Интерфейсный документ GPS, страница 24

The SV PRN code phase offset, uncorrected by clock correctioncoefficient updates, is given by equation 2 in paragraph 20.3.3.3.3.1 (see para. 30.3.3.2.3). If the matched pair ofDC data for the subject SV is available, the user may apply clock correction coefficient update values by;∆tsv = (af0 + δaf0) + (af1 + δaf1)(t − toc) + af2(t − toc )2 + ∆tr,where δaf0 and δaf1, (see Table 30-X), are given in message types 34 or 13, and all other terms are as stated inparagraph 20.3.3.3.3.1.Clock-related DC data shall not be applied to any SV transmitting clock correctionparameters message(s) containing a top value greater than the top-D value of messages types 34 or 13 containing theclock-related DC data.30.3.3.7.4 Application of Orbit-Related DC Data.

The DC data packet includes corrections to parameters thatcorrect the state estimates for ephemeris parameters transmitted in the message types 10 and 11 (broadcast by theSV to which the DC data packet applies). The user will update the ephemeris parameters utilizing a variation of thealgorithm expressed in the following equations. The user will then incorporate the updated quasi-Keplerian elementset in all further calculations of SV position, as represented by the equations in Table 30-II (see para. 30.3.3.1.3).Ephemeris-related DC data shall not be applied to any SV transmitting message types 10 and 11 containing a topvalue greater than the top-D value of message types 34 or 14 containing the ephemeris-related DC data.The user will construct a set of initial (uncorrected) elements by:Ai=A0ei=enii=i0-nΩi=Ω0-nαi=en cos(ωn)βi=en sin(ωn)γi=M0-n + ωnIS-GPS-200D7 Dec 2004184where A0, en, i0-n, Ω0-n, ωn and M0-n are obtained from the applicable SV’s message types 10 and 11 data.

The termsαi, βi, and γi form a subset of stabilized ephemeris elements which are subsequently corrected by ∆α, ∆β and∆γthe values of which are supplied in the message types 34 or 14 — as follows:αc=αi + ∆αβc=βi + ∆βγc=γi + ∆γThe quasi-Keplerian elements are then corrected byAc=Ai + ∆Aec=(αc2 + βc2)1/2ic=ii + ∆iΩc=Ωi + ∆Ωωc=tan-1 (βc/αc)M0-c=γc − ωc + ∆M0where ∆A, ∆i and ∆Ω are provided in the EDC data packet of the message type 34 or 14 and ∆M0 is obtained from∆M0=−3µAc 2[(toe) − (tOD )].The corrected quasi-Keplerian elements above are applied to the user algorithm for determination of antenna phasecenter position in Section 30.3.3.1.3, Table 30-II.IS-GPS-200D7 Dec 2004185•30.3.3.7.5 SV Differential Range Accuracy Estimates.

The UDRAop-D and UDRA shall give the differential userrange accuracy for the SV. It must be noted that the two parameters provide estimated accuracy after both clock and•ephemeris DC are applied. The UDRA and UDRA indices are signed, two’s complement integers in the range of+15 to –16 and has the following relationship:Index Value1514131211109876543210-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16UDRAop-D (meters)•UDRA (10-6 m/sec)6144.00UDRAop-DUDRAop-D ≤ 6144.003072.00UDRAop-D ≤ 3072.001536.00UDRAop-D ≤ 1536.00768.00UDRAop-D ≤768.00384.00UDRAop-D ≤384.00192.00UDRAop-D ≤192.0096.00UDRAop-D ≤96.0048.00UDRAop-D ≤48.0024.00UDRAop-D ≤24.0013.65UDRAop-D ≤13.659.65UDRAop-D ≤9.656.85UDRAop-D ≤6.854.85UDRAop-D ≤4.853.40UDRAop-D ≤3.402.40UDRAop-D ≤2.401.70UDRAop-D ≤1.701.20UDRAop-D ≤1.200.85UDRAop-D ≤0.850.60UDRAop-D ≤0.600.43UDRAop-D ≤0.430.30UDRAop-D ≤0.300.21UDRAop-D ≤0.210.15UDRAop-D ≤0.150.11UDRAop-D ≤0.110.08UDRAop-D ≤0.080.06UDRAop-D ≤0.060.04UDRAop-D ≤0.040.03UDRAop-D ≤0.030.02UDRAop-D ≤0.020.01UDRAop-D ≤0.010.005No accuracy prediction available—use at own risk6144.003072.001536.00768.00384.00192.0096.0048.0024.0013.659.656.854.853.402.401.701.200.850.600.430.300.210.150.110.080.060.040.030.020.01<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<For any time, tk, other than top-D, UDRA is found by,•UDRA = UDRAop-D + UDRA (tk – top-D)IS-GPS-200D7 Dec 200418630.3.3.8 Message Type 35 GPS/GNSS Time Offset.

Message type 35, Figure 30-8, contains the GPS/GlobalNavigation Satellite System (GNSS) Time Offset (GGTO) parameters. The contents of message type 35 are definedbelow. The validity period of the GGTO shall be 1 day as a minimum.30.3.3.8.1 GPS/GNSS Time Offset Parameter Content. Message Type 35 provides SV clock correction parameters(ref. Section 30.3.3.2) and also, shall contain the parameters related to correlating GPS time with other GNSS time.Bits 155 through 157 of message type 35 shall identify the other GPS like navigation system to which the offset dataapplies.

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 range, and the units of the GGTO parameters are given in Table 30XI. See Figure 30-8 for complete bit allocation in message type 35.30.3.3.8.2 GPS and GNSS Time. The GPS/GNSS-time relationship is given by,tGNSS = tE – (A0GGTO + A1GGTO (tE – totGGTO + 604800 (WN – WNotGGTO) + A2GGTO (tE – totGGTO + 604800 (WN – WNotGGTO))2)where tGNSS is in seconds, tE and WN are as defined in Section 20.3.3.5.2.4, and the remaining parameters are asdefined in Table 30-XI.IS-GPS-200D7 Dec 2004187Table 30-XI.GPS/GNSS Time Offset ParametersParameterNo. ofBits**ScaleFactor(LSB)EffectiveRange***UnitsA0GGTOBias coefficient of GPS time scalerelative to GNSS time scale16*2-35secondsA1GGTODrift coefficient of GPS time scalerelative to GNSS time scale13*2-51sec/secA2GGTODrift rate correction coefficient ofGPS time scale relative to GNSStime scale7*2-68sec/sec2totGGTOTime data reference Time of Week1624WNotGGTOTime data reference Week Number1320GNSS IDGNSS Type ID3******604,784secondsweekssee textParameters so indicated shall be two's complement with the sign bit (+ or -) occupyingthe MSB;See Figure 30-8 for complete bit allocation;Unless otherwise indicated in this column, effective range is the maximum rangeattainable with indicated bit allocation and scale factor.30.3.3.9 Message Types 36 and 15 Text Messages.

Text messages are provided either in message type 36, Figure30-9, or type 15, Figure 30-14. The specific contents of text message will be at the discretion of the OperatingCommand. Message type 36 can accommodate the transmission of 18 eight-bit ASCII characters. Message type 15can accommodate the transmission of 29 eight-bit ASCII characters. The requisite bits shall occupy bits 39 through270 of message type 15 and bits 128 through 275 of message type 36.

The eight-bit ASCII characters shall belimited to the set described in paragraph 20.3.3.5.1.8.IS-GPS-200D7 Dec 200418830.3.4 Timing Relationships. The following conventions shall apply.30.3.4.1 Paging and Cutovers. Broadcast system of messages is completely arbitrary, but sequenced to provideoptimum user performance. Message types 10 and 11 shall be broadcast at least once every 48 seconds. All othermessages shall be broadcast in-between, not exceeding the maximum broadcast interval in Table 30-XII. Messagetype 15 will be broadcast as needed, but will not reduce the maximum broadcast interval of the other messages.Type 15 messages that are longer than one page will not 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 Almanac37120 min**EOP32*30 minUTC33*288 sec34* or 13 & 1430 min***35*288 sec36* or 15As neededMessage DataEphemerisClockISC, IONOReduced AlmanacDiff CorrectionGGTOText* Also contains SV clock correction parameters.** Complete set of SVs in the constellation.*** When Differential Corrections are available.†The intervals specified are maximum.

As such, the broadcast intervals may be shorter than thespecified value.IS-GPS-200D7 Dec 200418930.3.4.2 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 (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 SV time.30.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the aboveparagraphs isc = 2.99792458 x 108meters 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 200419030.3.5 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 randomerrors with a probability of undetected error ≤ 2-24 = 5.96×10-8 for all channel bit error probabilities ≤ 0.5. TheCRC 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 bymeans of a code that is generated by the polynomial24g(X ) = ∑ g i 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 binarypolynomial 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 theinformation sequence m(X) is expressed asm(X ) = m k + m k −1X + m k − 2 X 2 + ⋅ ⋅ ⋅ + m1X k −1IS-GPS-200D7 Dec 2004191The result is a quotient and a remainder R(X) of degree < 24.

The bit sequence formed by this remainder representsthe parity check sequence. Parity bit pi, for any i from 1 to 24, is the coefficient 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 polynomial g(X) has afactor 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. The fraction oferror 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.IS-GPS-200D7 Dec 2004192(This page intentionally left blank.)IS-GPS-200D7 Dec 2004193.