IS-GPS-800D (797938), страница 6
Текст из файла (страница 6)
The block interleaver is conceptually described using a two-dimensional array of 38rows and 46 columns, as depicted in Figure 3.2-6. The LDPC encoded subframe 2 symbols arewritten first (MSB first) into the interleaver from left to right starting at Row 1. After Row 1 isfilled, Row 2 is filled from left to right and this process continues until the 1748th symbol (LSBof LDPC encoded subframe 3) is written into the rightmost cell of the last (38th) row. Once all1748 symbols are written into the array, the symbols are sequentially read out of the array, forbroadcast to user, from top to bottom starting at Column 1. After reading out the last symbol ofthe 38th row in Column 1, Column 2 symbols are read out from top to bottom and this processcontinues until the last symbol in the 38th row of the last column (46th) is read out.24IS-GPS-800D24 Sep 2013Figure 3.2-6.
Conceptual Block Interleaver3.3 Signal ModulationThe signals modulated on the L1 RF carrier include C/A, P(Y), M, and L1C, which consists oftwo components: L1CP and L1CD. The modulation used is binary offset carrier (BOC) asdescribed in reference document [2]. The modulation is essentially shaped BPSK where asubcarrier is used to shape the spectrum.25IS-GPS-800D24 Sep 2013The bitstream of the L1CP signal is constructed by modulo-2 addition of L1CP-code and L1COcode. The overlay code, L1CO, is described in section 3.2.2.1.2. The PRN ranging codes, L1CPand L1CD, are described in section 3.2.2.1.1.
The bitstream of L1CD is constructed by modulo-2addition of L1CD-code and the L1C message symbol train, DL1C(t). The timing relationship ofL1C signal components is described in Figure 3.3-1.The bitstream of the L1CD signal is modulated on L1 carrier frequency using BOC (1, 1)modulation, with a subcarrier frequency of 1.023 MHz and a chipping rate of 1.023 Mbps. Eachbit of the bitstream is applied to a BOC (1,1) spreading symbol consisting of one cycle of a 1.023MHz squarewave, defined as binary 10 (1= positive binary bit value, see Figure 3.3-2a) withtotal duration 1/1.023 microseconds.
The BOC (1,1) spreading symbols are defined using sinephasing, so they are aligned with the bits of the L1CD-code. Contrary to convention, a “0” is inphase with the carrier and a “1” is 180 degrees out of phase with the carrier.The bitstream of the L1CP signal is modulated on L1 carrier frequency using TMBOCmodulation technique. The L1CP TMBOC technique uses a mixture of BOC (1, 1) spreadingsymbols and BOC (6,1) spreading symbols, where each BOC (6,1) spreading symbol consists of6 cycles of a 6 x 1.023 MHz squarewave, defined as binary 101010101010 (1= binary bit value),with total duration 1/1.023 microseconds (see Figure 3.3-2b).The pattern of BOC (1,1) and BOC (6,1) spreading symbols repeats every 10230 spreadingsymbols corresponding to a new bit of L1CO-code.
Let the index of the spreading symbols forL1CP be t = 0, 1, . . ., 10229, where t = 0 is the first spreading symbol in the next bit of L1COcode. Write t as t = ut + 33 vt, where ut = 0, . . ., 32 and vt = 0, . . ., 309. Then all spreadingsymbols in L1CP are BOC (1,1), except for those that are BOC (6,1) that occur for those t with ut= 0, 4, 6, and 29 (i.e, t = 0, 4, 6, 29, 33, 37, 39, 62, …, 10197, 10201, 10203, 10226).
Thispattern is shown in Figure 3.3-2c.The BOC (1,1) and BOC (6,1) spreading symbols are defined using sine-phasing, so they arealigned with the bits of L1CP-code. The phase relationship between L1CD and L1CP is defined insection 3.2.1.6.26IS-GPS-800D24 Sep 2013Figure 3.3-1. L1CP-/L1CD-/L1CO-Code Timing Relationships27IS-GPS-800D24 Sep 2013Figure 3.3-2. Subcarrier Polarity Mapping28IS-GPS-800D24 Sep 20133.4 Interface CriteriaThe following specifies additional criteria for the SS/US interface for the GPS L1C signal.3.4.1 GPS TimeGPS time is established by the GPS Control Segment and is referenced to Coordinated UniversalTime (UTC) as realized by the U.S.
Naval Observatory (UTC (USNO)) with zero time-pointdefined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largestunit used in stating GPS time is one week defined as 604,800 seconds, concatenated with theGPS week number. GPS time may differ from UTC because GPS time is a continuous timescale, while UTC is corrected periodically with an integer number of leap seconds. There also isan inherent but bounded drift rate between the UTC and GPS time scales.
The GPS CS shallcontrol the GPS time scale to be within 50 nanoseconds (95% probability) of UTC (USNO)(modulo one second).The L1C message (henceforth referred to as CNAV-2) contains the requisite data for relatingGPS time to UTC. The accuracy of this data during the transmission interval shall be such that itrelates GPS time to UTC (USNO) to within 20 nanoseconds (one sigma). This data is generatedby the GPS CS; therefore, the accuracy of this relationship may degrade if for some reason theGPS CS is unable to upload data to a SV.The CNAV-2 data contains thirteen bits representing the sequential number assigned to thecurrent GPS week (see paragraph 6.2.2).
The range of this count is from 0 to 8191 with its zerostate being defined as the GPS week number zero and every integer multiple of 8192 weeksthereafter (i.e. 0, 8192, 16384, etc.).3.4.2 SV Time vs. GPS TimeIn controlling the SVs and uploading of data, the CS shall allow for the following timingrelationships:Each SV operates on its own SV time;All TOI and Interval Time of Week (ITOW) in the CNAV-2 messages shall be in SV-time;All other data in the CNAV-2 messages shall be relative to GPS-time;The acts of transmitting the CNAV-2 messages shall be executed by the SV on SV time.3.4.3 Speed of LightThe speed of light used by the CS for generating the data described in the above paragraphs is29IS-GPS-800D24 Sep 2013c = 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 oflight in all computations.3.5 Message DefinitionAs shown in Figure 3.2-3, the L1C message, CNAV-2, structure utilizes three different subframeformats.
This section defines and specifies the content of each subframe.3.5.1 Message ContentSubframe 1 provides TOI count defined as being equal to the number of 18-second epochs thathave occurred since the start of current Interval Time of Week (ITOW) epoch (reference Section3.5.3).Subframe 2 provides clock and ephemeris data which is nominally invariant over multipleframes.
Subframe 2 also provides the ITOW count defined as being equal to the number of twohour epochs that have occurred since the transition from the previous week.Subframe 3 provides other navigation data which is commutated over multiple pages. Each pageof subframe 3 provides different data as shown in Figures 3.5-2 through 3.5-8. Additionalsubframe 3 pages may be defined in the future. It shall be noted that the broadcast sequence ofsubframe 3 pages is variable and, as such, users must not expect a fixed pattern of pagesequence. Subframe 3 provides an 8-bit PRN number of the transmitting SV with a range of 0(00000000) to 255 (11111111).In the event of message generation failure, the SV shall replace each affected subframe 2 and/or3 with the default message.
The data content of the default message shall be alternating ones andzeros beginning with one and the message shall contain a proper CRC block. In addition, in theevent of message generation failure of either subframe 2 or 3, the SV shall replace the content ofsubframe 1 with the default TOI count (111111111). The SV shall also broadcast the defaultTOI count in the event of a subframe 1 message generation failure.3.5.2 Subframe 1The TOI count utilizes a 9-bit data word that represents SV time at the start of the next 18second frame. The count represents the number of 18-second epochs that have occurred sincethe start of the two-hour period represented by ITOW count in the subframe 2 of the next 18second frame.
The TOI count range is from 0 (000000000) to 399 (110001111). The beginningepoch of a two-hour period shall correspond to a start of subframe 1 and TOI count one(000000001) shall correspond to the start of the next 18-second frame following the beginning ofa two-hour period. The TOI data is nominally the same on all SVs (for those SVs that broadcastTOI data).30IS-GPS-800D24 Sep 2013The 9-bit subframe 1 data is channel encoded into 52 symbols for broadcast as specified inparagraph 3.2.3.2.31IS-GPS-800D24 Sep 2013114DIRECTION OF DATA FLOW FROM SV100 BITS50343922WNITOWtop13 BITS8 BITS11 BITS5BITSMSB FIRST76•toe∆AA11 BITS26 BITS25 BITSURA ED INDEXL1C HEALTH – 1 BITDIRECTION OF DATA FLOW FROM SV100 BITS101MSB FIRST14111 8∆∆n 0•n174023 BITS17 BITSM 0 -nen33 BITS27 MSBsDIRECTION OF DATA FLOW FROM SV100 BITS201207MSB FIRST27324 0enωnΩ0 - nio -n6LSBs33 BITS33 BITS28 MSBsDIRECTION OF DATA FLOW FROM SV100 BITS301306323354338•MSB FIRST•370394i 0 -n∆Ωi o- nCis - nC ic -nC rs - nC rc -n5LSBs17 BITS15 BITS16 BITS16 BITS24 BITS7 MSBsDIRECTION OF DATA FLOW FROM SV100 BITS401418439497471Cuc - n21 BITS17 LSBs465460C us -nC rc - nMSB FIRST5BITS21 BITSaf0 -naf1 - n26 BITS4MSBsURANED2INDEX – 3 BITSURA NED0INDEXURA NED1INDEX – 3 BITSDIRECTION OF DATA FLOW FROM SV100 BITS501517566553540527MSB FIRST575567577af1 -naf2 -nT GDISC L1CPISC L1C DWNOPCRC16 LSBs10 BITS13 BITS13 BITS13 BITS8 BITS24 BITSISF – 1 BITRESERVED – 2 BITSFigure 3.5-1.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
