 30 six-bit ERD slots in bits 11 through 24 of word three, bits 1 through 24 ofwords four through nine, and bits 1 through 22 of word ten of page 13 of subframe 4 willcorrespond to an ERD value for a particular SV ID.
There are 31 possible SV IDs that theseERD slots may correspond to, ranging from SV ID 1 to SV ID 31. SV ID 32 is not a valid SVID for any of the slots in an NMCT. The correspondence between the 30 ERD slots and the 31possible SV IDs depends on the SV ID of the particular transmitting SV in accordance with thefollowing two rules: 1) the CS shall ensure via upload that no SV shall transmit an NMCTcontaining an ERD value which applies to its own SV ID, and 2) the CS shall ensure via uploadthat all ERD values shall be transmitted in ascending numerical slot order of the correspondingSV ID. To illustrate: the SV operating as SV ID 1 will transmit (in order) ERD values whichcorrespond to SV ID 2 through SV ID 31 in ERD slots 1 through 30 respectively, while the SVoperating as SV ID 31 will transmit ERD values which correspond to SV ID 1 through SV ID 30in ERD slots 1 through 30 respectively.Each ERD value contained in an NMCT ERD slot shall be represented as a six-bit two’scomplement field with the sign bit occupying the MSB and an LSB of 0.3 meters for an effectiverange of ±9.3 m.
A binary value of “100000” shall indicate that no valid ERD for thecorresponding SV ID is present in that slot.20.3.3.5.2 Algorithms Related to Subframe 4 and 5 Data.The following algorithms shall apply when interpreting Almanac, Coordinated Universal Time,Ionospheric Model, and NMCT data in the NAV message.20.3.3.5.2.1 Almanac.The almanac is a subset of the clock and ephemeris data, with reduced precision. The useralgorithm is essentially the same as the user algorithm used for computing the precise ephemerisfrom the one subframe 1, 2, and 3 parameters (see Table 20-IV).
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