IS-GPS-200H (797934), страница 9
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Naval ObservatoryUTC-Coordinated Universal TimeWGS 84-World Geodetic System 1984WN-Week NumberWNe-Extended Week Number47IS-GPS-200H24 Sep 20136.2Definitions6.2.1 User Range Accuracy.User Range Accuracy (URA) is a statistical indicator of the GPS ranging accuracy obtainablewith a specific signal and SV. URA provides a conservative RMS estimate of the user rangeerror (URE) in the associated navigation data for the transmitting SV. It includes all errors forwhich the Space and Control Segments are responsible. Whether the integrity status flag is 'off'or 'on', 4.42 times URA bounds the instantaneous URE with 1-(1e-5) per hour probability(‘legacy’ level of integrity assurance). When the integrity status flag is 'on', 5.73 times URAbounds the instantaneous URE with 1-(1e-8) per hour probability (‘enhanced’ level of integrityassurance).
Integrity properties of the URA are specified with respect to the scaled (multipliedby either 4.42 or 5.73 as appropriate) upper bound values of the URA index or to the scaledcomposite of the upper bound values of all component URA indexes.Note #1: URA applies over the transmission interval that is applicable to the NAV data fromwhich the URA is read, for the worst-case location within the satellite footprint.Note #2: The URA for a particular signal may be represented by a single index in the NAV dataor by a composite of more than one index representing components of the total URA.
SpecificURA indexes and formulae for calculating the total URA for each signal are defined in appendix20 for the LNAV message and appendix 30 for the CNAV message.Note #3: The URA is not required to bound the instantaneous URE when: (a) an alert is issuedto the users before the instantaneous URE exceeds either of the scaled URA bounds; or (b) if theintegrity status flag is 'off' an alert is issued to the users no more than 8.0 seconds after theinstantaneous URE exceeds the 4.42 times URA bound; or (c) if the integrity status flag is 'on' analert is issued to the users no more than 8.0 seconds after the instantaneous URE exceeds the4.42 times URA bound; or (d) if the integrity status flag is 'on' an alert is issued to users no morethan 5.2 seconds after the instantaneous URE exceeds the 5.73 times URA bound.
In thiscontext, 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.6.2.1.1 User Differential Range Accuracy.User Differential Range Accuracy (UDRA) is a statistical indicator of the GPS ranging accuracyobtainable with a specific signal and SV after the application of the associated differentialcorrections (DC parameters).
UDRA provides a conservative RMS estimate of the differentialuser range errors in the navigation data for that satellite. It includes all errors for which theSpace and Control Segments are responsible.48IS-GPS-200H24 Sep 20136.2.2 SV Block Definitions.The following block definitions are given to facilitate discussion regarding the capability of thevarious blocks of GPS satellites to support the SV-to-US interface.6.2.2.1 Developmental SVs.The original concept validation satellites developed by Rockwell International and designated assatellite vehicle numbers (SVNs) 1-11 are termed "Block I" SVs.
These SVs were designed toprovide 3-4 days of positioning service without contact from the CS. These SVs transmitted aconfiguration code of 000 (reference paragraph 20.3.3.5.1.4). There are no longer any activeBlock I SVs in the GPS constellation. The last Block I SV was decommissioned in 1995.6.2.2.2 Operational SVs.The operational satellites are designated Block II, Block IIA, Block IIR, Block IIR-M, Block IIFand GPS III SVs. Characteristics of these SVs are provided below. Modes of operation for theseSVs and accuracy of positioning services provided are described in paragraphs 6.3.2 through6.3.4. These SVs transmit configuration codes as specified in paragraph 20.3.3.5.1.4. Thenavigation signal provides no direct indication of the type of the transmitting SV.6.2.2.2.1 Block II SVs.The first block of full scale operational SVs developed by Rockwell International are designatedas SVNs 13-21 and are termed "Block II" SVs.
These SVs were designed to provide 14 days ofpositioning service without contact from the CS.6.2.2.2.2 Block IIA SVs.The second block of full scale operational SVs developed by Rockwell International aredesignated as SVNs 22-40 and are termed "Block IIA" SVs. These SVs are capable of providing60 days of positioning service without contact from the CS.6.2.2.2.3 Block IIR SVs.The block of operational replenishment SVs developed by Lockheed Martin are designated asSVNs 41-61 and are termed "Block IIR" SVs.
These SVs have the capability of storing at least60 days of navigation data with current memory margins, while operating in a IIA mode, toprovide positioning service without contact from the CS for that period. (Contractualrequirements for these SVs specify transmission of correct data for only 14 days to support shortterm extended operations while in IIA mode.) The IIR SV will provide a minimum of 60 days ofpositioning service without contact from the CS when operating in autonomous navigation(Autonav) mode.6.2.2.2.4 Block IIR-M SVs.The subset of operational replenishment SVs developed by Lockheed Martin which are“Modernized” configuration of “Block IIR” SVs are termed “Block IIR-M”.49IS-GPS-200H24 Sep 20136.2.2.2.5 Block IIF SVs.The block of operational replenishment SVs developed by Boeing are designated as SVNs 62-73and are termed “Block IIF” SVs.
This is the first block of operational SVs that transmit the L5Civil signal. These SVs will provide at least 60 days of positioning service without contact fromthe CS.6.2.2.2.6 GPS III SVs.The block of operational replenishment SVs are designated as SVNs 74-105. This is the firstblock of operational SVs that transmit the L1C signal. These SVs will provide at least 60 daysof positioning service without contact from the CS.6.2.3 Operational Interval Definitions.The following three operational intervals have been defined.
These labels will be used to refer todifferences in the interface definition as time progresses from SV acceptance of the lastnavigation data upload.6.2.3.1 Normal Operations.The SV is undergoing normal operations whenever the fit interval flag (reference paragraph20.3.3.4.3.1) is zero.6.2.3.2 Short-term Extended Operations.The SV is undergoing short-term extended operations whenever the fit interval flag is one andthe IODE (reference paragraph 20.3.4.4) is less than 240.6.2.3.3 Long-term Extended Operations.The SV is undergoing long-term extended operations whenever the fit interval flag is one and theIODE is in the range 240-255.6.2.4 GPS Week Number.The GPS week numbering system is established with week number zero (0) being defined as thatweek which started with the X1 epoch occurring at midnight UTC (USNO) on the night ofJanuary 5, 1980/ morning of January 6, 1980. The GPS week number continuously incrementsby one (1) at each end/start of week epoch without ever resetting to zero.
Users must recognizethat the week number information contained in the Nav Message may not necessarily reflect thecurrent full GPS week number (see paragraphs 20.3.3.3.1.1, 20.3.3.5.1.5, 20.3.3.5.2.4, and30.3.3.1.1.1).6.2.5 L5 Civil Signal.L5 is the GPS downlink signal at a nominal carrier frequency of 1176.45 MHz. The L5 signal isonly available on Block IIF and subsequent blocks of SVs and the signal is specified/described ininterface specification IS-GPS-705.50IS-GPS-200H24 Sep 20136.3Supporting Material.6.3.1 Received Signals.The guaranteed minimum user-received signal levels are defined in paragraph 3.3.1.6.
Asadditional supporting material, Figure 6-1 illustrates an example variation in the minimumreceived power of the near-ground user-received L1 and L2 signals from Block II/IIA/IIR SVs asa function of SV elevation angle.Higher received signals levels can be caused by such factors as SV attitude errors, mechanicalantenna alignment errors, transmitter power output variations due to temperature variations,voltage variations and power amplifier variations, and due to a variability in link atmosphericpath loss. For Block II/IIA and IIR SVs, the maximum received signal levels as a result of thesefactors is not expected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) andC/A components of the L1 channel, nor -158.0 dBW for either signal on the L2 channel.
ForBlock IIR-M and IIF SVs, the maximum received signal levels as a result of these factors is notexpected to exceed -155.5 dBW and -153.0 dBW, respectively, for the P(Y) and C/Acomponents of the L1 channel and L2 channel. In addition, due to programmable power outputcapabilities of Block IIR-M and IIF SVs, under certain operational scenarios, individual signalcomponents of Block IIR-M/IIF SVs may exceed the previously stated maximum but are notexpected to exceed -150 dBW.51IS-GPS-200H24 Sep 2013-155.5C/A - L1RECEIVED POWER AT 3dBiLINEARLY POLARIZEDANTENNA (dBW)-158.5P - L1-161.5P - L2 or-164.5C/A - L20o 5o20o40o60o80o90o100oUSER ELEVATION ANGLE (DEG)Figure 6-1.User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR)6.3.2 Extended Navigation Mode (Block II/IIA).The Block II and IIA SVs are capable of being uploaded by the CS with a minimum of 60 daysof navigation data to support a 60 day positioning service. Due to memory retention limitations,the Block II SVs may not transmit correct data for the entire 60 days but are guaranteed totransmit correct data for at least 14 days to support short-term extended operations.
Under52IS-GPS-200H24 Sep 2013normal conditions the CS will provide daily uploads to each SV, which will allow the SV tomaintain normal operations as defined in paragraph 6.2.3.1 and described within this IS. Duringnormal operations, the SVs will have a user range error that is at or below a level required tosupport a positioning accuracy of 16 meters spherical error probable (SEP). In addition, thealmanac data, UTC parameters and ionospheric data will be maintained current to meet theaccuracy specified in this IS.If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept andprocess the upload), each SV will individually transition to short-term extended operations andeventually to long-term extended operations (based on time from each SV's last upload) asdefined in paragraphs 6.2.3.2 and 6.2.3.3, and as further described throughout this IS. As timefrom upload continues through these three operational intervals, the user range error of the SVwill increase, causing a positioning service accuracy degradation.
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