IS-GPS-200F (811524), страница 22
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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.20.3.4.5 Reference Times.
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.133IS-GPS-200F21 Sep 2011Each 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.The coefficients of expansion are obviously dependent upon choice of epoch, and thus the epoch time and expansioncoefficients must be treated as an inseparable parameter set.
Note that a user applying current navigation data willnormally be working with negative values of (t-toc) and (t-toe) in evaluating the expansions.The CS (Block II/IIA/IIR/IIR-M/IIF) and SS (GPS III) shall assure that the toe value, for at least the first data settransmitted by an SV after a new upload, is different from that transmitted prior to the cutover (see paragraph20.3.4.4). As such, when a new upload is cutover for transmission, the CS (Block IIA/IIR/IIR-M/IIF) and SS (GPSIII) shall introduce a small deviation in the toe resulting in the toe value that is offset from the hour boundaries (seeTable 20-XIII).
This offset toe will be transmitted by an SV in the first data set after a new upload cutover and thesecond data 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. a new uploadcutover has occurred in the recent past), then the CS (Block II/IIA/IIR/IIR-M/IIF) and SS (GPS III) shall introducean additional deviation to the toe when a new upload 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.134IS-GPS-200F21 Sep 2011Table 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 20.3.4.4.135IS-GPS-200F21 Sep 201120.3.5 Data Frame Parity The data signal shall contain parity coding according to the following conventions.20.3.5.1 User Frame Parity.
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 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.Table 20-XIV.D1=d1 ⊕ D30D2=d2 ⊕ D30D3=d3 ⊕ D30•••••••Parity Encoding Equations•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.136IS-GPS-200F21 Sep 2011ENTERCOMPLEMENTD1 .
. . D24TO OBTAINd1 . . . d24YES* = 1?IS D30NODO NOTCOMPLEMENTD1 . . . D24TO OBTAINd1 . . . d24SUBSTITUTE d1 . . . d24,* & D * INTOD2930PARITY EQUATIONS(TABLE 20-XIV)ARE COMPUTEDD25 . . . D30EQUAL TO CORRESPONDINGRECEIVEDD25 . . . D30?NOYESPARITY CHECKFAILSPARITY CHECKPASSESFAILEXITPASSEXITFigure 20-5.Example Flow Chart for User Implementation of Parity Algorithm137IS-GPS-200F21 Sep 201130.
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 PublicationsNone30.2.2 Non-Government Documents.
In addition to the documents listed in paragraph 2.2, the following documentsof the issue specified 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 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.138IS-GPS-200F21 Sep 201130.3.2 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.Block IIR-M and IIF SVs have the capability of storing at least 48 hours of CNAV navigation data, with currentmemory margins, to provide CNAV positioning service without contact from the CS for that period. GPS III SVshave the capability of providing up to 60 days of CNAV positioning service without contact from the CS.
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