IS-GPS-705D (797936), страница 15
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The three bits are defined as follows;000 = no data available,001 = Galileo,010 = GLONASS,011 through 111 = reserved for other systems.The number of bits, the scales factor (LSB), the ranges, and the units of the GGTO parametersare given in Table 20-XI. See Figure 20-8 for complete bit allocation in message type 35.20.3.3.8.2 GPS and GNSS Time.The GPS/GNSS-time relationship is given by,tGNSS = tE - (A0GGTO + A1GGTO (tE - tGGTO + 604800 (WN - WNGGTO)) + A2GGTO (tE - tGGTO +604800 (WN - WNGGTO))2)90IS-GPS-705D24 Sep 2013where tGNSS is in seconds, tE and WN are as defined in Section 20.3.3.5.2.4 of IS-GPS-200, andthe remaining parameters are as defined in Table 20-XI.Table 20-XI.
GPS/GNSS Time Offset ParametersParameterSymbolParameter DescriptionNo. ofScaleFactorEffectiveBits**(LSB)Range***Units-35secondsA0GGTOBias coefficient of GPS timescale relative to GNSS time scale16*2A1GGTODrift coefficient of GPS timescale relative to GNSS time scale13*2-51sec/secA2GGTODrift rate correction coefficient ofGPS time scale relative to GNSStime scale7*2-68sec/sec2tGGTOTime data reference Time ofWeek16241320WNGGTOTime data reference WeekNumberGNSS IDGNSS Type ID3******604,784secondsweekssee textParameters so indicated shall be two's complement with the sign bit (+ or -) occupyingthe MSB;See Figure 20-8 for complete bit allocation;Unless otherwise indicated in this column, effective range is the maximum rangeattainable with indicated bit allocation and scale factor.20.3.3.9 Message Types 36 and 15 Text Messages.Text messages are provided either in message type 36, Figure 20-9, or type 15, Figure 20-14.The specific contents of text message will be at the discretion of the Operating Command.Message type 36 can accommodate the transmission of 18 eight-bit ASCII characters.
Messagetype 15 can accommodate the transmission of 29 eight-bit ASCII characters. The requisite bitsshall occupy bits 39 through 274 of message type 15 and bits 128 through 275 of message type36. The eight-bit ASCII characters shall be limited to the set described in paragraph 20.3.3.5.1.8of IS-GPS-200.20.3.4 Timing Relationships.The following conventions shall apply.20.3.4.1 Paging and Cutovers.Broadcast system of messages is completely arbitrary, but sequenced to provide optimum userperformance. Message types 10 and 11 shall be broadcast at least once every 24 seconds. Allother messages shall be broadcast in-between, not exceeding the maximum broadcast interval in91IS-GPS-705D24 Sep 2013Table 20-XII. Message type 15 will be broadcast as needed, but will not reduce the maximumbroadcast interval of the other messages.
Type 15 messages that are longer than one page willnot necessarily be broadcast consecutively.Table 20-XII.Message Broadcast IntervalsMessage Type NumberMaximum Broadcast Intervals †10 & 1124 secType 30’s24 sec30*144 sec31* or 1210 min**,****Midi Almanac37*60 min**EOP32*15 min****UTC33*144 sec34* or 13 & 1415 min***,****35*144 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.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 (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.20.3.4.3 Speed of Light.The speed of light used by the CS for generating the data described in the above paragraphs is92IS-GPS-705D24 Sep 2013c = 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 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 20.3.4.1) 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.20.3.4.5 Reference TimesThe CNAV reference time information may be found in paragraph 30.3.4.5 in IS-GPS-200.20.3.5 Data Frame Parity.The data signal contains parity coding according to the following conventions.20.3.5.1 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 x 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 thepolynomial.93IS-GPS-705D24 Sep 201324g ( X ) = ∑ gi X ii =0whereg 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 + X 17 + X 13 + X 12 + X 11 + 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 ) = mk + mk −1 X + mk − 2 X 2 + ⋅ ⋅ ⋅ + m1 X 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.94IS-GPS-705D24 Sep 20135) 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.95IS-GPS-705D24 Sep 2013.
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