IS-GPS-200F (811524), страница 11
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The SV is undergoing normal operations whenever the fit interval flag (referenceparagraph 20.3.3.4.3.1) is zero.6.2.3.2 Short-term Extended Operations. The SV is undergoing short-term extended operations whenever the fitinterval flag is one and the 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 fitinterval flag is one and the IODE is in the range 240-255.6.2.4 GPS Week Number. The GPS week numbering system is established with week number zero (0) beingdefined as that week which started with the X1 epoch occurring at midnight UTC (USNO) on the night of January 5,1980/ morning of January 6, 1980.
The GPS week number continuously increments by one (1) at each end/start ofweek epoch without ever resetting to zero. Users must recognize that the week number information contained in theNav Message may not necessarily reflect the current full GPS week number (see paragraphs 20.3.3.3.1.1,20.3.3.5.1.5, 20.3.3.5.2.4, and 30.3.3.1.1.1).6.2.5 L5 Civil Signal. L5 is the GPS downlink signal at a nominal carrier frequency of 1176.45 MHz. The L5signal is only available on Block IIF and subsequent blocks of SVs and the signal is specified/described in interfacespecification IS-GPS-705.6.3 Supporting 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 minimum received power of thenear-ground user-received L1 and L2 signals from Block II/IIA/IIR SVs as a function of SV elevation angle.Higher received signals levels can be caused by such factors as SV attitude errors, mechanical antenna alignmenterrors, transmitter power output variations due to temperature variations, voltage variations and power amplifier52IS-GPS-200F21 Sep 2011variations, and due to a variability in link atmospheric path loss. For Block II/IIA and IIR SVs, the maximumreceived signal levels as a result of these factors is not expected to exceed -155.5 dBW and -153.0 dBW,respectively, for the P(Y) and C/A components of the L1 channel, nor -158.0 dBW for either signal on the L2channel. For Block 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/A components of the L1 channeland L2 channel.
In addition, due to programmable power output capabilities of Block IIR-M and IIF SVs, undercertain operational scenarios, individual signal components of Block IIR-M/IIF SVs may exceed the previouslystated maximum but are not expected to exceed -150 dBW.53IS-GPS-200F21 Sep 2011-155.5C/A - L1POLARIZEDP - L1-161.5ANTENNA (dBW)LINEARLYRECEIVED POWER AT 3dBi-158.5P - L2 or-164.5C/A - L20o 5o20o40o60o80o90oUSER ELEVATION ANGLE (DEG)Figure 6-1. User Received Minimum Signal Level Variations (Example, Block II/IIA/IIR)54IS-GPS-200F21 Sep 2011100o6.3.2 Extended Navigation Mode (Block II/IIA).
The Block II and IIA SVs are capable of being uploaded by the CSwith a minimum of 60 days of navigation data to support a 60 day positioning service. Due to memory retentionlimitations, the Block II SVs may not transmit correct data for the entire 60 days but are guaranteed to transmitcorrect data for at least 14 days to support short-term extended operations. Under normal conditions the CS willprovide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph6.2.3.1 and described within this IS.
During normal operations, the SVs will have a user range error that is at orbelow a level required to support a positioning accuracy of 16 meters spherical error probable (SEP). In addition,the almanac data, UTC parameters and ionospheric data will be maintained current to meet the accuracy specified inthis IS.If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload),each SV will individually transition to short-term extended operations and eventually to long-term extendedoperations (based on time from each SV's last upload) as defined in paragraphs 6.2.3.2 and 6.2.3.3, and as furtherdescribed throughout this IS.
As time from upload continues through these three operational intervals, the userrange error of the SV will increase, causing a positioning service accuracy degradation. The rate of accuracydegradation is slow over the short-term extended operations interval, such that at the end of this interval(approximately 14 days after upload) the US will be able to achieve a positioning accuracy of 425 meters SEP.
Therate of accuracy degradation increases in the long-term extended interval, such that by the 180th day after the lastupload, the positioning errors will have grown to 10 kilometers SEP. During these intervals the URA will continueto provide the proper estimate of the user range errors.During short-term and long-term extended operations (approximately day 2 through day 62 after an upload), thealmanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracyfrom the time of last upload.6.3.3 Block IIA Mode (Block IIR/IIR-M). The Block IIR/IIR-M SVs, when operating in the Block IIA mode, willperform similarly to the Block IIA SVs and have the capability of storing at least 60 days of navigation data, withcurrent memory margins, to provide positioning service without contact from the CS for that period (through shortterm and long-term extended operations).
(Contractual requirements for these SVs specify transmission of correctdata for only 14 days to support short-term extended operations while in IIA mode.) Under normal conditions, theCS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined inparagraph 6.2.3.1 and described within this IS.If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload),each SV will individually transition to short-term extended operations and eventually to long-term extendedoperations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as furtherdescribed throughout this IS.
As time from upload continues through these three operational intervals, the user rangeerror (URE) of the SV will increase, causing a positioning service accuracy degradation.55IS-GPS-200F21 Sep 20116.3.4 Extended Navigation Mode (Block II-F). The Block II-F SVs shall be capable of being uploaded by the CSwith a minimum of 60 days of data to support a 60 day positioning service. Under normal conditions, the CS willprovide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph6.2.3.1 and described within this IS.If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload),each SV shall individually transition to short-term extended operations and eventually to long-term extendedoperations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as furtherdescribed throughout this IS.
As time from upload continues through these three operational intervals, the user rangeerror (URE) of the SV will increase, causing a positioning service accuracy degradation.6.3.5 Extended Navigation Mode (GPS III). The GPS III SVs shall be capable of being uploaded by the CS with aminimum of 60 days of data to support a 60 day positioning service. Under normal conditions, the CS will providedaily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 anddescribed within this IS.If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload),each SV shall individually transition to short-term extended operations and eventually to long-term extendedoperations (based on time from each SV’s last upload) as defined in paragraph 6.2.3.2 and 6.2.3.3, and as furtherdescribed throughout this IS.
As time from upload continues through these three operational intervals, the user rangeerror (URE) of the SV will increase, causing a positioning service accuracy degradation.6.3.6 Autonomous Navigation Mode. The Block IIR/IIR-M, Block IIF, and directional crosslink-capable GPS IIISV in conjunction with a sufficient number of other Block IIR/IIR-M, Block IIF or directional crosslink-capableGPS III SVs, operates in an Autonav mode when commanded by the CS. Each Block IIR/IIR-M/IIF/directionalcrosslink-capable GPS III SV in the constellation determines its own ephemeris and clock correction parameters viaSV-to-SV ranging, communication of data, and on-board data processing which updates data uploaded by the CS.In the Autonav mode the Block IIR/IIR-M/IIF/directional crosslink-capable GPS III SV will maintain normaloperations as defined in paragraph 6.2.3.1 and as further described within this IS, and will have a URE of no largerthan 6 meters, one sigma for Block IIR/IIR-M.
URE of 6 meters, one sigma, is expected to support 16 meter SEPaccuracy under a nominal position dilution of precision. If the CS is unable to upload the SVs, the Block IIR/IIRM/IIF/directional crosslink-capable GPS III SVs will maintain normal operations for period of at least 60 days afterthe last upload.In the Autonav mode, the almanac data, UTC parameters and ionospheric data are still calculated and maintainedcurrent by the CS and uploaded to the SV as required.
If the CS is unable to upload the SVs, the almanac data, UTCparameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of thelast upload.56IS-GPS-200F21 Sep 20116.3.7 Additional PRN Code Sequences. The additional PRN sequences provided in this section are for informationonly. The additional PRN sequences identified in this section are not applicable to GPS SVs.
In addition, thecurrent valid ranges for GPS PRN signal number for C/A- and P-code are 1-37 and 38-63 as specified in Table 3-Iaand Table 3-Ib. The PRN sequences provided in this section are for other L1/L2 signal applications, such asSatellite Based Augmentation System (SBAS) satellite signals.6.3.7.1 Additional C/A-code PRN sequences. The PRN C/A-code is described in Section 3.2.1.3 and 62 uniqueC/A-code sequences are assigned by SV ID number in Tables 3-Ia and 3-Ib. An additional set of 147 C/A-codePRN sequences are selected and assigned with PRN numbers in this section as shown in Table 6-I.Among the 147 additional sequences; PRN numbers 120 through 158 are reserved for SBAS; PRN numbers 64through 119 and PRN numbers 159 through 210 are reserved for other Global Navigation Satellite System (GNSS)applications.For GPS application, the CNAV data, Dc(t), will be modulo-2 added to the C/A-code sequences of PRN numbers 38through 63.Any assignment of a C/A-code PRN number and its code sequence for any additional SV and/or other L1/L2 signalapplications, such as SBAS satellite signals, will be selected from the sequences of Table 6-I and will be approved,controlled, and managed by the GPS Directorate.It should be noted that, in Table 6-I, the C/A-code sequences are identified by “G2 Delay” and “Initial G2 Setting”which are not the same as the method used in Table 3-Ia.
The two-tap coder implementation method referenced andused in Table 3-Ia is not used in Table 6-I due to its limitation in generating C/A-code sequences. The “G2 Delay”specified in Table 6-I may be accomplished by using the “Initial G2 Setting” as the initialization vector for the G2shift register of Figure 3-9. For higher order PRNs ( > 37) the two-tap output mask is not used and the output of theG2 register becomes tap -10 (10th tap) which is labeled as the "output" in Figure 3-9.57IS-GPS-200F21 Sep 20116.3.7.2 Additional P-Code PRN sequences.
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