Интерфейсный документ GPS (1015430), страница 17

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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.

Some userequipment may be designed to approximate UTC by decrementing the running count of time within severalseconds after the event, thereby promptly returning to a proper time indication. Whenever a leap second event isencountered, the user equipment must consistently implement carries or borrows into any year/week/day counts.c. Whenever the effectivity time of the leap second event, as indicated by the WNLSF and DN values, is inthe "past" (relative to the user's current time), and the user’s current time does not fall in the time span as givenabove in 20.3.3.5.2.4b, the relationship previously given for tUTC in 20.3.3.5.2.4a above is valid except that thevalue of ∆tLSF is substituted for ∆tLS.

The CS will coordinate the update of UTC parameters at a future upload so asto maintain a proper continuity of the tUTC time scale.IS-GPS-200D7 Dec 200412120.3.3.5.2.5 Ionospheric Model. The "two frequency" (L1 and L2) user shall correct the time received from the SVfor ionospheric effect by utilizing the time delay differential between L1 and L2 (reference paragraph 20.3.3.3.3.3).The "one frequency" user, however, may use the model given in Figure 20-4 to make this correction.

It is estimatedthat the use of this model will provide at least a 50 percent reduction in the single - frequency user's RMS error dueto ionospheric propagation effects. During extended operations, or for the SVs in the Autonav mode if the CS isunable to upload the SVs, the use of this model will yield unpredictable results.20.3.3.5.2.6 NMCT Data. For each SV, the ERD value in the NMCT is an estimated pseudorange error. Each ERDvalue is computed by the CS and represents the radial component of the satellite ephemeris error minus the speed oflight times the satellite clock error.

The satellite ephemeris and clock errors are computed by subtracting thebroadcast from current estimates. Therefore, the ERD value may be used as follows to correct the user's measuredpseudorange:PRc = PR – ERDwhere,PRc = pseudorange corrected with the ERD value from the NMCTPR = measured pseudorangeNote that as described above, the ERD values are actually error estimates rather than differential corrections and soare subtracted rather than added in the above equation.IS-GPS-200D7 Dec 2004122The ionospheric correction model is given byTiono x 2 x 4 F ∗ 5.0 ∗ 10 − 9 + (AMP)1 −, x < 1.57 +224 =   (sec)−9, x ≥ 1.57 F ∗ 5.0 ∗ 10()whereTiono is referred to the L1 frequency; if the user is operating on the L2 frequency, the correction term mustbe multiplied by γ (reference paragraph 20.3.3.3.3.2), 3n α n φ m , AMP ≥ 0 AMP = n = 0if AMP < 0, AMP = 0∑x=2π (t - 50400 )PER(sec)(radians) 3n β n φ m , PER ≥ 72,000PER = n = 0 (sec)if PER < 72,000, PER = 72,000∑F = 1.0 + 16.0 [0.53 - E]3and αn and βn are the satellite transmitted data words with n = 0, 1, 2, and 3.Figure 20-4.

Ionospheric Model (Sheet 1 of 3)IS-GPS-200D7 Dec 2004123Other equations that must be solved areφm = φi + 0.064cos(λi - 1.617)(semi-circles)λ i = λu +ψ sinAcos φ iφi = φ i ≤ 0.416 if φ i > +0.416, then φ i = +0.416if φ i < −0.416, then φ i = −0.416ψ=(semi-circles)φ u + ψ cosA,0.0137- 0.022E + 0.11t = 4.32 (104) λi + GPS time(semi-circles)(semi-circles)(sec)where0 ≤ t < 86400: therefore, if t ≥ 86400 seconds, subtract 86400 seconds;if t < 0 seconds, add 86400 seconds.Figure 20-4. Ionospheric Model (Sheet 2 of 3)IS-GPS-200D7 Dec 2004124The terms used in computation of ionospheric delay are as follows:• Satellite Transmitted Termsαn-the coefficients of a cubic equation representing the amplitude of the verticaldelay (4 coefficients - 8 bits each)βn-the coefficients of a cubic equation representing the period of the model(4 coefficients - 8 bits each)• Receiver Generated TermsE-elevation angle between the user and satellite (semi-circles)A-azimuth angle between the user and satellite, measured clockwise positive fromthe true North (semi-circles)φu-user geodetic latitude (semi-circles) WGS-84λu-user geodetic longitude (semi-circles) WGS-84GPS time-receiver computed system timeX-phase (radians)F-obliquity factor (dimensionless)t-local time (sec)φm-geomagnetic latitude of the earth projection of the ionospheric intersection• Computed Termspoint (mean ionospheric height assumed 350 km) (semi-circles)λi-geodetic longitude of the earth projection of the ionospheric intersection point(semi-circles)φi-geodetic latitude of the earth projection of the ionospheric intersection point(semi-circles)ψ-earth's central angle between the user position and the earth projection ofionospheric intersection point (semi-circles)Figure 20-4.

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