Интерфейсный документ GPS (1014597), страница 17

Просмтор этого файла доступен только зарегистрированным пользователям. Но у нас супер быстрая регистрация: достаточно только электронной почты!

Текст из файла (страница 17)

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.

Ionospheric Model (Sheet 3 of 3)IS-GPS-200D7 Dec 200412520.3.4 Timing Relationships. The following conventions shall apply.20.3.4.1 Paging and Cutovers. At end/start of week (a) the cyclic paging of subframes 1 through 5 shall restartwith subframe 1 regardless of which subframe was last transmitted prior to end/start of week, and (b) the cycling ofthe 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes, regardless of which page wasthe last to be transmitted prior to the end/start of week. Cutovers to newly updated data for subframes 1, 2, and 3occur on frame boundaries (i.e., modulo 30 seconds relative to end/start of week). Newly updated data forsubframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes.20.3.4.2 SV Time vs.

GPS Time. In controlling the SVs and uploading of data, the CS shall allow for thefollowing timing relationships:a.Each SV operates on its own SV time;b.All time-related data in the TLM word and the HOW shall be in SV-time;c.All other data in the NAV message shall be relative to GPS time;d.The acts of transmitting the NAV message shall be executed by the SV on SV time.20.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the aboveparagraphs isc = 2.99792458 x 108 meters per secondwhich is the official WGS-84 speed of light.

The user shall use the same value for the speed of light in allcomputations.IS-GPS-200D7 Dec 200412620.3.4.4 Data Sets. The IODE is an 8 bit number equal to the 8 LSBs of the 10 bit IODC of the same data set. Thefollowing rules govern the transmission of IODC and IODE values in different data sets: (1) The transmittedIODC will be different from any value transmitted by the SV during the preceding seven days; (2) The transmittedIODE will be different from any value transmitted by the SV during the preceding six hours. The range of IODCwill be as given in Table 20-XI for Block II/IIA SVs and Table 20-XII for Block IIR/IIR-M/IIF SVs.Cutovers to new data sets will occur only on hour boundaries except for the first data set of a new upload.

The firstdata set may be cut-in (reference paragraph 20.3.4.1) at any time during the hour and therefore may be transmittedby the SV for less than one hour. During short-term operations, cutover to 4-hour sets and subsequent cutovers tosucceeding 4-hour data sets will always occur modulo 4 hours relative to end/start of week. Cutover from 4-hourdata sets to 6-hour data sets shall occur modulo 12 hours relative to end/start of week. Cutover from 12-hour datasets to 24-hour data sets shall occur modulo 24 hours relative to end/start of week. Cutover from a data settransmitted 24 hours or more occurs on a modulo 24-hour boundary relative to end/start of week.The start of the transmission interval for each data set corresponds to the beginning of the curve fit interval for thedata set.

Each data set remains valid for the duration of its curve fit interval.Normal Operations. The subframe 1, 2, and 3 data sets are transmitted by the SV for periods of two hours. Thecorresponding curve fit interval is four hours. SVs operating in the Autonav mode will deviate. They will transmitsubframe 1, 2, and 3 data sets for periods of one hour. The corresponding curve-fit interval will be four hours.Short-term and Long-term Extended Operations.The transmission intervals and curve fit intervals with theapplicable IODC ranges are given in Tables 20-XI and 20-XII.IS-GPS-200D7 Dec 2004127Table 20-XI.IODC Values and Data Set Lengths (Block II/IIA)DaysSpannedTransmission Interval(hours)(Note 4)Curve FitInterval(hours)IODC Range(Note 1)12-1415-1617-2021-2728-4142-5960-6324612244872964681426507498(Note 2)(Note 2)240-247248-255, 496 (Note 3)497-503504-510511, 752-756757Note 1: For transmission intervals of 6 hours or greater, the IODC values shown will be transmitted inincreasing order.Note 2: IODC values for blocks with 2- or 4-hour transmission intervals (at least the first 14 days afterupload) shall be any numbers in the range 0 to 1023 excluding those values of IODC thatcorrespond to IODE values in the range 240-255, subject to the constraints on re-transmissiongiven in paragraph 20.3.4.4.Note 3: The ninth 12-hour data set may not be transmitted.Note 4: The first data set of a new upload may be cut-in at any time and therefore the transmission intervalmay be less than the specified value.IS-GPS-200D7 Dec 2004128Table 20-XII.

IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF)DaysSpanned12-1415-1617-2021-62Transmission Interval(hours)(Note 5)Curve FitInterval(hours)IODC Range4681426(Note 2)(Note 2)240-247 (Note 1)248-255, 496 (Note 1) (Note 3)497-503, 1021-10232 (Note 4)461224Note 1: For transmission intervals of 6 and 12 hours, the IODC values shown will be transmitted inincreasing order.Note 2: IODC values for blocks with 1-, 2- or 4-hour transmission intervals (at least the first 14 days afterupload) shall be any numbers in the range 0 to 1023 excluding those values of IODC thatcorrespond to IODE values in the range 240-255, subject to the constraints on re-transmissiongiven in paragraph 20.3.4.4.Note 3: The ninth 12-hour data set may not be transmitted.Note 4: SVs operating in the Autonav mode will have transmission intervals of 1 hour per paragraph20.3.4.4.Note 5: The first data set of a new upload may be cut-in at any time and therefore the transmission intervalmay be less than the specified value.IS-GPS-200D7 Dec 200412920.3.4.5 Reference Times.

Характеристики

Список файлов учебной работы