IS-GPS-200F (811524), страница 15
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The format shall be as shown in Figure 20-2. Bit 1 is transmitted first. EachTLM word shall begin with a preamble, followed by the TLM message, the integrity status flag, one reserved bit,and six parity bits. The TLM message contains information needed by the precise positioning service (PPS) user(authorized user) and by the CS, as described in the related SS/CS interface documentation.Bit 23 of each TLM word is the Integrity Status Flag (ISF). A "0" in bit position 23 indicates that the conveyingsignal is provided with the legacy level of integrity assurance. That is, the probability that the instantaneous URE ofthe conveying signal exceeds 4.42 times the upper bound value of the current broadcast URA index, for more than5.2 seconds, without an accompanying alert, is less than 1E-5 per hour.
A "1" in bit-position 23 indicates that the89IS-GPS-200F21 Sep 2011conveying signal is provided with an enhanced level of integrity assurance. That is, the probability that theinstantaneous URE of the conveying signal exceeds 5.73 times the upper bound value of the current broadcast URAindex, for more than 5.2 seconds, without an accompanying alert, is less than 1E-8 per hour. The probabilitiesassociated with the nominal and 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, as specifiedelsewhere in this document, which signifies to users that the conveying signal may be invalid 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 eachsubframe/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 beginswith the 17 MSBs of the time-of-week (TOW) count. (The full TOW count consists of the 19 LSBs of the 29-bit Zcount).
These 17 bits correspond to the TOW-count at the X1 epoch which occurs at the start (leading edge) of thenext following subframe (reference paragraph 3.3.4).Bit 18 is an "alert" flag. When this flag is raised (bit 18 = "1"), it shall indicate to the standard positioning service(SPS) user (unauthorized user) that the signal URA may be worse than indicated in subframe 1 and that he shall usethat 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 in that SV.Bits 20, 21, and 22 of the HOW provide the ID of the subframe in which that particular HOW is the second word;the ID code shall be as follows:SubframeID Code1001201030114100510190IS-GPS-200F21 Sep 2011Figure 20-2.TLM and HOW Formats20.3.3.3 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 astheir LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten for paritycomputation purposes. The remaining 190 bits of words three through ten shall contain the clock parameters andother data described in the following.The clock parameters describe the SV time scale during the period of validity. The parameters are applicable duringthe time in which they are transmitted.
The timing information for subframes, pages, and data sets is covered inSection 20.3.4.91IS-GPS-200F21 Sep 201120.3.3.3.1.1 Transmission Week Number. The ten MSBs of word three shall contain the ten LSBs of the WeekNumber as defined in 3.3.4. These ten bits shall be a modulo 1024 binary representation of the current GPS weeknumber at the start of the data set transmission interval (see paragraph 3.3.4(b)).
The GPS week number incrementsat each end/start of week epoch. For Block II SVs in long-term extended operations, beginning approximately 28days after upload, the transmission week number may not correspond to the actual GPS week number due to curvefit 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) commandedON for the L2 channel, 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 (referenceparagraph 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 URA expected over the entireephemeris curve fit interval. Except for Block IIR/IIR-M SVs in the Autonav mode, the URA index (N) is aninteger in the range of 0 through 15 and has the following 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 ≤96.0092IS-GPS-200F21 Sep 2011996.00< 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 areadvised to use the 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),• 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 standard positioning serviceuser 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 better than 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 signal-in-space (SIS) rangeerrors for accuracy-related purposes in the pseudorange domain (e.g., measurement de-weighting, receiverautonomous integrity monitoring (RAIM), figure of merit (FOM) computations).
Integrity properties of the URAare specified with respect to the scaled (multiplied by either 4.42 or 5.73 as appropriate) upper bound values of theURA index (see 20.3.3.1).URA accounts for SIS contributions to user range error which include, but are not limited to, the following: LSBrepresentation/truncation error; the net effect of clock correction polynomial error and code phase error in thetransmitted signal for single-frequency L1C/A or single-frequency L2C users who correct the code phase asdescribed in Section 30.3.3.3.1.1.1; the net effect of clock parameter, code phase, and inter-signal correction errorfor dual-frequency L1/L2 and L1/L5 users who correct for group delay and ionospheric effects as described inSection 30.3.3.3.1.1.2; ephemeris error; anisotropic antenna errors; and signal deformation error.
URA does not93IS-GPS-200F21 Sep 2011account for user range error contributions due to the inaccuracy of the broadcast ionospheric data parameters used inthe single-frequency ionospheric model or for other atmospheric 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 thetransmitting SV. 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 codes given in paragraph20.3.3.5.1.3.
The health indication shall be given relative to the "as designed" capabilities of each SV (as designatedby the configuration code - see paragraph 20.3.3.5.1.4). Accordingly, any SV which does not have a certaincapability will be indicated as "healthy" if the lack of this capability is inherent in its design or if it has beenconfigured into a mode which is 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 may differ from that shownin subframes 4 and/or 5 of other SVs since the latter may be updated at a different 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 theten-bit IODC term; bits one through eight of word eight in subframe 1 shall contain the eight LSBs of the IODC.The IODC indicates the issue number of the data set and thereby provides the user with a convenient means ofdetecting any change in the correction parameters. Constraints on the IODC as well as the relationship between theIODC and the IODE (issue of data, ephemeris) terms are defined in paragraph 20.3.4.4.Short-term and Long-term Extended Operations. Whenever the fit interval flag indicates a fit interval greater than 4hours, the IODC can be used to determine the actual fit interval of the data set (reference section 20.3.4.4).94IS-GPS-200F21 Sep 201120.3.3.3.1.6 Data Flag for L2 P-Code.
When bit 1 of word four is a "1", it shall indicate that the NAV data streamwas commanded OFF 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 correctionterm, TGD, for the benefit of "L1 only" or "L2 only" users; the related user algorithm is given in paragraph20.3.3.3.3.20.3.3.3.1.8 SV Clock Correction. Bits nine through 24 of word eight, bits one through 24 of word nine, and bitsone through 22 of word ten contain the parameters needed by the users for apparent SV clock correction (toc, af2, af1,af0). The related algorithm is given in paragraph 20.3.3.3.3.20.3.3.3.2 Subframe 1 Parameter Characteristics.
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