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Many of the parameters which describe the SV state vary with true time, and musttherefore be expressed as time functions with coefficients provided by the Navigation Message to be evaluated bythe user equipment. These include the following parameters as functions of GPS time:a.SV time,b.Mean anomaly,c.Longitude of ascending node,d.UTC,e.Inclination.Each of these parameters is formulated as a polynomial in time. The specific time scale of expansion can bearbitrary.
Due to the short data field lengths available in the Navigation Message format, the nominal epoch of thepolynomial is chosen near the midpoint of the expansion range so that quantization error is small. This results intime epoch values which can be different for each data set. Time epochs contained in the Navigation Message andthe different algorithms which utilize them are related as follows:EpochApplication Algorithm Referencetoc20.3.3.3.3.1toe20.3.3.4.3toa20.3.3.5.2.2 and 20.3.3.5.2.3tot20.3.3.5.2.4Table 20-XIII describes the nominal selection which will be expressed modulo 604,800 seconds in the NavigationMessage.IS-GPS-200D7 Dec 2004130The coefficients of expansion are obviously dependent upon choice of epoch, and thus the epoch time andexpansion coefficients must be treated as an inseparable parameter set.Note that a user applying currentnavigation data will normally be working with negative values of (t-toc) and (t-toe) in evaluating the expansions.The CS shall assure that the toe value, for at least the first data set transmitted by an SV after a new upload, isdifferent from that transmitted prior to the cutover (see paragraph 20.3.4.4).
As such, when a new upload is cutoverfor transmission, the CS shall introduce a small deviation in the toe resulting in the toe value that is offset from thehour boundaries (see Table 20-XIII). This offset toe will be transmitted by an SV in the first data set after a newupload cutover and the second data set, following the first data set, may also continue to reflect the same offset inthe toe.When the toe, immediately prior to a new upload cutover, already reflects a small deviation (i.e. a new uploadcutover has occurred in the recent past), then the CS shall introduce an additional deviation to the toe when a newupload is cutover for transmission.A change from the broadcast reference time immediately prior to cutover is used to indicate a change of values inthe data set.
The user may use the following example algorithm to detect the occurrence of a new upload cutover:DEV = toe [modulo 3600]If DEV ≠ 0, then a new upload cutover has occurred within past 4 hours.IS-GPS-200D7 Dec 2004131Table 20-XIII. Reference TimesHours After First Valid Transmission TimeFit Interval (hours)TransmissionInterval (hours)toc(clock)toe(ephemeris)42*22643386441412772624131350482525747237379896494912212061611461447373144 (6 days)> 144 (6 days)*toa(almanac)tot(UTC)1447070> 1447070Some SVs will have transmission intervals of 1 hour per paragraph 20.3.4.4.IS-GPS-200D7 Dec 200413220.3.5 Data Frame Parity.
The data signal shall contain parity coding according to the following conventions.20.3.5.1 SV/CS Parity Algorithm. This algorithm links 30-bit words within and across subframes of ten wordsusing the (32,26) Hamming Code described in Table 20-XIV.20.3.5.2 User Parity Algorithm. As far as the user is concerned, several options are available for performing datadecoding and error detection.
Figure 20-5 presents an example flow chart that defines one way of recovering data(dn) and checking parity. The parity bit D30* is used for recovering raw data. The parity bits D29* and D30*, alongwith the recovered raw data (dn) are modulo-2 added in accordance with the equations appearing in Table 20-XIVfor D25 . . . D30, which provide parity to compare with transmitted parity D25 . . . D30.IS-GPS-200D7 Dec 2004133Table 20-XIV.
Parity Encoding EquationsD1=d1 ⊕ D30D2=d2 ⊕ D30D3=d3 ⊕ D30••••••••D24=d24 ⊕ D30D25=D29 ⊕ d1 ⊕ d2 ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d10 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d17 ⊕ d18 ⊕ d20 ⊕ d23D26=D30 ⊕ d2 ⊕ d3 ⊕ d4 ⊕ d6 ⊕ d7 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d18 ⊕ d19 ⊕ d21 ⊕ d24D27=D29 ⊕ d1 ⊕ d3 ⊕ d4 ⊕ d5 ⊕ d7 ⊕ d8 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16⊕ d19 ⊕ d20 ⊕ d22D28=D30 ⊕ d2 ⊕ d4 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d20 ⊕ d21 ⊕ d23D29=D30 ⊕ d1⊕ d3⊕ d5⊕ d6⊕ d7⊕ d9 ⊕ d10 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d18 ⊕ d21 ⊕ d22⊕ d24D30=D29 ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d10 ⊕ d11 ⊕ d13 ⊕ d15 ⊕ d19 ⊕ d22 ⊕ d23 ⊕ d24Whered1, d2, ..., d24 are the source data bits;the symbol is used to identify the last 2 bits of the previous word of the subframe;D25, D26, ..., D30 are the computed parity bits;D1, D2, ..., D29, D30 are the bits transmitted by the SV;⊕ is the "modulo-2" or "exclusive-or" operation.IS-GPS-200D7 Dec 2004134ENTERCOMPLEMENTD1 . . .
D24TO OBTAINd1 . . . d24YES* = 1?IS D30NODO NOTCOMPLEMENTD1 . . . D24TO OBTAINd1 . . . d24SUBSTITUTE d1 . . . d24,* & D30* INTOD29PARITY EQUATIONS(TABLE 20-XIV)NOARE COMPUTEDD25 . . . D30EQUAL TO CORRESPONDINGRECEIVEDD25 . . . D30?YESPARITY CHECKFAILSPARITY CHECKPASSESFAILEXITPASSEXITFigure 20-5. Example Flow Chart for User Implementation of Parity AlgorithmIS-GPS-200D7 Dec 2004135(This page intentionally left blank.)IS-GPS-200D7 Dec 200413630. APPENDIX III. GPS NAVIGATION DATA STRUCTURE FOR CNAV DATA, DC(t)30.1 Scope.
This appendix describes the specific GPS CNAV data structure denoted as DC(t).30.2 Applicable Documents.30.2.1 Government Documents. In addition to the documents listed in paragraph 2.1, the following documents ofthe issue specified contribute to the definition of the CNAV data related interfaces and form a part of this Appendixto the extent specified herein.SpecificationsNoneStandardsNoneOther PublicationsNone30.2.2Non-Government Documents.In addition to the documents listed in paragraph 2.2, the followingdocuments of the issue specified contribute to the definition of the CNAV data related interfaces and form a part ofthis Appendix to the extent specified herein.SpecificationsNoneOther PublicationsNoneIS-GPS-200D7 Dec 2004137(This page intentionally left blank.)IS-GPS-200D7 Dec 200413830.3 Requirements.30.3.1 Data Characteristics. The CNAV data, DC(t), is a higher precision representation and nominally containsmore accurate data than the NAV data, D(t), described in Appendix II.
Also, the CNAV data stream uses a differentparity algorithm.Users are advised that the CNAV data, DC(t), described in this appendix and the NAV data, D(t), described inAppendix II, should not be mixed in any user algorithms or applications. Each of the two data sets should be treatedas a set and used accordingly.30.3.2 Message Structure. As shown in Figures 30-1 through 30-14, the CNAV message structure utilizes a basicformat of twelve-second 300-bit long messages.
Each message contains a Cyclic Redundancy Check (CRC) parityblock consisting of 24 bits covering the entire twelve-second message (300 bits) (reference Section 30.3.5).Message type 0 (zero) is defined to be the default message. In the event of message generation failure, the SV shallreplace each affected message type with the default message type. In the event that a particular message is notassigned (by the CS) a particular message type for broadcast, the SV shall generate and broadcast the defaultmessage type in that message slot.Currently undefined and unused message types are reserved for future use.30.3.3 Message Content.
Each message starts with an 8-bit preamble – 10001011, followed by a 6-bit PRN numberof the transmitting SV, a 6-bit message type ID with a range of 0 (000000) to 63 (111111), and the 17-bit messagetime of week (TOW) count. When the value of the message TOW count is multiplied by 6, it represents SV time inseconds at the start of the next 12-second message. An “alert” flag, when raised (bit 38 = “1”), indicates to the userthat the SV URA and/or the SV User Differential Range Accuracy (UDRA) may be worse than indicated in therespective message types, and the SV should be used at the user’s own risk. For each default message (MessageType 0), bits 39 through 276 shall be alternating ones and zeros and the message shall contain a proper CRC parityblock.IS-GPS-200D7 Dec 2004139(This page intentionally left blank.)IS-GPS-200D7 Dec 2004140211591PRN6BITS8 BITSDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS38526639556BITSMESSAGETOW COUNT*"ALERT" FLAG - 1 BITPREAMBLE11 BITS13BITS17 BITSMESSAGE TYPE IDtopWNnL1 HEALTH - 1 BITL2 HEALTH - 1 BITL5 HEALTH - 1 BIT133108∆A•A7 LSBs25 BITS206M0-n∆A19 MSBs11 BITSURA oe INDEXMSB FIRST150173∆ n0•∆ n0M0-n17 BITS23 BITS28 MSBsMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS2018271toe5BITSDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101MSB FIRST272239277enωnCRC33 BITS33 BITS24 BITS5 LSBsRESERVED – 5 BITs* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-1.
Message Type 10 - Ephemeris 1IS-GPS-200D7 Dec 20041411591PRN6BITS8 BITS6BITSMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS3850392183MESSAGETOW COUNT*toeΩ 0-ni0-n17 BITS11 BITS33 BITS18 MSBsMESSAGE TYPE IDPREAMBLE"ALERT" FLAG - 1 BITMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101133116148164180i0-n•∆Ωi0-n - DOTc is-nc ic-nc rs-n15 LSBs17 BITS15 BITS16 BITS16 BITS21 MSBsMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS201204228270249c rc-nc us-nc uc-n24 BITS21 BITS21 BITSc rs-n - 3 LSBs277CRC7 BITS24 BITSRESERVED* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-2.
Message Type 11 - Ephemeris 2IS-GPS-200D7 Dec 2004142211591PRN6BITS8 BITS6BITSMESSAGETOW COUNT*top17 BITS11 BITS5BITStoca f0-n11 BITS26 BITSa f1-n – 3 MSBsURA oc INDEX"ALERT" FLAG - 1 BITMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS1019872URA oc2 INDEX - 3 BITSURA oc1 INDEX - 3 BITSMESSAGE TYPE IDPREAMBLEMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS385055 58 6139128118141154167180193a f1-na f2-nTGDISC L1C/AISC L2CISC L5I5ISC L5Q5α017 LSBs10 BITS13 BITS13 BITS13 BITS13 BITS13 BITS8 BITSDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS209201217225233241277257249MSB FIRSTα1α2α3β0β1β2β3RESERVEDCRC8 BITs8 BITS8 BITS8 BITS8 BITS8 BITS8 BITS20 BITS24 BITS* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-3.
Message Type 30 - Clock, IONO & Group DelayIS-GPS-200D7 Dec 2004143211591PRN6BITS8 BITS6BITSMESSAGETOW COUNT*top17 BITS11 BITS5BITStoca f0-n11 BITS26 BITSa f1-n – 3 MSBsURA oc INDEX"ALERT" FLAG - 1 BITMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101128118141149180Reduced AlmanacPacket 1a f1-na f2-nWN a-ntoa17 LSBs10 BITS13 BITS8 BITS10 LSBs211Reduced AlmanacPacket 231 BITSDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS2019872URA oc2 INDEX - 3 BITSURA oc1 INDEX - 3 BITSMESSAGE TYPE IDPREAMBLEMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS385055 58 613921 MSBsMSB FIRST273 277242Reduced AlmanacPacket 3Reduced AlmanacPacket 431 BITS31 BITSReduced Almanac Packet 2CRC4BITS24 BITSRESERVED* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-4. Message Type 31 - Clock & Reduced AlmanacIS-GPS-200D7 Dec 2004144211591PRN6BITS8 BITS6BITSMESSAGETOW COUNT*top17 BITS11 BITS5BITStoca f0-n11 BITS26 BITSa f1-n – 3 MSBsURA oc INDEX"ALERT" FLAG - 1 BITMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101144128118165180a f1-na f2-ntEOPPM-X•PM-X17 LSBs10 BITS16 BITS21 BITS15 BITS15 BITS266247216•PM-YPM-Y21 BITSMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS2019872URA oc2 INDEX - 3 BITSURA oc1 INDEX - 3 BITSMESSAGE TYPE IDPREAMBLEMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS385055 58 6139277∆UT1•∆UT1RESERVEDCRC31 BITS19 BITS11 BITS24 BITS* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-5.
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