IS-GPS-200H (797934), страница 28
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Type 15 messages that are longer than one page willnot necessarily be broadcast consecutively.Table 30-XII.Message Broadcast IntervalsMessage Type NumberMaximum Broadcast Intervals †10 & 1148 secType 30’s48 sec30*288 sec31* or 1220 min**,****Midi Almanac37*120 min**,****EOP32*30 min****UTC33*288 sec34* or 13 & 1430 min***,****35*288 sec****36* or 15As needed****Message DataEphemerisClockISC, IONOReduced AlmanacDiff CorrectionGGTOText**********†Also contains SV clock correction parameters.Complete set of SVs in the constellation.When Differential Corrections are available.Optional (interval applies if/when broadcast).The intervals specified are maximum.
As such, the broadcast intervals may be shorter than thespecified value.30.3.4.2 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 (TOW) in the messages 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 messages shall be executed by the SV on SVtime.190IS-GPS-200H24 Sep 201330.3.4.3 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.30.3.4.4 Data SetsThe toe shall be equal to the toc of the same CNAV data set. The following rules govern thetransmission of toe and toc values in different data sets: (1) The transmitted toc will be differentfrom any value transmitted by the SV during the preceding seven days; (2) The transmitted toewill be different from any value transmitted by the SV during the preceding six hours.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 30.3.4.1) at any time duringthe hour and therefore may be transmitted by the SV for less than one hour.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 remains valid for the duration of its transmissioninterval, and nominally also remains valid for the duration of its curve fit interval. A data set isrendered invalid before the end of its curve fit interval when it is superseded by the SV cuttingover to the first data set of a new upload.Normal Operations. The message type 10, 11, and 30-37 data sets are transmitted by the SV forperiods of two hours.
The corresponding curve fit interval is three hours.30.3.4.5 Reference TimesMany of the parameters which describe the SV state vary with true time, and must therefore beexpressed as time functions with coefficients provided by the Navigation Message to beevaluated by the user equipment. These include the following parameters as functions of GPStime:a. SV time,b. Semi-major axis,c. Mean anomaly,d. Longitude of ascending node,e. Inclination,f. UTC,191IS-GPS-200H24 Sep 2013g.
URANED,h. EOP,i. Differential corrections,j. GGTO.Each of these parameters is formulated as a polynomial in time. The specific time scale ofexpansion can be arbitrary. Due to the short data field lengths available in the NavigationMessage format, the epoch of the polynomial is chosen near the midpoint of the expansion rangeso that quantization error is small.
This results in time epoch values which can be different foreach data set. Time epochs contained in the Navigation Message and the different algorithmswhich utilize them are related as follows:Epoch Application Algorithm Referencetoc20.3.3.3.3.1toe20.3.3.4.3toa20.3.3.5.2.2 and 20.3.3.5.2.3tot20.3.3.5.2.4 and 30.3.3.6.2top30.3.3.2.4tEOP30.3.3.5.1tOD30.3.3.7tGGTO 30.3.3.8.2For those parameters for which fit interval and transmission interval are relevant, Table 30-XIIIspecifies the fit interval, the nominal transmission interval, and the nominal selection of the fitpoint (which will be expressed modulo 604,800 seconds in the Navigation Message).The coefficients of expansion are obviously dependent upon choice of epoch, and thus the epochtime and expansion coefficients must be treated as an inseparable parameter set.
Note that a userapplying current navigation data will normally be working with negative values of (t-toc) and (ttoe) in evaluating the expansions.192IS-GPS-200H24 Sep 2013The CS (Block IIR-M/IIF) and SS (GPS III) shall assure that the toe value, for at least the firstdata set transmitted by an SV after a new upload, is different from that transmitted prior to thecutover (see paragraph 30.3.4.4). As such, when a new upload is cutover for transmission, the CS(Block IIR-M/IIF) and SS (GPS III) shall introduce a small deviation in the toe resulting in the toevalue that is offset from the nominal location of 1.5 hours into the fit interval (see Table 30XIII).
This offset toe will be transmitted by an SV in the first data set after a new upload cutoverand the second data set, following the first data set, may also continue to reflect the same offsetin the toe.When the toe, immediately prior to a new upload cutover, already reflects a small deviation (i.e.
anew upload cutover has occurred in the recent past), then the CS (Block IIR-M/IIF) and SS (GPSIII) shall introduce an additional deviation to the toe when a new upload is cutover fortransmission.For CNAV data, the user may use the following example algorithm to detect the occurrence of anew upload cutover:DEV = toe [modulo 7200]If DEV ≠ 5400, then a new upload cutover has occurred within the past 4 hours.Table 30-XIII. Reference TimesFit IntervalTransmission(hours)Interval (hours)3*2*144 (6 days)144≥144 (6 days)≥144* Defined in Section 30.3.3.1.1Hours After First Valid Transmission Timetoctoetoatot(clock)(ephemeris)(almanac)(UTC)1.51.57070707030.3.5 Data Frame Parity.The data signal contains parity coding according to the following conventions.30.3.5.1 Parity Algorithm.Twenty-four bits of CRC parity will provide protection against burst as well as random errorswith a probability of undetected error ≤ 2-24 = 5.96×10-8 for all channel bit error probabilities ≤0.5.
The CRC word is calculated in the forward direction on a given message using a seed of 0.The sequence of 24 bits (p1,p2,...,p24) is generated from the sequence of information bits(m1,m2,...,m276) in a given message. This is done by means of a code that is generated by thepolynomial24g(X ) = ∑ g i X ii =0193IS-GPS-200H24 Sep 2013whereg i = 1 for i = 0,1,3, 4,5,6,7,10,11,14,17,18, 23, 24= 0 otherwiseThis code is called CRC-24Q. The generator polynomial of this code is in the following form(using binary polynomial algebra):g(X ) = (1 + X )p(X )where p(X) is the primitive and irreducible polynomialp(X ) = X 23 + X17 + X13 + X12 + X11 + X 9 + X 8 + X 7 + X 5 + X 3 + 1When, by the application of binary polynomial algebra, the above g(X) is divided into m(X)X24,where the information sequence m(X) is expressed asm(X ) = m k + m k −1X + m k − 2 X 2 + ⋅ ⋅ ⋅ + m1X k −1The result is a quotient and a remainder R(X) of degree < 24.
The bit sequence formed by thisremainder represents the parity check sequence. Parity bit pi, for any i from 1 to 24, is thecoefficient of X24-i in R(X).This code has the following characteristics:1) It detects all single bit errors per code word.2) It detects all double bit error combinations in a codeword because the generator polynomialg(X) has a factor of at least three terms.3) It detects any odd number of errors because g(X) contains a factor 1+X.4) It detects any burst error for which the length of the burst is ≤ 24 bits.5) It detects most large error bursts with length greater than the parity length r = 24 bits. Thefraction of error bursts of length b > 24 that are undetected is:a) 2-24 = 5.96 × 10-8, if b > 25 bits.b) 2-23 = 1.19 × 10-7, if b = 25 bits.194IS-GPS-200H24 Sep 201340 APPENDIX IV GPS NAVIGATION DATA STRUCTURE FORLNAV DATA, D(t), FOR PRN 33-6340.1 ScopeThis appendix describes the specific legacy navigation (LNAV) data structure denoted by dataID number 2 for the upper set of PRN numbers (PRN 33-63).
This data ID number, whentransmitted as part of the LNAV data, shall be represented by the two-bit binary notation as 01.Data ID number 1 is no longer in use. The LNAV data structure for the upper set of PRNnumbers is denoted as LNAV-U. The LNAV data structure for the lower set of PRN numbers(LNAV-L) is described in Appendix II.40.2 Applicable DocumentsApplicable documents shall be as specified in Appendix II, Section 20.2.40.3 Requirements40.3.1 Data CharacteristicsThe data characteristics shall be as specified in Appendix II, Section 20.3.1.40.3.2 Message StructureThe message structure shall be as specified in Appendix II, Section 20.3.2 except as indicated byFigure 40-1.195IS-GPS-200H24 Sep 2013DIRECTION OF DATA FLOW FROM SV150 BITSWORD 1MSB FIRST3 SECONDSWORD 2WORD 3WORD 4WORD 571SUBFRAMENO.PAGENO.1N/A131TLM61HOWC P22 BITS73 77 8391121WNt22 BITSPP10BITS23 BITS***C/A OR P ON L2 - 2 BITSURA INDEX - 4 BITSSV HEALTH - 6 BITSP2 MSBs150 BITSIODC - 10 BITS TOTAL1N/A18124 BITS***P16BITS***WORD 8197T GDMSB FIRST3 SECONDSWORD 7151PL2 P DATA FLAG - 1 BITDIRECTION OF DATA FLOW FROM SVWORD 624 BITS***WORD 9211 219P8 BITSWORD 10241t oc16 BITSPa f28BITS8 LSBs271af116 BITSa f0PtP22 BITSIODC - 10 BITS TOTAL*** RESERVEDP = 6 PARITY BITSt = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5)C = TLM BITS 23 AND 24.
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