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By means of Modulo-2addition of G2 with different phase shift and G1, a ranging code is generated foreach satellite. The phase assignment of G2 sequence is shown in Table 4-2.7BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeTable 4-2Phase assignment of G2 sequenceNo.Satellite typeRanging codenumberPhase assignment ofG2 sequence1GEO satellite11⊕32GEO satellite21⊕43GEO satellite31⊕54GEO satellite41⊕65GEO satellite51⊕86MEO/IGSO satellite61⊕97MEO/IGSO satellite71⊕108MEO/IGSO satellite81⊕119MEO/IGSO satellite92⊕710MEO/IGSO satellite103⊕411MEO/IGSO satellite113⊕512MEO/IGSO satellite123⊕613MEO/IGSO satellite133⊕814MEO/IGSO satellite143⊕915MEO/IGSO satellite153⊕1016MEO/IGSO satellite163⊕1117MEO/IGSO satellite174⊕518MEO/IGSO satellite184⊕619MEO/IGSO satellite194⊕820MEO/IGSO satellite204⊕921MEO/IGSO satellite214⊕1022MEO/IGSO satellite224⊕1123MEO/IGSO satellite235⊕624MEO/IGSO satellite245⊕825MEO/IGSO satellite255⊕926MEO/IGSO satellite265⊕1027MEO/IGSO satellite275⊕1128MEO/IGSO satellite286⊕829MEO/IGSO satellite296⊕930MEO/IGSO satellite306⊕1031MEO/IGSO satellite316⊕118BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeNo.Satellite typeRanging codenumberPhase assignment ofG2 sequenceNo.Satellite typeRanging code numberPhase assignment of G2sequence32MEO/IGSO satellite328⊕933MEO/IGSO satellite338⊕1034MEO/IGSO satellite348⊕1135MEO/IGSO satellite359⊕1036MEO/IGSO satellite369⊕1137MEO/IGSO satellite3710⊕115 NAV Message5.1General5.1.1 NAV Message ClassificationNAV messages are formatted in D1 and D2 based on their rate andstructure.
The rate of D1 NAV message which is modulated with 1 kbpssecondary code is 50 bps. D1 NAV message contains basic NAV information(fundamental NAV information of the broadcasting satellites, almanacinformation for all satellites as well as the time offsets from other systems);while D2 NAV message contains basic NAV and augmentation serviceinformation (the BDS integrity, differential and ionospheric grid information)and its rate is 500 bps.The NAV message broadcast by MEO/IGSO and GEO satellites is D1 andD2 respectively.5.1.2 NAV Message Information Type and BroadcastingThe NAV message information type and broadcasting are shown in Table5-1. The detailed structure, bits allocations, contents and algorithms will bedescribed in later chapters.9BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeTable 5-1NAV message information contents and their broadcastingNo.
ofMessage information contentBroadcastingBits11Subframe ID (FraID)3Seconds of week (SOW)20Week number (WN)User range accuracy index(URAI)Autonomous satellite healthflag (SatH1)Equipmentgroupdifferential (TGD1,TGD2)delay41Clock correction parameters(toc, a0, a1, a2)74Ionosphere model parameters(αn, βn, n=0~3)Alamanac parameters , δi,(toa, A , e, ω, M0, Ω0, a0, a1)Week number of alamanac(WNa)Health information for 30satellites（Heai, i=1~30）D1: broadcast in subframes 1, 2 and 3,repeated every 30 seconds.205Page number (Pnum)Almanac13Age of data, clock (AODC)Age of data, ephemeris(AODE)Ephemeris parameters,(toe, A , e, ω, Δn, M0, Ω0, i0, IDOT, Cuc, Cus, Crc, Crs, Cic,Cis)Occurring every subframeD2: broadcast in the first five wordsof pages 1~10 of subframe 1, repeatedevery 30 seconds.5Updating rate: every 1 hour.37164717689×3010D1: broadcast in subframe 4 andsubframe 5.D2: broadcast in subframe 5.D1: broadcasting in pages 1~24 ofsubframe 4 and pages 1~6 ofsubframe 5, repeated every 12minutes.D2: broadcast in pages 37~60,95~100 of subframe 5, repeated every6 minutes.Updating period: less than 7 days.D1: broadcast in pages 7~8 ofsubframe 5, repeated every 12minutes.D2: broadcast in pages 35~36 ofsubframe 5, repeated every 6 minutes.Updating period: less than 7 days.Basic NAV information, broadcast in every satelliteFundamental NAV information of the broadcasting satellitePreamble (Pre)BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeMessage information contentD1: broadcast in pages 9~10 ofsubframe 5, repeating every 12minutes.D2: broadcast in pages 101~102 ofsubframe 5, repeated every 6 minutes.Updating period: less than 7 days.3030304D2: broadcast in pages 1~10 ofsubframe 1.4D2: broadcast in pages 1~6 ofsubframe 2.2Integrity and differentialcorrection information of BDS1×30D2: broadcast in pages 1~6 ofsubframe 2.Updating rate: every 3 seconds.D2: broadcast in pages 1~6 ofsubframe 2.Updating rate: every 3 seconds.User differential range errorindex (UDREIi, i=1~18)4×18Regional user range accuracyindex (RURAIi, i=1~18)4×18Equivalent clock correction(Δti, i=1~18)13×18Ionospherc gridinformatioinTime offsets from othersystems88BroadcastingVertical ionospheric delay at9×320grid point (dτ)D2: broadcast in pages 1~13, 61~73of subframe 5.Grid ionospheric vertical delay4×320 Updating rate: every 6 minutes.error indiex (GIVEI)D2: broadcast in subframe 2.Updating rate: every 3 seconds.D2: broadcast in subframe 2 andsubframe 3.Updating rate: every 18 seconds.Integrity and differential correction information and ionospheric grid information arebroadcast by GEO satellites only.Time parameters relative toUTC (A0UTC, A1UTC, ΔtLS,ΔtLSF,WNLSF, DN)Time parameters relative toGPS time (A0GPS, A1GPS）Time parameters relative toGalileo time (A0Gal, A1Gal)Time parameters relative toGLONASS time(A0GLO, A1GLO)Page number for basic NAVinformation (Pnum1)Page number for integrity anddifferential correction information(Pnum2)Satellite health flag for integrity anddifferential correction information(SatH2)BDS Satellite identification forintegrity and differential correctioninformation (BDIDi, i=1~30)No.
ofBits5.1.3 Data Error Correction Coding ModeThe NAV message encoding involves both error control of BCH(15,11,1)and interleaving. The BCH code is 15 bits long with 11 information bits and11BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation Officeerror correction capability of 1 bit. The generator polynomial is g(X)=X4+X+1.The NAV message bits are grouped every 11 bits in sequence first. Theserial/parallel conversion is made and the BCH(15,11,1) error correctionencoding is performed in parallel. Parallel/serial conversion is then carried outfor every two parallel blocks of BCH codes by turns of 1 bit to form aninterleaved code of 30 bits length. The implementation is shown in Figure 5-1.InputFig 5-1Serial/parallelconvertingfor eachblock of11 bitsBCH(15,11,1) encodingParallel/serialOutputconvertingby turnsof 1 bitBCH(15,11,1) encodingError correction encoding and interleaving of down-link NAV messageThe implementation of BCH (15,11,1) encoder is shown in Figure 5-2.Initially the four stages of the shift register are all reset to zero, Gate1 is on andGate2 is off.
The 11 bits of information block X are sent into a dividing circuitg(X). Meantime the information bits are sent out of the encoder through gate―or‖ as the output. The dividing operation finishes when all the 11 bits havebeen sent in and then the states of the four register stages represent the paritycheck bits. Now switch Gate 1 off and Gate 2 on. The four parity check bits areshifted out of the encoder through gate ―or‖ to form a 15 bits code incombination with the output 11 bits of information block.
Then switch Gate1 onand Gate2 off and send in the next information block and the procedure above isrepeated again.12BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeGate1D0D1D2D3Gate2OutputORInput information block XFig 5-2 BCH(15, 11, 1) encoderFor the received NAV message by receivers near ground a serial/parallelconversion by turns of 1 bit is required first, followed by an error correctiondecoding of BCH(15,11,1) in parallel. Then a parallel/serial conversion iscarried out for each 11 bits block to form a 22 bits information code in sequence.The processing is shown in Figure 5-3.BCH(15,11,1) decodingInputSerial/parallelconvertingby turnsof 1 bitparallel/SerialOutputtransformingfor each 11bitsBCH(15,11,1) decodingFig 5-3Processing of received down-link NAV messageThe decoding logic of BCH(15,11,1) is shown in Figure 5-4.
The initialstates of the four register stages are all zeros. BCH codes are sent in bit by bitinto a division circuit and a fifteen stages buffer simultaneously. When allfifteen bits of a BCH code are inputted, the ROM list circuit forms a fifteen-bittable based on the states D3, D2, D1 and D0 of the four register stages. Then the15 bits in the table and 15 bits in the buffer are Modulo-2 added and an errorcorrected information code obtained is output. The ROM table list is shown inTable 5-2.13BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation OfficeBCH code inputGate1D0D1D2D3ROM list circuit15 stage bufferDecoder outputFig 5-4Table 5-2BCH(15,11,1) decoding logicROM table list for error correctionD3D2D1D015 bits data for error correction0000000000000000000000100000000000000100100000000000000100011000000000010000010000000000000010001010000001000000000110000000000100000011100001000000000010000000000000010001001100000000000000101000000100000000010110000000100000001100000000001000000110101000000000000011100001000000000001111001000000000000The interleaving pattern of 30 bits code is as follows:X11X12X12X22…X111X112P1114P21P12P22P13P23P14P24BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation Officewhere Xij is the information bit, subscript i stands for the bit in BCH codeof block i and i=1 or 2; superscript j stands for the information bit j in block iand j=1 to 11; Pim is the check parity bit, subscript i stands for the bit in BCHcode of block i and i=1 or 2; superscript m stands for the parity bit m in BCHcode of block i and m=1 to 4.5.2D1 NAV Message5.2.1 Secondary Code Modulated on D1For D1 NAV message in format D1 of rate 50 bps a secondary code ofNeumann-Hoffman (NH) code is modulated on ranging code.
The period of NHcode is selected as long as the duration of a NAV message bit. The bit durationof NH code is the same as one period of the ranging code. Shown as in Figure5-5, the duration of one NAV message bit is 20 milliseconds and the rangingcode period is 1 millisecond. Thus the NH code (0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 0,1, 0, 0, 1, 1, 1, 0) with length of 20 bits, rate 1 kbps and bit duration of 1millisecond is adopted. It is modulated on the ranging code synchronously withNAV message bit.NH codeNAV messageRangingcodeNAVmessage0010120 msPeriod (1 bit duration of NAV message)NHcodeRangingcode0 0 0 0 0 1 0 0 1 1 0 1 0 1 0 0 1 1 1 01msRanging code period (1 bit duration of NH code）Fig 5-5Secondary code and its timing15BDS-SIS-ICD-2.02013-12 2013 China Satellite Navigation Office5.2.2 D1 NAV Message Frame StructureThe NAV message in format D1 is structured in the superframe, frame andsubframe.
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