Интерфейсный документ GPS, страница 14

In an ECI coordinate system, GPS signalspropagate in straight lines at the constant speed c* (reference paragraph 20.3.4.3). A stable ECI coordinate systemof convenience may be defined as being coincident with the ECEF coordinate system at a given time t0. The x, y, zcoordinates in the ECEF coordinate system at some other time t can be transformed to the x′, y′, z′ coordinates in theselected ECI coordinate system of convenience by the simple** rotation:x′ = x cos(θ) – y sin(θ)y′ = x sin(θ) + y cos(θ)z′ = zwhere•θ = Ω e (t – t0)* The propagation speed c is constant only in a vacuum.

The gravitational potential also has a smalleffect on the propagation speed, but may be neglected by most users.** Neglecting effects due to polar motion, nutation, and precession which may be neglected by most usersfor small values of (t – t0).20.3.3.4.3.4 Geometric Range. The user shall account for the geometric range (D) from satellite to receiver in anECI coordinate system. D may be expressed as,→→D = | r (tR) - R (tT)|wheretT and tR are the GPS system times of transmission and reception, respectively,and where,→R (tT) = position vector of the GPS satellite in the selected ECI coordinate system at time tT,→r (tR) = position vector of the receiver in the selected ECI coordinate system at time tR.IS-GPS-200D7 Dec 200410020.3.3.4.4 NMCT Validity Time. Users desiring to take advantage of the NMCT data provided in page 13 ofsubframe 4 shall first examine the AODO term currently provided in subframe 2 of the NAV data from thetransmitting SV. If the AODO term is 27900 seconds (i.e., binary 11111), then the NMCT currently availablefrom the transmitting SV is invalid and shall not be used.

If the AODO term is less than 27900 seconds, then theuser shall compute the validity time for that NMCT (tnmct) using the ephemeris toe parameter and the AODO termfrom the current subframe 2 as follows:OFFSET = toe [modulo 7200]if OFFSET = 0, then tnmct = toe - AODOif OFFSET > 0, then tnmct = toe - OFFSET + 7200 - AODONote that the foregoing computation of tnmct must account for any beginning or end of week crossovers; forexample,if t* - tnmct > 302,400 then tnmct = tnmct + 604,800if t* - tnmct < -302,400 then tnmct = tnmct - 604,800* t is GPS system time at time of transmission.Users are advised that different SVs will transmit NMCTs with different tnmct and that the best performance willgenerally be obtained by applying data from the NMCT with the latest (largest) tnmct.

As a result, users shouldcompute and examine the tnmct values for all visible and available SVs in order to find and use the NMCT with thelatest tnmct. If the same latest (largest) tnmct is provided by two or more visible and available SVs, then the NMCTfrom any SV with the latest tnmct may be selected and used; however, the estimated range deviation (ERD) valueprovided by the selected NMCT for the other SVs with the same tnmct shall be set to zero if those SVs are used inthe positioning solution. It should be noted that the intended positioning solution accuracy improvement will not beobtained if the data from two different NMCTs are applied simultaneously or if the data from a given NMCT isapplied to just a subset of the SVs used in the positioning solution (i.e., mixed mode operation results in potentiallydegraded solution accuracy).It should be noted that the NMCT information shall be supported by the Block IIR SV only when operating in theIIA like mode of operation including the Autonav Test mode.IS-GPS-200D7 Dec 200410120.3.3.5 Subframes 4 and 5.

Both subframe 4 and 5 are subcommutated 25 times each; the 25 versions of thesesubframes are referred to as pages 1 through 25 of each subframe. With the possible exception of "reserved forsystem use" pages and explicit repeats, each page contains different specific data in words three through ten.

Asshown in Figure 20-1, the pages of subframe 4 utilize seven different formats, while those of subframe 5 use two.The content of words three through ten of each page is described below, followed by algorithms and materialpertinent to the use of the data.20.3.3.5.1 Content of Subframes 4 and 5. Words three through ten of each page contain six parity bits as theirLSBs; in addition, two non-information bearing bits are provided as bits 23 and 24 of word ten in each page forparity computation purposes. The data contained in the remaining bits of words three through ten of the variouspages in subframes 4 and 5 are described in the following subparagraphs.IS-GPS-200D7 Dec 2004102A brief summary of the various data contained in each page of subframes 4 and 5 is as follows:a.Subframe 4:•Pages 1, 6, 11, 16 and 21: (reserved);•Pages 2, 3, 4, 5, 7, 8, 9 and 10:almanac data for SV 25 through 32 respectively;•Pages 12, 19, 20, 22, 23 and 24:(reserved);•Page 13:•Pages 14 and 15:•Page 17:special messages;•Page 18:ionospheric and UTC data;•Page 25:A-S flags/SV configurations for 32 SVs, plus SV health for SV 25NMCT;reserved for system use;through 32.b.Subframe 5:•Pages 1 through 24:•Page 25:almanac data for SV 1 through 24;SV health data for SV 1 through 24, the almanac reference time, thealmanac reference week number.IS-GPS-200D7 Dec 200410320.3.3.5.1.1 Data ID and SV ID.

The two MSBs of word three in each page shall contain data ID. Data ID numbertwo (denoted by binary code 01) denotes the NAV data structure of D(t) which is described in this Appendix.Future data IDs will be defined as necessary.As shown in Table 20-V, the data ID is utilized to provide one of two indications:(a) for those pages which areassigned to contain the almanac data of one specific SV, the data ID defines the data structure utilized by that SVwhose almanac data are contained in that page; and (b) for all other pages, the data ID denotes the data structure ofthe transmitting SV.The SV ID is given by bits three through eight of word three in each page as shown in Table 20-V. Specific IDsare reserved for each page of subframes 4 and 5. The SV IDs are utilized in two different ways: (a) for thosepages which contain the almanac data of a given SV, the SV ID is the same number that is assigned to the PRN codephase of that SV (reference Table 3-I), and (b) for all other pages the SV ID assigned in accordance with Table 20-Vserves as the "page ID".

IDs 1 through 32 are assigned to those pages which contain the almanac data of specificSVs (pages 1-24 of subframe 5 and pages 2-5 and 7-10 of subframe 4). The "0" ID (binary all zeros) is assigned toindicate a dummy SV, while IDs 51 through 63 are utilized for pages containing other than almanac data of aspecific SV. The remaining IDs (33 through 50) are unassigned.Pages which carry the same SV ID (e.g., in subframe 4, pages 1, 6, 11, 16 and 21 carry an ID of 57, while pages 12and 24 are designated by an ID of 62) may not be considered to contain identical data.

The data in the pages withthe same SV ID can be different.IS-GPS-200D7 Dec 2004104Table 20-V.Data IDs and SV IDs in Subframes 4 and 5Subframe 4Page12345678910111213141516171819202122232425Subframe 5Data IDSV ID*Data IDSV ID*Note(2)Note(1)Note(1)Note(1)Note(1)Note(2)Note(1)Note(1)Note(1)Note(1)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)Note(2)57252627285729303132576252535457555658 Note(3)59 Note(3)5760 Note(3)61 Note(3)6263Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(1)Note(2)12345678910111213141516171819202122232451* Use "0" to indicate "dummy" SV.

When using "0" to indicate dummy SV, use the data ID of the transmittingSV.Note 1:Note 2:Note 3:Data ID of that SV whose SV ID appears in that page.Data ID of transmitting SV.SV ID may vary (except for IIR/IIR-M/IIF SVs).IS-GPS-200D7 Dec 200410520.3.3.5.1.2 Almanac Data. Pages 1 through 24 of subframe 5, as well as pages 2 through 5 and 7 through 10 ofsubframe 4 contain the almanac data and a SV health word for up to 32 SVs (the health word is discussed inparagraph 20.3.3.5.1.3).

The almanac data are a reduced-precision subset of the clock and ephemeris parameters.The data occupy all bits of words three through ten of each page except the eight MSBs of word three (data ID andSV ID), bits 17 through 24 of word five (SV health), and the 50 bits devoted to parity. The number of bits, thescale factor (LSB), the range, and the units of the almanac parameters are given in Table 20-VI.

The algorithmsand other material related to the use of the almanac data are given in paragraph 20.3.3.5.2.The almanac message for any dummy SVs shall contain alternating ones and zeros with valid parity.The almanac parameters shall be updated by the CS at least once every 6 days while the CS is able to upload theSVs. If the CS is unable to upload the SVs, the accuracy of the almanac parameters transmitted by the SVs willdegrade over time.For Block II and IIA SVs, three sets of almanac shall be used to span at least 60 days. The first and second sets willbe transmitted for up to six days each; the third set is intended to be transmitted for the remainder of the 60 daysminimum, but the actual duration of transmission will depend on the individual SV's capability to retain data inmemory.

All three sets are based on six-day curve fits that correspond to the first six days of the transmissioninterval. For Block IIR/IIR-M and IIF SVs, multiple sets of almanac parameters shall be uploaded to span at least60 days.IS-GPS-200D7 Dec 2004106ParameterTable 20-VI.Almanac ParametersScaleFactor(LSB)EffectiveRange***No. ofBits**-21Unitse162toa8212δi****16*2-19semi-circles16*2-38semi-circles/secA242-11Ω024*2-23semi-circlesω24*2-23semi-circlesM024*2-23semi-circlesaf011*2-20secondsaf111*2-38sec/sec•Ωdimensionless602,112secondsmeters* Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;** See Figure 20-1 for complete bit allocation in subframe;*** Unless otherwise indicated in this column, effective range is the maximum range attainable withindicated bit allocation and scale factor;**** Relative to i0 = 0.30 semi-circles.IS-GPS-200D7 Dec 200410720.3.3.5.1.3 SV Health.

Subframes 4 and 5 contain two types of SV health data: (a) each of the 32 pages whichcontain the clock/ephemeris related almanac data provide an eight-bit SV health status word regarding the SVwhose almanac data they carry, and (b) the 25th page of subframe 4 and of subframe 5 jointly contain six-bit healthstatus data for up to 32 SVs.The three MSBs of the eight-bit health words indicate health of the NAV data in accordance with the code given inTable 20-VII.