IS-GPS-200H (797934), страница 20
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Note that when a leap second is added, unconventional time values of the form23:59:60.xxx are encountered. Some user equipment may be designed to approximate UTC bydecrementing the running count of time within several seconds after the event, thereby promptlyreturning to a proper time indication. Whenever a leap second event is encountered, the userequipment 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 DNvalues, is in the "past" (relative to the user's current time), and the user’s current time does notfall in the time span as given above in 20.3.3.5.2.4b, the relationship previously given for tUTC in20.3.3.5.2.4a above is valid except that the value of ∆tLSF is substituted for ∆tLS.
The CS willcoordinate the update of UTC parameters at a future upload so as to maintain a proper continuityof the tUTC time scale.20.3.3.5.2.5 Ionospheric Model.The "two frequency" (L1 and L2) user shall correct the time received from the SV forionospheric effect by utilizing the time delay differential between L1 and L2 (referenceparagraph 20.3.3.3.3.3). The "one frequency" user, however, may use the model given in Figure20-4 to make this correction.
It is estimated that the use of this model will provide at least a 50percent reduction in the single - frequency user's RMS error due to ionospheric propagation125IS-GPS-200H24 Sep 2013effects. During extended operations, or for the SVs in the Autonav mode if the CS is unable toupload 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 ERD valueis computed by the CS and represents the radial component of the satellite ephemeris error minusthe speed of light times the satellite clock error. The satellite ephemeris and clock errors arecomputed by subtracting the broadcast from current estimates.
Therefore, the ERD value may beused as follows to correct the user's measured pseudorange:PRc = PR - ERDwhere,PRc = pseudorange corrected with the ERD value from the NMCTPR = measured pseudorange.Note that as described above, the ERD values are actually error estimates rather than differentialcorrections and so are subtracted rather than added in the above equation.126IS-GPS-200H24 Sep 2013The 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), 3n α n φ m , AMP ≥ 0 AMP = n = 0if AMP < 0, AMP = 0∑(sec)x=2π (t - 50400 )PER(radians) 3n β n φ m , PER ≥ 72,000PER = 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)127IS-GPS-200H24 Sep 2013Other equations that must be solved areφm = φi + 0.064cos(λi - 1.617)λ i = λu +φi = ψ sinAcos φ i(semi-circles)(semi-circles)φ i ≤ 0.416 if φ i > +0.416, then φ i = +0.416if φ i < −0.416, then φ i = −0.416 φ u + ψ cosA,ψ=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)128IS-GPS-200H24 Sep 2013The 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)129IS-GPS-200H24 Sep 201320.3.4 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 restart with subframe 1regardless of which subframe was last transmitted prior to end/start of week, and (b) the cyclingof the 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes,regardless of which page was the last to be transmitted prior to the end/start of week. Cutoversto newly updated data for subframes 1, 2, and 3 occur on frame boundaries (i.e., modulo 30seconds relative to end/start of week). Newly updated data for subframes 4 and 5 may start to betransmitted 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 the following timingrelationships: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 SVtime.20.3.4.3 Speed of Light.The speed of light used by the CS for generating the data described in the above paragraphs 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 oflight in all computations.20.3.4.4 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) Thetransmitted IODC will be different from any value transmitted by the SV during the precedingseven days; (2) The transmitted IODE will be different from any value transmitted by the SV130IS-GPS-200H24 Sep 2013during the preceding six hours. The range of IODC will be as given in Table 20-XI for BlockII/IIA SVs and Table 20-XII for Block IIR/IIR-M/IIF and GPS III SVs.Cutovers to new data sets will occur only on hour boundaries except for the first data set of anew upload. The first data set may be cut-in (reference paragraph 20.3.4.1) at any time duringthe hour and therefore may be transmitted by the SV for less than one hour.
During short-termoperations, cutover to 4-hour sets and subsequent cutovers to succeeding 4-hour data sets willalways occur modulo 4 hours relative to end/start of week. Cutover from 4-hour data sets to 6hour data sets shall occur modulo 12 hours relative to end/start of week.
Cutover from 12-hourdata sets to 24-hour data sets shall occur modulo 24 hours relative to end/start of week. Cutoverfrom a data set transmitted 24 hours or more occurs on a modulo 24-hour boundary relative toend/start of week.The start of the transmission interval for each data set corresponds to the beginning of the curvefit interval for the data set. Each data set nominally remains valid for the duration of its curve fitinterval. A data set may be rendered invalid before the end of its curve fit interval when it issuperseded by the SV cutting over to new data.Normal Operations.
The subframe 1, 2, and 3 data sets are transmitted by the SV for periods oftwo hours. The corresponding curve fit interval is four hours. SVs operating in the Autonavmode will deviate. They will transmit subframe 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 fitintervals with the applicable IODC ranges are given in Tables 20-XI and 20-XII.131IS-GPS-200H24 Sep 2013Table 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.132IS-GPS-200H24 Sep 2013Table 20-XII.DaysSpanned12-1415-1617-2021-62IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF & GPS III)Transmission 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.
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