Интерфейсный документ GPS (1014597), страница 16
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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.
Note also thatcutover to a new upload may occur between the almanac pages of interest and page 25 of subframe 5 (referenceparagraph 20.3.4.1), and thus there may be a temporary inconsistency between toa, in the almanac page of interest,and in word 3 of page 25 of subframe 5. The toa mismatch signifies that this WNa may not apply to the almanac ofinterest and that the user must not apply almanac data until the pages with identical values of toa are obtained.Normal and Short-term Extended Operations.
The almanac reference time, toa, is some multiple of 212 secondsoccurring approximately 70 hours after the first valid transmission time for this almanac data set (reference20.3.4.5). The almanac is updated often enough to ensure that GPS time, t, shall differ from toa by less than 3.5days during the transmission period.
The time from epoch tk shall be computed as described in Table 20-IV,except that toe shall be replaced with toa.Long-term Extended Operations. During long-term extended operations or if the user wishes to extend the usetime of the almanac beyond the time span that it is being transmitted, one must account for crossovers into timespans where these computations of tk are not valid.This may be accomplished without time ambiguity byrecognizing that the almanac reference time (toa) is referenced to the almanac reference week (WNa), both of whichare given in word three of page 25 of subframe 5 (see paragraph 20.3.3.5.1.5).IS-GPS-200D7 Dec 200411820.3.3.5.2.3 Almanac Time Parameters. The almanac time parameters shall consist of an 11-bit constant term (af0)and an 11-bit first order term (af1). The applicable first order polynomial, which shall provide time to within 2microseconds of GPS time (t) during the interval of applicability, is given byt=tsv - ∆tsvt=GPS system time (seconds),tsv=effective SV PRN code phase time at message transmission time (seconds),∆tsv=SV PRN code phase time offset (seconds).whereThe SV PRN code phase offset is given by∆tsv=af0 + af1 tkwhere the computation of tk is described in paragraph 20.3.3.5.2.2, and the polynomial coefficients af0 and af1 aregiven in the almanac.
Since the periodic relativistic effect is less than 25 meters, it need not be included in thetime scale used for almanac evaluation. Over the span of applicability, it is expected that the almanac timeparameters will provide a statistical URE component of less than 135 meters, one sigma. This is partially due tothe fact that the error caused by the truncation of af0 and af1 may be as large as 150 meters plus 50 meters/dayrelative to the toa reference time.During extended operations (short-term and long-term) the almanac time parameter may not provide the specifiedtime accuracy or URE component.20.3.3.5.2.4 Coordinated Universal Time (UTC). Page 18 of subframe 4 includes: (1) the parameters needed torelate GPS time to UTC, and (2) notice to the user regarding the scheduled future or recent past (relative to NAVmessage upload) value of the delta time due to leap seconds (∆tLSF), together with the week number (WNLSF) andthe day number (DN) at the end of which the leap second becomes effective.
"Day one" is the first day relative tothe end/start of week and the WNLSF value consists of eight bits which shall be a modulo 256 binary representationof the GPS week number (see paragraph 6.2.4) to which the DN is referenced. The user must account for thetruncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full weeknumber (see paragraph 3.3.4(b)). The CS shall manage these parameters such that, when ∆tLS and ∆tLSF differ, theabsolute value of the difference between the untruncated WN and WNLSF values shall not exceed 127.IS-GPS-200D7 Dec 2004119Depending upon the relationship of the effectivity date to the user's current GPS time, the following three differentUTC/GPS-time relationships exist:a. Whenever the effectivity time indicated by the WNLSF and the DN values is not in the past (relative tothe user's present time), and the user's present time does not fall in the time span which starts at six hours prior tothe effectivity time and ends at six hours after the effectivity time, the UTC/GPS-time relationship is given by=(tE - ∆tUTC) [modulo 86400 seconds]∆tUTC=∆tLS + A0 + A1 (tE - tot + 604800 (WN - WNt)), seconds;tE=GPS time as estimated by the user after correcting tSV for factorstUTCwhere tUTC is in seconds anddescribed in paragraph 20.3.3.3.3 as well as for selective availability(SA) (dither) effects;∆tLS=delta time due to leap seconds;A0 and A1=constant and first order terms of polynomial;tot=reference time for UTC data (reference 20.3.4.5);WN=current week number (derived from subframe 1);WNt=UTC reference week number.The estimated GPS time (tE) shall be in seconds relative to end/start of week.
During the normal and short-termextended operations, the reference time for UTC data, tot, is some multiple of 212 seconds occurring approximately70 hours after the first valid transmission time for this UTC data set (reference 20.3.4.5). The reference time forUTC data (tot) shall be referenced to the start of that week whose number (WNt) is given in word eight of page 18 insubframe 4. The WNt value consists of eight bits which shall be a modulo 256 binary representation of the GPSweek number (see paragraph 6.2.4) to which the tot is referenced. The user must account for the truncated nature ofthis parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (see paragraph3.3.4(b)). The CS shall manage these parameters such that the absolute value of the difference between theuntruncated WN and WNt values shall not exceed 127.IS-GPS-200D7 Dec 2004120b.
Whenever the user's current time falls within the time span of six hours prior to the effectivity time to sixhours after the effectivity time, proper accommodation of the leap second event with a possible week number transitionis provided by the following expression for UTC:tUTC=W[modulo (86400 + ∆tLSF - ∆tLS)], seconds;W=(tE - ∆tUTC - 43200)[modulo 86400] + 43200, seconds;whereand the definition of ∆tUTC (as given in 20.3.3.5.2.4a above) applies throughout the transition period. Note thatwhen a leap second is added, unconventional time values of the form 23:59:60.xxx are encountered.
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