IS-GPS-200H (797934), страница 21
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The CS can define the GPS III SV time of transition from the 4 hourcurve fits into extended navigation (beyond 4 hour curve fits). Following the transition time, theSV will follow the timeframes defined in the table, including appropriately setting IODC values.Note 3: The ninth 12-hour data set may not be transmitted.Note 4: SVs operating in the Autonav mode will have transmission intervals of 1 hour per paragraph20.3.4.4.Note 5: The first data set of a new upload may be cut-in at any time and therefore the transmission intervalmay be less than the specified value.133IS-GPS-200H24 Sep 201320.3.4.5 Reference Times.Many 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.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 ofexpansion can be arbitrary. Due to the short data field lengths available in the NavigationMessage format, the nominal epoch of the polynomial is chosen near the midpoint of theexpansion range so that quantization error is small. This results in time epoch values which canbe different for each data set. Time epochs contained in the Navigation Message and thedifferent algorithms which 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 20.3.3.5.2.3tot20.3.3.5.2.4Table 20-XIII describes the nominal selection which will be expressed modulo 604,800 secondsin 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.134IS-GPS-200H24 Sep 2013The CS (Block II/IIA/IIR/IIR-M/IIF) and SS (GPS III) shall assure that the toe value, for at leastthe first data set transmitted by an SV after a new upload, is different from that transmitted priorto the cutover (see paragraph 20.3.4.4). As such, when a new upload is cutover for transmission,the CS (Block IIA/IIR/IIR-M/IIF) and SS (GPS III) shall introduce a small deviation in the toeresulting in the toe value that is offset from the hour boundaries (see Table 20-XIII). This offsettoe will be transmitted by an SV in the first data set after a new upload cutover and the seconddata set, following the first data set, may also continue to reflect the same offset in 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 II/IIA/IIR/IIR-M/IIF)and SS (GPS III) shall introduce an additional deviation to the toe when a new upload is cutoverfor transmission.A change from the broadcast reference time immediately prior to cutover is used to indicate achange of values in the data set. The user may use the following example algorithm to detect theoccurrence of a new upload cutover:DEV = toe [modulo 3600]If DEV ≠ 0, then a new upload cutover has occurred within past 4 hours.135IS-GPS-200H24 Sep 2013Table 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 20.3.4.4.20.3.5 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 words using 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 data decoding anderror detection.
Figure 20-5 presents an example flow chart that defines one way of recoveringdata (dn) and checking parity. The parity bit D30* is used for recovering raw data. The parity bitsD29* and D30*, along with the recovered raw data (dn) are modulo-2 added in accordance with the136IS-GPS-200H24 Sep 2013equations appearing in Table 20-XIV for D25 . .
. D30, which provide parity to compare withtransmitted parity D25 . . . D30.Table 20-XIV.D1=d1 ⊕ D30D2=d2 ⊕ D30D3=d3 ⊕ D30••••••••Parity Encoding EquationsD24=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.137IS-GPS-200H24 Sep 2013ENTERCOMPLEMENTD1 .
. . D24TO OBTAINd1 . . . d24YES* = 1?IS D30NODO NOTCOMPLEMENTD1 . . . D24TO OBTAINd1 . . . d24SUBSTITUTE d1 . . . d24,* & D * INTOD2930PARITY EQUATIONS(TABLE 20-XIV)NOARE COMPUTEDD25 . . . D30EQUAL TO CORRESPONDINGRECEIVEDD25 . . . D30?YESPARITY CHECKFAILSPARITY CHECKPASSESFAILEXITPASSEXITFigure 20-5.Example Flow Chart for User Implementation of Parity Algorithm138IS-GPS-200H24 Sep 201330 APPENDIX III. GPS NAVIGATION DATA STRUCTURE FORCNAV 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 of the issuespecified contribute to the definition of the CNAV data related interfaces and form a part of thisAppendix to the extent specified herein.SpecificationsNoneStandardsNoneOther PublicationsNone30.2.2 Non-Government Documents.In addition to the documents listed in paragraph 2.2, the following documents of the issuespecified contribute to the definition of the CNAV data related interfaces and form a part of thisAppendix to the extent specified herein.SpecificationsNoneOther PublicationsNone30.3 Requirements.30.3.1 Data Characteristics.The CNAV data, DC(t), is a higher precision representation and nominally contains moreaccurate data than the NAV data, D(t), described in Appendix II.
Also, the CNAV data streamuses a different parity algorithm.139IS-GPS-200H24 Sep 2013Users are advised that the CNAV data, DC(t), described in this appendix and the NAV data, D(t),described in Appendix II, should not be mixed in any user algorithms or applications. Each ofthe two data sets should be treated as 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 basic format oftwelve-second 300-bit long messages.
Each message contains a Cyclic Redundancy Check(CRC) parity block 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 theevent of message generation failure, the SV shall replace each affected message type with thedefault message type.
In the event that a particular message is not assigned (by the CS) aparticular message type for broadcast, the SV shall generate and broadcast the default messagetype in that message slot.Currently undefined and unused message types are reserved for future use.Block IIR-M and IIF SVs have the capability of storing at least 48 hours of CNAV navigationdata, with current memory margins, to provide CNAV positioning service without contact fromthe CS for that period. GPS III SVs have the capability of providing up to 60 days of CNAVpositioning service without contact from the CS. The timeframe is defined by the CS.30.3.3 Message Content.Each message starts with an 8-bit preamble - 10001011, followed by a 6-bit PRN number of thetransmitting SV, a 6-bit message type ID with a range of 0 (000000) to 63 (111111), and the 17bit message time of week (TOW) count.
When the value of the message TOW count ismultiplied by 6, it represents SV time in seconds at the start of the next 12-second message. An“alert” flag, when raised (bit 38 = “1”), indicates to the users that the signal URA componentsmay be worse than indicated in the associated message types and that the users may use at theirown risk. For each default message (Message Type 0), bits 39 through 276 shall be alternatingones and zeros and the message shall contain a proper CRC parity block.140IS-GPS-200H24 Sep 2013121159PRN6BITS8 BITSDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS3852 5566396BITSMESSAGETOW COUNT*WN n17 BITS13 BITsMESSAGE TYPE ID"ALERT" FLAG - 1 BITPREAMBLEMSB FIRST827111 BITS5BITSL1 HEALTH - 1 BITL2 HEALTH - 1 BITL5 HEALTH - 1 BIT133108∆A•A7 LSBs25 BITS19 MSBs11 BITSURAED INDEXMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101∆At oet op173150•∆ n0∆ n0M0-n23 BITS17 BITS28 MSB sMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS272201206M0-n277239enωnCRC33 BITS33 BITS24 BITsIntegrity Status Flag – 1 BIT5 LSB sL2C Phasing – 1 BITRESERVED – 3 BITs* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12 SECONDMESSAGEFigure 30-1.Message Type 10 - Ephemeris 1141IS-GPS-200H24 Sep 20131915PRN6BITS8 BITS6BITSMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS3850392183MESSAGETOW COUNT*toeΩ0-ni0-n17 BITS11 BITS33 BITS18 MSBsMESSAGE TYPE IDPREAMBLE"ALERT" FLAG - 1 BITMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS101133116148164180i0-n•∆Ωi0-n - DOTcis-ncic-ncrs-n15 LSBs17 BITS15 BITS16 BITS16 BITS21 MSBsMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS201204228270249crc-ncus-ncuc-n24 BITS21 BITS21 BITScrs-n - 3 LSBs277CRC24 BITS7 BITSRESERVED* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-2.Message Type 11 - Ephemeris 2142IS-GPS-200H24 Sep 2013192115PRN6BITS8 BITS6BITSMESSAGETOW COUNT*top17 BITS11 BITS5BITS"ALERT" FLAG - 1 BITtocaf0-n11 BITS26 BITSMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS1011281189872URANED2 INDEX - 3 BITSURANED1 INDEX - 3 BITSaf1-n – 3 MSBsURANED0 INDEXMESSAGE TYPE IDPREAMBLEMSB FIRSTDIRECTION OF DATA FLOW FROM SV4 SECONDS100 BITS385055 58 6139141167154180193af1-naf2-nTGDISCL1C/AISCL2CISCL5I5ISCL5Q5α017 LSBs10 BITS13 BITS13 BITS13 BITS13 BITS13 BITS8 BITSMSB FIRSTDIRECTION OF DATA FLOW FROM SV100 BITS4 SECONDS201209217225233241257249α1α2α3β0β1β2β38 BITs8 BITS8 BITS8 BITS8 BITS8 BITS8 BITSWNOP8 BITS265277RESERVEDCRC12 BITS24 BITS* MESSAGE TOW COUNT = 17 MSB OF ACTUAL TOW COUNT AT START OF NEXT 12-SECOND MESSAGEFigure 30-3.
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