Интерфейсный документ GPS (1015430), страница 16
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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.
There are 31 possible SV IDs that these ERD slots may correspond to, ranging from SV ID 1 to SV ID 31.SV ID 32 is not a valid SV ID for any of the slots in an NMCT. The correspondence between the 30 ERD slotsand the 31 possible 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 NMCT containing an ERDvalue which applies to its own SV ID, and 2) the CS shall ensure via upload that all ERD values shall betransmitted in ascending numerical slot order of the corresponding SV ID.
To illustrate: the SV operating as SVID 1 will transmit (in order) ERD values which correspond to SV ID 2 through SV ID 31 in ERD slots 1 through30 respectively, while the SV operating as SV ID 31 will transmit ERD values which correspond to SV ID 1through SV ID 30 in ERD slots 1 through 30 respectively.Each ERD value contained in an NMCT ERD slot shall be represented as a six-bit two’s complement field with thesign bit occupying the MSB and an LSB of 0.3 meters for an effective range of ±9.3 m.
A binary value of“100000” shall indicate that no valid ERD for the corresponding SV ID is present in that slot.IS-GPS-200D7 Dec 200411620.3.3.5.2 Algorithms Related to Subframe 4 and 5 Data. The following algorithms shall apply when interpretingAlmanac, 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 ephemeris from the onesubframe 1, 2, and 3 parameters (see Table 20-IV). The almanac content for one SV is given in Table 20-VI. Aclose inspection of Table 20-VI will reveal that a nominal inclination angle of 0.30 semicircles is implicit and thatthe parameter δ i (correction to inclination) is transmitted, as opposed to the value computed by the user.
All otherparameters appearing in the equations of Tables 20-IV, but not included in the content of the almanac, are set tozero for SV position determination. In these respects, the application of the Table 20-IV equations differs betweenthe almanac and the ephemeris computations.The user is cautioned that the sensitivity to small perturbations in the parameters is even greater for the almanacthan for the ephemeris, with the sensitivity of the angular rate terms over the interval of applicability on the orderof 1014 meters/(semicircle/second). An indication of the URE provided by a given almanac during each of theoperational intervals is as follows:*Operational IntervalAlmanac Ephemeris URE(estimated by analysis)1 sigma (meters)Normal900*Short-term Extended900 - 3,600*Long-term Extended3600 - 300,000*,†URE values generally tend to degrade quadratically over time.
Larger errors may be encounteredduring eclipse seasons and whenever a propulsive event has occurred.†After the CS is unable to upload the SVs, URE values for the SVs operating in the Autonav modetend to degrade quadratically such that the URE will approach 300,000 meters 1 sigma at 180days.IS-GPS-200D7 Dec 200411720.3.3.5.2.2 Almanac Reference Time. Within each upload, the CS shall ensure that all toa values in subframes 4and 5 shall be the same for a given almanac data set and shall differ for successive data sets which contain changesin almanac parameters or SV health. In addition, the Block IIR/IIR-M SVs will also ensure that, based on a validCS upload, all toa values in subframes 4 and 5 will be the same for a given almanac data set and will differ forsuccessive data sets which contain changes in almanac parameters.Note that cutover to a new upload may continue to indicate the same toa values in subframes 4 and 5 as prior to thecutover but the new almanac data set may contain changes in almanac parameters or SV health.
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