IS-GPS-200H (797934), страница 19
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The almanac content for oneSV is given in Table 20-VI. A close inspection of Table 20-VI will reveal that a nominalinclination angle of 0.30 semicircles is implicit and that the parameter δi (correction toinclination) is transmitted, as opposed to the value computed by the user. All other parametersappearing in the equations of Tables 20-IV, but not included in the content of the almanac, are121IS-GPS-200H24 Sep 2013set to zero for SV position determination.
In these respects, the application of the Table 20-IVequations differs between the almanac and the ephemeris computations.The user is cautioned that the sensitivity to small perturbations in the parameters is even greaterfor the almanac than for the ephemeris, with the sensitivity of the angular rate terms over theinterval of applicability on the order of 1014 meters/(semicircle/second). An indication of theURE provided by a given almanac during each of the operational 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 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 and 5 shall be the samefor 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 CS upload, all toa values in subframes 4 and 5 will be the same for a givenalmanac data set and will differ for successive data sets which contain changes in almanacparameters.Note that cutover to a new upload may continue to indicate the same toa values in subframes 4and 5 as prior to the cutover but the new almanac data set may contain changes in almanacparameters or SV health.
Note also that cutover to a new upload may occur between the almanacpages of interest and page 25 of subframe 5 (reference paragraph 20.3.4.1), and thus there maybe a temporary inconsistency between toa, in the almanac page of interest, and in word 3 of page25 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 toaare obtained.Normal and Short-term Extended Operations.
The almanac reference time, toa, is somemultiple of 212 seconds occurring approximately 70 hours after the first valid transmission timefor this almanac data set (reference 20.3.4.5). The almanac is updated often enough to ensurethat GPS time, t, shall differ from toa by less than 3.5 days during the transmission period. Thetime from epoch tk shall be computed as described in Table 20-IV, except that toe shall bereplaced with toa.122IS-GPS-200H24 Sep 2013Long-term Extended Operations. During long-term extended operations or if the user wishesto extend the use time of the almanac beyond the time span that it is being transmitted, one mustaccount for crossovers into time spans where these computations of tk are not valid. This may beaccomplished without time ambiguity by recognizing that the almanac reference time (toa) isreferenced to the almanac reference week (WNa), both of which are given in word three of page25 of subframe 5 (see paragraph 20.3.3.5.1.5).20.3.3.5.2.3 Almanac Time Parameters.The almanac time parameters shall consist of an 11-bit constant term (af0) and an 11-bit firstorder 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 polynomialcoefficients af0 and af1 are given in the almanac. Since the periodic relativistic effect is less than25 meters, it need not be included in the time scale used for almanac evaluation.
Over the spanof applicability, it is expected that the almanac time parameters will provide a statistical UREcomponent of less than 135 meters, one sigma. This is partially due to the fact that the errorcaused by the truncation of af0 and af1 may be as large as 150 meters plus 50 meters/day relativeto the toa reference time.During extended operations (short-term and long-term) the almanac time parameter may notprovide the specified time accuracy or URE component.
Additionally, occasional CS actions tomanage clock offsets may also inhibit the ability to provide specified almanac time parameteraccuracies.20.3.3.5.2.4 Coordinated Universal Time (UTC).123IS-GPS-200H24 Sep 2013Page 18 of subframe 4 includes: (1) the parameters needed to relate GPS time to UTC, and (2)notice to the user regarding the scheduled future or recent past (relative to NAV message 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 thefirst day relative to the end/start of week and the WNLSF value consists of eight bits which shallbe a modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to whichthe DN is referenced. The user must account for the truncated nature of this parameter as well astruncation 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, when ∆tLS and ∆tLSF differ, theabsolute value of the difference between the untruncated WN and WNLSF values shall not exceed127.Depending upon the relationship of the effectivity date to the user's current GPS time, thefollowing three different UTC/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's present time), and the user's present time does not fall in the time spanwhich starts at six hours prior to the effectivity time and ends at six hours after the effectivitytime, 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 factorsdescribed in paragraph 20.3.3.3.3 as well as for selectiveavailability (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 normaland short-term extended operations, the reference time for UTC data, tot, is some multiple of 212seconds occurring approximately 70 hours after the first valid transmission time for this UTC124IS-GPS-200H24 Sep 2013data set (reference 20.3.4.5).
The reference time for UTC data (tot) shall be referenced to thestart of that week whose number (WNt) is given in word eight of page 18 in subframe 4. TheWNt 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 thetruncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rolloverof full week number (see paragraph 3.3.4(b)). The CS shall manage these parameters such thatthe absolute value of the difference between the untruncated WN and WNt values shall notexceed 127.b.
Whenever the user's current time falls within the time span of six hours prior to the effectivitytime to six hours after the effectivity time, proper accommodation of the leap second event witha possible week number transition is provided 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 transitionperiod.
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