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Ionospheric Model (Sheet 3 of 3)IS-GPS-200D7 Dec 200412520.3.4 Timing Relationships. The following conventions shall apply.20.3.4.1 Paging and Cutovers. At end/start of week (a) the cyclic paging of subframes 1 through 5 shall restartwith subframe 1 regardless of which subframe was last transmitted prior to end/start of week, and (b) the cycling ofthe 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes, regardless of which page wasthe last to be transmitted prior to the end/start of week.
Cutovers to newly updated data for subframes 1, 2, and 3occur on frame boundaries (i.e., modulo 30 seconds relative to end/start of week). Newly updated data forsubframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes.20.3.4.2 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 in the TLM word and the HOW 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 message shall be executed by the SV on SV time.20.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the aboveparagraphs isc = 2.99792458 x 108 meters 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 200412620.3.4.4 Data Sets.
The IODE is an 8 bit number equal to the 8 LSBs of the 10 bit IODC of the same data set. Thefollowing rules govern the transmission of IODC and IODE values in different data sets: (1) The transmittedIODC will be different from any value transmitted by the SV during the preceding seven days; (2) The transmittedIODE will be different from any value transmitted by the SV during the preceding six hours. The range of IODCwill be as given in Table 20-XI for Block II/IIA SVs and Table 20-XII for Block IIR/IIR-M/IIF SVs.Cutovers to new data sets will occur only on hour boundaries except for the first data set of a new upload. The firstdata set may be cut-in (reference paragraph 20.3.4.1) at any time during the hour and therefore may be transmittedby the SV for less than one hour.
During short-term operations, cutover to 4-hour sets and subsequent cutovers tosucceeding 4-hour data sets will always occur modulo 4 hours relative to end/start of week. Cutover from 4-hourdata sets to 6-hour data sets shall occur modulo 12 hours relative to end/start of week. Cutover from 12-hour datasets to 24-hour data sets shall occur modulo 24 hours relative to end/start of week. Cutover from a data settransmitted 24 hours or more occurs on a modulo 24-hour boundary relative to end/start of week.The start of the transmission interval for each data set corresponds to the beginning of the curve fit interval for thedata set. Each data set remains valid for the duration of its curve fit interval.Normal Operations.
The subframe 1, 2, and 3 data sets are transmitted by the SV for periods of two hours. Thecorresponding curve fit interval is four hours. SVs operating in the Autonav mode will deviate. They will transmitsubframe 1, 2, and 3 data sets for periods of one hour. The corresponding curve-fit interval will be four hours.Short-term and Long-term Extended Operations.The transmission intervals and curve fit intervals with theapplicable IODC ranges are given in Tables 20-XI and 20-XII.IS-GPS-200D7 Dec 2004127Table 20-XI.IODC Values and Data Set Lengths (Block II/IIA)DaysSpannedTransmission Interval(hours)(Note 4)Curve FitInterval(hours)IODC Range(Note 1)12-1415-1617-2021-2728-4142-5960-6324612244872964681426507498(Note 2)(Note 2)240-247248-255, 496 (Note 3)497-503504-510511, 752-756757Note 1: For transmission intervals of 6 hours or greater, the IODC values shown will be transmitted inincreasing order.Note 2: IODC values for blocks with 2- or 4-hour transmission intervals (at least the first 14 days afterupload) shall be any numbers in the range 0 to 1023 excluding those values of IODC thatcorrespond to IODE values in the range 240-255, subject to the constraints on re-transmissiongiven in paragraph 20.3.4.4.Note 3: The ninth 12-hour data set may not be transmitted.Note 4: 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.IS-GPS-200D7 Dec 2004128Table 20-XII.
IODC Values and Data Set Lengths (Block IIR/IIR-M/IIF)DaysSpanned12-1415-1617-2021-62Transmission Interval(hours)(Note 5)Curve FitInterval(hours)IODC Range4681426(Note 2)(Note 2)240-247 (Note 1)248-255, 496 (Note 1) (Note 3)497-503, 1021-10232 (Note 4)461224Note 1: For transmission intervals of 6 and 12 hours, the IODC values shown will be transmitted inincreasing order.Note 2: IODC values for blocks with 1-, 2- or 4-hour transmission intervals (at least the first 14 days afterupload) shall be any numbers in the range 0 to 1023 excluding those values of IODC thatcorrespond to IODE values in the range 240-255, subject to the constraints on re-transmissiongiven in paragraph 20.3.4.4.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.IS-GPS-200D7 Dec 200412920.3.4.5 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.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 .
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