IS-GPS-200F (811524), страница 20
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A binary value of “100000”shall indicate that no valid ERD for the corresponding 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 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 δi (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 to zerofor SV position determination.
In these respects, the application of the Table 20-IV equations differs between thealmanac and the ephemeris computations.The user is cautioned that the sensitivity to small perturbations in the parameters is even greater for the almanac thanfor the ephemeris, with the sensitivity of the angular rate terms over the interval of applicability on the order of 1014122IS-GPS-200F21 Sep 2011meters/(semicircle/second).
An indication of the URE provided by a given almanac during each of the operationalintervals 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 encountered duringeclipse 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 mode20.3.3.5.2.2 Almanac Reference Time.
Within each upload, the CS shall ensure that all toa values in subframes 4 and5 shall be the same for a given almanac data set and shall differ for successive data sets which contain changes inalmanac parameters or SV health. In addition, the Block IIR/IIR-M SVs will also ensure that, based on a valid CSupload, 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 3.5 daysduring the transmission period. The time from epoch tk shall be computed as described in Table 20-IV, except thattoe shall be replaced with toa.Long-term Extended Operations. During long-term extended operations or if the user wishes to extend the use timeof the almanac beyond the time span that it is being transmitted, one must account for crossovers into time spanswhere these computations of tk are not valid. This may be accomplished without time ambiguity by recognizing thatthe almanac reference time (toa) is referenced to the almanac reference week (WNa), both of which are given in wordthree of page 25 of subframe 5 (see paragraph 20.3.3.5.1.5).123IS-GPS-200F21 Sep 201120.3.3.5.2.3 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 - ∆tsvwheret =GPS system time (seconds),tsv =effective SV PRN code phase time at message transmission time (seconds),∆tsv =SV PRN code phase time offset (seconds).The 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 the timescale used for almanac evaluation.
Over the span of applicability, it is expected that the almanac time parameterswill provide a statistical URE component of less than 135 meters, one sigma. This is partially due to the fact that theerror caused by the truncation of af0 and af1 may be as large as 150 meters plus 50 meters/day relative to the toareference time.During extended operations (short-term and long-term) the almanac time parameter may not provide the specifiedtime accuracy or URE component.
Additionally, occasional CS actions to manage clock offsets may also inhibit theability to provide specified almanac time parameter accuracies.124IS-GPS-200F21 Sep 201120.3.3.5.2.4 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) and theday number (DN) at the end of which the leap second becomes effective. "Day one" is the first day relative to theend/start of week and the WNLSF value consists of eight bits which shall be a modulo 256 binary representation ofthe GPS week number (see paragraph 6.2.4) to which the DN is referenced.
The user must account for the truncatednature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (seeparagraph 3.3.4(b)). The CS shall manage these parameters such that, when ∆tLS and ∆tLSF differ, the absolute valueof the difference between the untruncated WN and WNLSF values shall not exceed 127.Depending 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 to the user'spresent time), and the user's present time does not fall in the time span which starts at six hours prior to theeffectivity time and ends at six hours after the effectivity time, the UTC/GPS-time relationship is given bytUTC=(tE - ∆tUTC) [modulo 86400 seconds]where tUTC is in seconds and∆tUTC=∆tLS + A0 + A1 (tE - tot + 604800 (WN - WNt)), seconds;tE=GPS time as estimated by the user after correcting tSV for factors described inparagraph 20.3.3.3.3 as well as for selective availability (SA) (dither) effects;∆tLS=A0 and A1 =delta time due to leap seconds;constant and first order terms of polynomial;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 paragraph125IS-GPS-200F21 Sep 20113.3.4(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.b. Whenever the user's current time falls within the time span of six hours prior to the effectivity time to six hoursafter the effectivity time, proper accommodation of the leap second event with a possible week number transition isprovided by the following expression for UTC:tUTC = W[modulo (86400 + ∆tLSF - ∆tLS)], seconds;whereW = (tE - ∆tUTC - 43200) [modulo 86400] + 43200, seconds;and the definition of ∆tUTC (as given in 20.3.3.5.2.4a above) applies throughout the transition period. Note that whena leap second is added, unconventional time values of the form 23:59:60.xxx are encountered. Some user equipmentmay be designed to approximate UTC by decrementing the running count of time within several seconds after theevent, thereby promptly returning to a proper time indication.
Whenever a leap second event is encountered,c. Whenever the effectivity time of the leap second event, as indicated by the WNLSF and DN values, is in the "past"(relative to the user's current time), and the user’s current time does not fall in the time span as given above in20.3.3.5.2.4b, the relationship previously given for tUTC in 20.3.3.5.2.4a above is valid except that the value of ∆tLSFis substituted for ∆tLS. The CS will coordinate the update of UTC parameters at a future upload so as to maintain aproper continuity of the tUTC time scale.126IS-GPS-200F21 Sep 201120.3.3.5.2.5 Ionospheric Model.
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