IS-GPS-200H (797934), страница 14
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Bit 1 is transmitted first.Each TLM word shall begin with a preamble, followed by the TLM message, the integrity statusflag, one reserved bit, and six parity bits. The TLM message contains information needed by theprecise positioning service (PPS) user (authorized user) and by the CS, as described in the relatedSS/CS interface documentation.Bit 23 of each TLM word is the Integrity Status Flag (ISF). A "0" in bit position 23 indicatesthat the conveying signal is provided with the legacy level of integrity assurance.
That is, theprobability that the instantaneous URE of the conveying signal exceeds 4.42 times the upperbound value of the current broadcast URA index, for more than 5.2 seconds, without anaccompanying alert, is less than 1E-5 per hour. A "1" in bit-position 23 indicates that theconveying signal is provided with an enhanced level of integrity assurance. That is, theprobability that the instantaneous URE of the conveying signal exceeds 5.73 times the upperbound value of the current broadcast URA index, for more than 5.2 seconds, without anaccompanying alert, is less than 1E-8 per hour.
The probabilities associated with the nominaland lower bound values of the current broadcast URA index are not defined.In this context, an "alert" is defined as any indication or characteristic of the conveying signal, asspecified elsewhere in this document, which signifies to users that the conveying signal may beinvalid or should not be used, such as the health bits not indicating operational-healthy,broadcasting non-standard code, parity error, etc.20.3.3.2 Handover Word (HOW).The HOW shall be 30 bits long and shall be the second word in each subframe/page,immediately following the TLM word. A HOW occurs every 6 seconds in the data frame.
Theformat and content of the HOW shall be as shown in Figure 20-2. The MSB is transmitted first.The HOW begins with the 17 MSBs of the time-of-week (TOW) count. (The full TOW countconsists of the 19 LSBs of the 29-bit Z-count). These 17 bits correspond to the TOW-count atthe X1 epoch which occurs at the start (leading edge) of the next following subframe (referenceparagraph 3.3.4).87IS-GPS-200H24 Sep 2013Bit 18 is an "alert" flag.
When this flag is raised (bit 18 = "1"), it shall indicate to the standardpositioning service (SPS) user (unauthorized user) that the signal URA may be worse thanindicated in subframe 1 and that he shall use that SV at his own risk.Bit 19 is an anti-spoof (A-S) flag. A "1" in bit-position 19 indicates that the A-S mode is ON inthat SV.Bits 20, 21, and 22 of the HOW provide the ID of the subframe in which that particular HOW isthe second word; the ID code shall be as follows:SubframeID Code1001201030114100510188IS-GPS-200H24 Sep 2013TLM Word1 = Reserved Bits2 = Integrity Status FlagMSBPreamble2TLM Message1 0 0 0 1 0 1 11 2 3 4 5 6 7 8ParityLSB9 10 11 12 13114 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30HOWSolved for bits to preserveparity check with zeros inbits 29 and 30Anti-Spoof Flag“Alert” FlagMSBParityLSBSubframeIDTOW-Count Message(Truncated)1 2 3 4 5 6 7 8 910 11 12 13 14 15 16 17 18 19Figure 20-2.0020 21 22 23 24 25 26 27 28 29 30TLM and HOW Formats89IS-GPS-200H24 Sep 201320.3.3.3 Subframe 1.The content of words three through ten of subframe 1 are defined below, followed by relatedalgorithms and material pertinent to use of the data.20.3.3.3.1 Subframe 1 Content.The third through tenth words of subframe 1 shall each contain six parity bits as their LSBs; inaddition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten forparity computation purposes.
The remaining 190 bits of words three through ten shall containthe clock parameters and other data described in the following.The clock parameters describe the SV time scale during the period of validity. The parametersare applicable during the time in which they are transmitted. The timing information forsubframes, pages, and data sets is covered in Section 20.3.4.20.3.3.3.1.1 Transmission Week Number.The ten MSBs of word three shall contain the ten LSBs of the Week Number as defined in 3.3.4.These ten bits shall be a modulo 1024 binary representation of the current GPS week number atthe start of the data set transmission interval (see paragraph 3.3.4(b)).
The GPS week numberincrements at each end/start of week epoch. For Block II SVs in long-term extended operations,beginning approximately 28 days after upload, the transmission week number may notcorrespond to the actual GPS week number due to curve fit intervals that cross week boundaries.20.3.3.3.1.2 Code(s) on L2 Channel.Bits 11 and 12 of word three shall indicate which code(s) is (are) commanded ON for the L2channel, as follows:00 = Reserved,01 = P code ON,10 = C/A code ON.20.3.3.3.1.3 SV Accuracy.Bits 13 through 16 of word three shall give the URA index of the SV (reference paragraph 6.2.1)for the standard positioning service user. While the URA may vary over the ephemeris curve fitinterval, the URA index (N) in the LNAV message shall correspond to the maximum URAexpected over the entire ephemeris curve fit interval. Except for Block IIR/IIR-M SVs in the90IS-GPS-200H24 Sep 2013Autonav mode, the URA index (N) is an integer in the range of 0 through 15 and has thefollowing relationship to the URA of the SV:URA INDEXURA (meters)00.00< URA ≤2.4012.40< URA ≤3.4023.40< URA ≤4.8534.85< URA ≤6.8546.85< URA ≤9.6559.65< URA ≤13.65613.65< URA ≤24.00724.00< URA ≤48.00848.00< URA ≤96.00996.00< URA ≤192.0010192.00< URA ≤384.0011384.00< URA ≤768.0012768.00< URA ≤1536.00131536.00< URA ≤3072.00143072.00< URA ≤6144.00156144.00< URA (or no accuracy prediction is available standard positioning service users are advised to usethe SV at their own risk.)For each URA index (N), users may compute a nominal URA value (X) as given by:• If the value of N is 6 or less, X = 2(1 + N/2),91IS-GPS-200H24 Sep 2013• If the value of N is 6 or more, but less than 15, X = 2(N - 2),• N = 15 shall indicate the absence of an accuracy prediction and shall advise the standardpositioning service user to use that SV at his own risk.For N = 1, 3, and 5, X should be rounded to 2.8, 5.7, and 11.3 meters, respectively.For Block IIR/IIR-M SVs in the Autonav mode, the URA shall be defined to mean “no betterthan X meters”, with “X” as defined above for each URA index.The nominal URA value (X) is suitable for use as a conservative prediction of the RMS signalin-space (SIS) range errors for accuracy-related purposes in the pseudorange domain (e.g.,measurement de-weighting, receiver autonomous integrity monitoring (RAIM), figure of merit(FOM) computations).
Integrity properties of the URA are specified with respect to the scaled(multiplied by either 4.42 or 5.73 as appropriate) upper bound values of the URA index (see20.3.3.1).URA accounts for SIS contributions to user range error which include, but are not limited to, thefollowing: LSB representation/truncation error; the net effect of clock correction polynomialerror and code phase error in the transmitted signal for single-frequency L1C/A or singlefrequency L2C users who correct the code phase as described in Section 30.3.3.3.1.1.1; the neteffect of clock parameter, code phase, and inter-signal correction error for dual-frequency L1/L2and L1/L5 users who correct for group delay and ionospheric effects as described in Section30.3.3.3.1.1.2; ephemeris error; anisotropic antenna errors; and signal deformation error.
URAdoes not account for user range error contributions due to the inaccuracy of the broadcastionospheric data parameters used in the single-frequency ionospheric model or for otheratmospheric effects.20.3.3.3.1.4 SV Health.The six-bit health indication given by bits 17 through 22 of word three refers to the transmittingSV. The MSB shall indicate a summary of the health of the NAV data, where0 = all NAV data are OK,1 = some or all NAV data are bad.The five LSBs shall indicate the health of the signal components in accordance with the codesgiven in paragraph 20.3.3.5.1.3.
The health indication shall be given relative to the "as designed"capabilities of each SV (as designated by the configuration code - see paragraph 20.3.3.5.1.4).92IS-GPS-200H24 Sep 2013Accordingly, any SV which does not have a certain capability will be indicated as "healthy" ifthe lack of this capability is inherent in its design or if it has been configured into a mode whichis normal from a user standpoint and does not require that capability.Additional SV health data are given in subframes 4 and 5. The data given in subframe 1 maydiffer from that shown in subframes 4 and/or 5 of other SVs since the latter may be updated at adifferent time.20.3.3.3.1.5 Issue of Data, Clock (IODC).Bits 23 and 24 of word three in subframe 1 shall be the two MSBs of the ten-bit IODC term; bitsone through eight of word eight in subframe 1 shall contain the eight LSBs of the IODC.
TheIODC indicates the issue number of the data set and thereby provides the user with a convenientmeans of detecting any change in the correction parameters. Constraints on the IODC as well asthe relationship between the IODC and the IODE (issue of data, ephemeris) terms are defined inparagraph 20.3.4.4.Short-term and Long-term Extended Operations.Whenever the fit interval flag indicates a fit interval greater than 4 hours, the IODC can be usedto determine the actual fit interval of the data set (reference section 20.3.4.4).20.3.3.3.1.6 Data Flag for L2 P-Code.When bit 1 of word four is a "1", it shall indicate that the NAV data stream was commandedOFF on the P-code of the L2 channel.20.3.3.3.1.7 Estimated Group Delay Differential.Bits 17 through 24 of word seven contain the L1-L2 correction term, TGD, for the benefit of "L1only" or "L2 only" users; the related user algorithm is given in paragraph 20.3.3.3.3.20.3.3.3.1.8 SV Clock Correction.Bits nine through 24 of word eight, bits one through 24 of word nine, and bits one through 22 ofword ten contain the parameters needed by the users for apparent SV clock correction (toc, af2,af1, af0).
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