BeiDou Navigation Satellite System Signal In Space Interface Control Document. Open Service Signal (Version 2.0)

Transcription

1 BeiDou Navigation Satellite System Signal In Space Interface Control Document Open Service Signal (Version 2.0) China Satellite Navigation Office December 203

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3 203 China Satellite Navigation Office Content Statement... 2 Scope... 3 BeiDou System Overview Space Constellation Coordinate System Time System Signal Specifications Signal Structure Signal Characteristics Carrier Frequency Modulation Mode olarization Mode Carrier hase Noise User-Received Signal ower Level Signal Multiplexing Mode Satellite Signal Bandwidth and Out-band Suppression Spurious Signal Coherence Equipment Group Delay Differential Ranging Code... 6 I

4 203 China Satellite Navigation Office 5 NAV Message General NAV Message Classification NAV Message Information Type and Broadcasting Data Error Correction Coding Mode D NAV Message Secondary Code Modulated on D D NAV Message Frame Structure D NAV Message Detailed Structure D NAV Message Content and Algorithm D2 NAV Message D2 NAV Message Frame Structure D2 NAV Message Detailed structure D2 NAV Message Content and Algorithm Acronyms II

5 203 China Satellite Navigation Office Statement BeiDou Navigation Satellite System Signal-In-Space Interface Control Document (hereafter referred to as ICD) is issued by the China Satellite Navigation Office, which reserves the right for final explanation. 2 Scope This ICD defines the specification related to open service signal BI and B2I between the space segment and the user segment of the BeiDou Navigation Satellite System. B2I will be gradually replaced by a better signal with the construction of global system. 3 BeiDou System Overview 3. Space Constellation BeiDou Navigation Satellite System is called BeiDou System for short, with the abbreviation as BDS. When fully deployed, the space constellation of BDS consists of five Geostationary Earth Orbit (GEO) satellites, twenty-seven Medium Earth Orbit (MEO) satellites and three Inclined Geosynchronous Satellite Orbit (IGSO) satellites. The GEO satellites are operating in orbit at an altitude of 35,786 ilometers and positioned at E, 80 E, 0.5 E, 40 E and 60 E respectively. The MEO satellites are operating in orbit at an altitude of 2,528 ilometers and an inclination of 55 to the equatorial plane. The IGSO satellites are operating in orbit at an altitude of 35,786 ilometers and an inclination of 55 to the equatorial plane. By the end of 202, there are five GEO, four MEO and five IGSO BeiDou navigation satellites in orbit.

6 203 China Satellite Navigation Office 3.2 Coordinate System BDS adopts the China Geodetic Coordinate System 2000 (CGCS2000), and the definition is listed below: The origin is located at the mass center of the Earth; The Z-axis is in the direction of the IERS (International Earth Rotation and Reference System Service) Reference ole (IR); The X-axis is directed to the intersection of IERS Reference Meridian (IRM) and the plane passing the origin and normal to the Z-axis; The Y-axis, together with Z-axis and X-axis, constitutes a right handed orthogonal coordinate system. The origin of the CGCS2000 is also the geometric center of the CGCS2000 ellipsoid, and the Z-axis is the rotation axis of the CGCS2000 ellipsoid. The parameters of the CGCS2000 ellipsoid are as follows: Semi-major axis: a = m Geocentric gravitational constant (mass of the earth atmosphere included): μ = m 3 /s 2 Flattening: f = / Rate of earth rotation: e = rad/s 3.3 Time System The time reference for the BDS uses the BeiDou navigation satellite system Time (BDT). BDT adopts international system of units (SI) seconds, rather than leap seconds, as the basic unit for continuous accumulation. The start epoch of BDT was 00:00:00 on January, 2006 of Coordinated Universal Time (UTC). BDT is counted with wee and seconds of wee (). BDT is related to the UTC through UTC(NTSC). BDT offset with respect to UTC is controlled within 00 nanoseconds (modulo second). The leap seconds are 2

7 203 China Satellite Navigation Office broadcast in navigation (NAV) message. 4 Signal Specifications 4. Signal Structure The signals on B and B2 are the sum of channel I and Q which are in phase quadrature of each other. The ranging code and NAV message are modulated on carrier. The signal is composed of the carrier frequency, ranging code and NAV message. The signals on B and B2 are expressed as follows: j j j j S () t A C (t)d (t)cos( 2πf t S B j B2 BI BI BI j j j () t A C (t)d (t)cos( 2πf t B2I B2I B2I 2 BI B2I ) A ) A Where, Superscript j: satellite number; A BI : amplitude of BI; A B2I : amplitude of B2I; A BQ : amplitude of BQ; A B2Q : amplitude of B2Q; C BI : ranging code of BI; C B2I : ranging code of B2I; C BQ : ranging code of BQ; C B2Q : ranging code of B2Q; D BI : data modulated on ranging code of BI; D B2I : data modulated on ranging code of B2I; D BQ : data modulated on ranging code of BQ; D B2Q : data modulated on ranging code of B2Q; f : carrier frequency of BI; BQ B2Q C j BQ C j B2Q (t)d (t)d j BQ j B2Q (t)sin(2 πf t j BQ j (t)sin(2 πf t 2 ) B2Q ) 3

8 203 China Satellite Navigation Office f 2 : carrier frequency of B2I; υ BI : carrier initial phase of BI; υ B2I : carrier initial phase of B2I; υ BQ : carrier initial phase of BQ; υ B2Q : carrier initial phase of B2Q. 4.2 Signal Characteristics 4.2. Carrier Frequency The carrier frequencies of BI and B2I shall be coherently derived from a common reference frequency source on board of the satellite. The nominal frequency of BI signal is MHz,and the nominal frequency of B2I signal is MHz Modulation Mode (QSK). The transmitted signal is modulated by Quadrature hase Shift Keying olarization Mode The transmitted signal shall be Right-Handed Circularly olarized (RHC). The signal polarization ellipticity is specified in Table 4-. Table 4- Signal polarization ellipticity Satellite type GEO MEO IGSO Signal polarization ellipticity Ellipticity is no worse than 2.9 db, angular range: 0 from boresight. Ellipticity is no worse than 2.9 db, angular range: 5 from boresight. Ellipticity is no worse than 2.9 db, angular range: 0 from boresight. 4

9 203 China Satellite Navigation Office Carrier hase Noise The phase noise spectral density of the unmodulated carrier is as follows: -60 f 0 ±0 Hz -75 f 0 ±00 Hz -80 f 0 ± Hz -85 f 0 ±0 Hz -95 f 0 ±00 Hz Where, f 0 is the carrier frequency of BI or B2I User-Received Signal ower Level The minimum user-received signal power level is specified to be -63dBW for channel I, which is measured at the output of a 0 db RHC receiving antenna (located near ground), when the satellite s elevation angle is higher than 5 degree Signal Multiplexing Mode The signal multiplexing mode is Code Division Multiple Access (CDMA) Satellite Signal Bandwidth and Out-band Suppression ()Bandwidth (db): MHz (centered at carrier frequency of BI); 20.46MHz (centered at carrier frequency of B2I). Bandwidth (3dB): 6MHz (centered at carrier frequency of BI); 36MHz (centered at carrier frequency of B2I). (2)Out-band suppression: no less than 5 db on f 0 ±30 MHz, where f 0 is the carrier frequency of BI or B2I signal. 5

10 203 China Satellite Navigation Office Spurious In-band spurious shall be at least 50 db below the unmodulated carrier over the satellite signal bandwidth ( db) Signal Coherence ()The random jitter of the ranging code phase difference (satellite transmitter time delay included) among 4 channels of I and Q on B, B2 is less than ns (σ). (2)The random jitter of the initial phase differenal between the ranging code modulated on the carrier and the carrier is less than 3 (σ) (relative to the carrier) for BI,B2I. (3)Carrier phase quadrature difference between channels I and Q:<5 (σ) Equipment Group Delay Differential Equipment group delay is defined as the delay between the antenna phase center of a satellite and the output of the satellite onboard frequency source. The reference equipment group delay is included in the cloc correction parameter a 0 in NAV message with uncertainty less than 0.5ns(σ).The equipment group delay differential of radiated signals on B and B2 with respect to that of reference is given in T GD and T GD2 respectively in NAV message with uncertainty less than ns(σ). 4.3 Ranging Code The chip rate of the BI and B2I ranging code is Mcps, and the 6

11 203 China Satellite Navigation Office length is 2046 chips. The BI and B2I ranging code (hereinafter referred to as C BI and C B2I ) is a balanced Gold code truncated with the last one chip. The Gold code is generated by means of Modulo-2 addition of G and G2 sequences which are respectively derived from two -bit linear shift registers. The generator polynomials for G and G2 are as follows: G(X)=+X+X 7 +X 8 +X 9 +X 0 +X G2(X)=+X+X 2 +X 3 +X 4 +X 5 +X 8 +X 9 +X The initial phases of G and G2 are: G: G2: The generator of C BI and C B2I is shown in Figure G sequence Reset control cloc Shift control cloc Set to initial phases hase selection logic G2 sequence Ranging code Figure 4- The generator of C BI and C B2I The different phase shift of G2 sequence is accomplished by respective tapping in the shift register generating G2 sequence. By means of Modulo-2 addition of G2 with different phase shift and G, a ranging code is generated for each satellite. The phase assignment of G2 sequence is shown in Table

12 203 China Satellite Navigation Office Table 4-2 hase assignment of G2 sequence Satellite type Ranging code number hase assignment of G2 sequence GEO satellite 3 2 GEO satellite GEO satellite GEO satellite GEO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite 8 9 MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite 3 6 8

13 203 China Satellite Navigation Office Satellite type Ranging code number Satellite type Ranging code number hase assignment of G2 sequence hase assignment of G2 sequence 32 MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite MEO/IGSO satellite NAV Message 5. General 5.. NAV Message Classification NAV messages are formatted in D and D2 based on their rate and structure. The rate of D NAV message which is modulated with bps secondary code is 50 bps. D NAV message contains basic NAV information (fundamental NAV information of the broadcasting satellites, almanac information for all satellites as well as the time offsets from other systems); while D2 NAV message contains basic NAV and augmentation service information (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 D and D2 respectively NAV Message Information Type and Broadcasting The NAV message information type and broadcasting are shown in Table 5-. The detailed structure, bits allocations, contents and algorithms will be described in later chapters. 9

14 Almanac Fundamental NAV information of the broadcasting satellite Basic NAV information, broadcast in every satellite 203 China Satellite Navigation Office Table 5- NAV message information contents and their broadcasting of Message information content Broadcasting Bits reamble (re) ID () 3 Seconds of wee () 20 Wee number (WN) 3 User range accuracy index (URAI) Autonomous satellite health flag (SatH) Equipment group delay differential (T GD,T GD2 ) Age of data, cloc (AODC) 5 Cloc correction parameters (t oc, a 0, a, a 2 ) Age of data, ephemeris (AODE) Ephemeris parameters (t oe, A, e, ω, Δn, M 0, Ω 0,, i 0, IDOT, C uc, C us, C rc, C rs, C ic, C is ) Ionosphere model parameters (α n, β n, n=0~3) age number (num) 7 Alamanac parameters (t oa, A, e, ω, M 0, Ω 0,, δ i, a 0, a ) Wee number of alamanac (WN a ) Health information for 30 satellites(hea i, i=~30) Occurring every subframe D: broadcast in subframes, 2 and 3, repeated every 30 seconds. D2: broadcast in the first five words of pages ~0 of subframe, repeated every 30 seconds. Updating rate: every hour. D: broadcast in subframe 4 and subframe 5. D2: broadcast in subframe 5. D: broadcasting in pages ~24 of subframe 4 and pages ~6 of subframe 5, repeated every 2 minutes. D2: broadcast in pages 37~60, 95~00 of subframe 5, repeated every 6 minutes. Updating period: less than 7 days. D: broadcast in pages 7~8 of subframe 5, repeated every 2 minutes. D2: broadcast in pages 35~36 of subframe 5, repeated every 6 minutes. Updating period: less than 7 days. 0

15 Ionospherc grid informatioin Integrity and differential correction information of BDS Time offsets from other systems Integrity and differential correction information and ionospheric grid information are broadcast by GEO satellites only. 203 China Satellite Navigation Office Message information content Time parameters relative to UTC (A 0UTC, A UTC, Δt LS, Δt LSF,WN LSF, DN) Time parameters relative to GS time (A 0GS, A GS ) Time parameters relative to Galileo time (A 0Gal, A Gal ) Time parameters relative to GLONASS time(a 0GLO, A GLO ) age number for basic NAV information (num) age number for integrity and differential correction information (num2) Satellite health flag for integrity and differential correction information (SatH2) BDS Satellite identification for integrity and differential correction information (BDID i, i=~30) of Bits Broadcasting 88 D: broadcast in pages 9~0 of subframe 5, repeating every 2 30 minutes. D2: broadcast in pages 0~02 of subframe 5, repeated every 6 minutes. Updating period: less than 7 days D2: broadcast in pages ~0 of subframe. D2: broadcast in pages ~6 of subframe 2. D2: broadcast in pages ~6 of subframe 2. Updating rate: every 3 seconds. D2: broadcast in pages ~6 of subframe 2. Updating rate: every 3 seconds. User differential range error index (UDREI i, i=~8) 4 8 D2: broadcast in subframe 2. Updating rate: every 3 seconds. Regional user range accuracy index (RURAI i, i=~8) Equivalent cloc correction (Δt i, i=~8) D2: broadcast in subframe 2 and subframe 3. Updating rate: every 8 seconds. Vertical ionospheric delay at grid point (dτ) Grid ionospheric vertical delay error indiex (GIVEI) D2: broadcast in pages ~3, 6~73 of subframe 5. Updating rate: every 6 minutes Data Error Correction Coding Mode The NAV message encoding involves both error control of BCH(5,,) and interleaving. The BCH code is 5 bits long with information bits and

16 203 China Satellite Navigation Office error correction capability of bit. The generator polynomial is g(x)=x 4 +X+. The NAV message bits are grouped every bits in sequence first. The serial/parallel conversion is made and the BCH(5,,) error correction encoding is performed in parallel. arallel/serial conversion is then carried out for every two parallel blocs of BCH codes by turns of bit to form an interleaved code of 30 bits length. The implementation is shown in Figure 5-. Input Serial/ parallel converting for each bloc of bits BCH(5,,) encoding BCH(5,,) encoding arallel/ serial converting by turns of bit Output Fig 5- Error correction encoding and interleaving of down-lin NAV message The implementation of BCH (5,,) encoder is shown in Figure 5-2. Initially the four stages of the shift register are all reset to zero, Gate is on and Gate2 is off. The bits of information bloc X are sent into a dividing circuit g(x). Meantime the information bits are sent out of the encoder through gate or as the output. The dividing operation finishes when all the bits have been sent in and then the states of the four register stages represent the parity chec bits. Now switch Gate off and Gate 2 on. The four parity chec bits are shifted out of the encoder through gate or to form a 5 bits code in combination with the output bits of information bloc. Then switch Gate on and Gate2 off and send in the next information bloc and the procedure above is repeated again. 2

17 203 China Satellite Navigation Office Gate D0 D D2 D3 Gate2 OR Output Input information bloc X Fig 5-2 BCH(5,, ) encoder For the received NAV message by receivers near ground a serial/parallel conversion by turns of bit is required first, followed by an error correction decoding of BCH(5,,) in parallel. Then a parallel/serial conversion is carried out for each bits bloc to form a 22 bits information code in sequence. The processing is shown in Figure 5-3. Input Serial/ parallel converting by turns of bit BCH(5,,) decoding BCH(5,,) decoding parallel/ Serial transforming for each bits Output Fig 5-3 rocessing of received down-lin NAV message The decoding logic of BCH(5,,) is shown in Figure 5-4. The initial states of the four register stages are all zeros. BCH codes are sent in bit by bit into a division circuit and a fifteen stages buffer simultaneously. When all fifteen bits of a BCH code are inputted, the ROM list circuit forms a fifteen-bit table based on the states D3, D2, D and D0 of the four register stages. Then the 5 bits in the table and 5 bits in the buffer are Modulo-2 added and an error corrected information code obtained is output. The ROM table list is shown in Table

18 203 China Satellite Navigation Office BCH code input Gate D0 D D2 D3 ROM list circuit 5 stage buffer Decoder output Fig 5-4 BCH(5,,) decoding logic Table 5-2 ROM table list for error correction D 3 D 2 D D 0 5 bits data for error correction The interleaving pattern of 30 bits code is as follows: X X 2 2 X 2 X 2 X X

19 203 China Satellite Navigation Office where X j i is the information bit, subscript i stands for the bit in BCH code of bloc i and i= or 2; superscript j stands for the information bit j in bloc i and j= to ; m i is the chec parity bit, subscript i stands for the bit in BCH code of bloc i and i= or 2; superscript m stands for the parity bit m in BCH code of bloc i and m= to D NAV Message 5.2. Secondary Code Modulated on D For D NAV message in format D of rate 50 bps a secondary code of Neumann-Hoffman (NH) code is modulated on ranging code. The period of NH code is selected as long as the duration of a NAV message bit. The bit duration of NH code is the same as one period of the ranging code. Shown as in Figure 5-5, the duration of one NAV message bit is 20 milliseconds and the ranging code period is millisecond. Thus the NH code (0, 0, 0, 0, 0,, 0, 0,,, 0,, 0,, 0, 0,,,, 0) with length of 20 bits, rate bps and bit duration of millisecond is adopted. It is modulated on the ranging code synchronously with NAV message bit. Ranging code NH code NAV message NAV message NH code ms ms eriod ( bit duration of NAV message) Ranging code Ranging code period ( bit duration of NH code) Fig 5-5 Secondary code and its timing 5

20 203 China Satellite Navigation Office D NAV Message Frame Structure The NAV message in format D is structured in the superframe, frame and subframe. Every superframe has bits and lasts 2 minutes. Every superframe is composed of 24 frames (24 pages). Every frame has 500 bits and lasts 30 seconds. Every frame is composed of 5 subframes. Every subframe has 300 bits and lasts 6 seconds. Every subframe is composed of 0 words. Every word has 30 bits and lasts 0.6 second. Every word consists of NAV message data and parity bits. In the first word of every subframe, the first 5 bits is not encoded and the following bits are encoded in BCH(5,,) for error correction. So there is only one group of BCH code contained and there are altogether 26 information bits in the word. For all the other 9 words in the subframe both BCH(5,,) encoding for error control and interleaving are involved. Each of the 9 words of 30 bits contains two blocs of BCH codes and there are altogether 22 information bits in it. (reference paragraph 5..3) The frame structure in format D is shown in Figure 5-6. Superframe bits, 2 min Frame Frame 2 Frame n Frame 24 Frame 500 bits, 30 sec bits, 6 sec Word Word 2 Word 0 Word, 30 bits, 0.6 sec 26 information bits 4 parity bits Word 2~0, 30 bits, 0.6 sec 22 information bits 8 parity bits Fig 5-6 Frame structure of NAV message in format D 6

21 203 China Satellite Navigation Office D NAV Message Detailed Structure The main information contents of NAV message in format D are basic NAV information, including fundamental NAV information of the broadcasting satellites (seconds of wee, wee number, user range accuracy index, autonomous satellite health flag, ionospheric delay model parameters, satellite ephemeris parameters and their age, satellite cloc correction parameters and their age and equipment group delay differential), almanac and BDT offsets from other systems (UTC and other navigation satellite systems). It taes 2 minutes to transmit the whole NAV message. The D frame structure and information contents are shown in Figure 5-7. The fundamental NAV information of the broadcasting satellite is in subframes, 2 and 3. The information contents in subframes 4 and 5 are subcommutated 24 times each via 24 pages. ages ~24 of subframe 4 and pages ~0 of subframe 5 shall be used to broadcast almanac and time offsets from other systems. ages ~24 of subframe 5 are reserved Fundamental NAV information of the broadcasting satellite Almanac and time offsets from other systems Fig 5-7 Frame structure and information contents of NAV message in format D The bits allocations of format D are shown in Figure 5-8~5-. 7

22 203 China Satellite Navigation Office age N/A Word re 2s SatH bit AODC 5bits URAI 6 WN 9s t oc 8s 9 t oc T GD 0bits 4s T GD2 (300 bits) bits allocation 6s 2 T GD2 α 0 α 5 α 2 α 3 6s first β 0 8 β 0 2s β β 2 4s β 3 4s 2 β 3 a 2 7s a 0 24 a 0 a 5bits 7s 5s 7s 27 a AODE 5bits Fig 5-8 Bits allocation of subframe in format D age 2 N/A Word 2 6 re 4bit Δn 0bits 2s 0s 6s 6 Δn C uc 6s 2 (300 bits) bits allocation 9 C uc 2s M 0 20bits 20s 2s 0s first M e 0 e C C us rc C C rc C rs rs A A 0bits 2 0bits 20bits 22s 4s 4s 0s 2s 20s 2s t oe Fig 5-9 Bits allocation of subframe 2 in format D 8

23 203 China Satellite Navigation Office age 3 N/A Word 2 6 re t oe 0bits 2s * 5s * These are data bits next to s and before s. 6 t oe 5bits i 0 7s 9 i 0 5bits 5s C ic 7s 3 (300 bits) bits allocation 2 5 C ic s s 3s C is 9bit 9s first C IDOT Ω 0 Ω 0 ω is IDOT ω bit 2bits 2bits bit 9s Fig 5-0 Bits allocation of subframe 3 in format D 3s 2s s s 2s first 4 5(300 bits)bits allocation 4 5 age ~24 ~6 re Word s bit num 2s A 53 6 A 2 22s a a s 2s e 3s i 8 i 3s t oa bit 2 ω 6s 6s 8s 24 ω M 0 4s 27 M 0 20bits 20s Fig 5-- Bits allocation of pages through 24 in subframe 4 and pages through 6 in subframe 5 of format D 9

24 203 China Satellite Navigation Office first Word 5 (300 bits) bits allocation age 5 7 re 27 3 bit num Hea 53 6 Hea Hea2 Hea3 9 Hea3 Hea Hea5 Hea6 bit Hea7 Hea8 27 Hea8 Hea9 2s 2s 7s 6s 3s 3s 6s Fig 5--2 Bits allocation of page 7 in subframe 5 of format D first Word 5(300 bits)bits allocation age 5 8 re 27 3 bit num Hea Hea20 Hea Hea27 Hea28 Hea29 Hea30 WNa t oa 5bits 2 t oa 6 2 arity of 3 words 2s 2s 7s 4s 5s 3s Fig 5--3 Bits allocation of page 8 in subframe 5 of format D 20

25 203 China Satellite Navigation Office first Word 5(300 bits) bits allocation age 5 9 re 27 3 bit num A 0GS A GS 2 A GS A 0Gal 5 A 0Gal A Gal 8 A 0GLO A GLO 2 A GLO 5 2 arity of 3 words 2s 2s 4s 6s 8s Fig 5--4 Bits allocation of page 9 in subframe 5 of format D age 5 0 re Word 27 3 bit num Δt LS 6 Δt LS 5(300bits) bits allocation Δt LSF WN LSF 9 A 0UTC 2 first 2 5 A 0UTC A UTC A UTC 0bits DN 90bits 40bits arity of 5 words 2s 2s 6s 22s 0s 2s 2s Fig 5--5 Bits allocation of page 0 in subframe 5 of format D 2

26 203 China Satellite Navigation Office Direction data of flow first Word 5(300 bits)bits allocation age 5 ~24 re 27 3 bit num 78 bits 7 arity of 9 words 2s Fig 5--6 Bits allocation of reserved pages ~24 in format D subframe 22

27 203 China Satellite Navigation Office D NAV Message Content and Algorithm reamble (re) The bits ~ of every subframe are preamble (re) of from modified Barer code of bits. count occurs at the leading edge of the preamble first bit which is for time scale synchronization identification () The bits 6, 7 and 8 of every subframe are for subframe identification (). The detailed definitions are as follows: Table 5-3 definitions Code Identification of subframe Seconds of Wee () The bits 9~26 and bits 3~42, altogether 20 bits of the each subframe are for seconds of wee () which is defined as the number of seconds that have occurred since the last Sunday, 00:00:00 of BDT. The count occurs at the leading edge of preamble first bit of the subframe Wee Number (WN) There are altogether 3 bits for wee number (WN) which is the integral wee count of BDT with the range of 0 through 89. Wee number count started from zero at 00:00:00 on Jan., 2006 of BDT. 23

28 203 China Satellite Navigation Office User Range Accuracy Index (URAI) The user range accuracy (URA) is used to describe the signal-in-space accuracy in meters. There are 4 bits for the user range accuracy index (URAI). The range of URAI is from 0 to 5. See Table 5-4 for the corresponding relationship between URAI and URA. Table 5-4 Corresponding relationship between URAI and URA Code URAI (N) URA range (meters, σ) < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA < URA URA > When an URAI is received by the user, the corresponding URA (X) is computed by the following equations: If 0 N < 6, X = 2 N/2+ ; If 6 N < 5, X = 2 N-2 ; If N=5, it means the satellite is in maneuver or there is no accuracy 24

29 203 China Satellite Navigation Office prediction; If N=, 3 and 5, X should be rounded to 2.8, 5.7, and.3 meters, respectively Autonomous Satellite Health flag (SatH) The autonomous satellite health flag (SatH) occupies bit. 0 means broadcasting satellite is good and means not Ionospheric Delay Model arameters (α n, β n ) There are 8 parameters, altogether 64 bits for ionospheric delay model. All the 8 parameters are in two s complement. See Table 5-5 for details. Table 5-5 Ionospheric delay model parameters arameter of bits Scale factor () Units α 0 8 * 2-30 s α 8 * 2-27 s/π α 2 8 * 2-24 s/π 2 α 3 8 * 2-24 s/π 3 β 0 8 * 2 s β 8 * 2 4 s/π β 2 8 * 2 6 s/π 2 β 3 8 * 2 6 s/π 3 * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. The user computers the vertical ionospheric delay correction (t) I ' z with the 8 parameters and Klobuchar model as follows: 5 0 ' I z (t) A 2 2π(t 50400) cos[ ], t A 4/4 A 25 4, t A /4 4

30 203 China Satellite Navigation Office Where I ' z (t) is the vertical ionospheric delay in seconds for BI, t is the local time (range 0~86400 sec) for the place under the intersection point (M) of ionosphere and the direction from receiver to satellite. A 2 is the amplitude of Klobuchar cosine curve in the day time computed from the α n. β n.. A 2 3 n 0 0 α n n M,, A A A 4 is the period of cosine curve in seconds. It is computed from the 72800, A n 4 n M 4 n0 A, A , A Where, is the geographic latitude of earth projection of the M ionosphere intersection point in semi-circles (π). The geographic latitude and longitude M λ M M arcsin sin λ u Where, λ M of the intersection point M are computed as: u cosψ cos sinψ sina arcsin cos M u sinψ cosa u is the user s geographic latitude in radians. A is the satellte azimuth from the user location in radians. ψ is the earth s central angle in radians between the user location and ionospheric intersection point. It is computed as: ψ π 2 R E arcsin cos E R h Where,R is the mean radius of the earth (6378 m). E is the satellite elevation from the user s location in radians. h is the height of ionosphere (375 m). I ' z (t) can be converted to the ionospheric delay along the BI 26

31 203 China Satellite Navigation Office propagation path I BI (t) through the equation as follows and the unit is seconds. I BI (t) R - cos E R h 2 I (t) z For B2I, users need to multiply a factor (f) to calculate the ionospheric delay along the B2I propagation path, and its value is as follows: f (f) f Where, f refers to the nominal carrier frequency of BI, f 2 refers to the nominal carrier frequency of B2I, and the unit is MHz. The dual-frequency (BI and B2I) user shall correct for the group delay due to ionospheric effects by applying the expression: R R B2I -(f) R BI C (TGD2 -(f) T GD) -(f) (f) where, R: pseudorange corrected for ionospheric effects; R BI : pseudorange measured on BI(corrected by the satellite cloc correction parameters and T GD ); R B2I : pseudorange measured on B2I(corrected by the satellite cloc correction parameters and T GD2 ); T GD : equipment group delay differential on BI; T GD2 : equipment group delay differential on B2I; C: the light speed, and its value is m/s. 27

32 203 China Satellite Navigation Office Note: When user adopts the ionospheric delay model in the south hemisphere, the ionospheric correction accuracy is slightly worse than that in the north Equipment Group Delay Differential (T GD,T GD2 ) The equipment group delay differential (T GD,T GD2 ) in the satellite is 0 bits long respectively. It is in two s complement with sign bit (+ or ) occupying. Sign bit 0 means positive and means negative. The scale factor is 0. and the unit is nanoseconds,and the detailed algorithm is defined in paragraph Age of Data, Cloc (AODC) Age of data, cloc (AODC) is updated at the start of each hour in BDT, and it is 5 bits long with definitions as follows: Table 5-6 AODC definitions AODC Definition < 25 Age of the satellite cloc correction parameters in hours 25 Age of the satellite cloc correction parameters is two days 26 Age of the satellite cloc correction parameters is three days 27 Age of the satellite cloc correction parameters is four days 28 Age of the satellite cloc correction parameters is five days 29 Age of the satellite cloc correction parameters is six days 30 Age of the satellite cloc correction parameters is seven days 3 Age of the satellite cloc correction parameters is over seven days Cloc Correction arameters (t oc, a 0, a, a 2 ) Cloc correction parameters are t oc, a 0, a and a 2 in 74 bits altogether. t oc is the reference time of cloc parameters in seconds with the effective range of 0~ Other 3 parameters are two s complement. 28

33 203 China Satellite Navigation Office 5-7. The definitions of cloc correction parameters are listed in Table Table 5-7 Cloc correction parameters arameter of bits Scale factor () Effective range Units t oc s a 0 24 * 2-33 s a 22 * 2-50 s/s a 2 * 2-66 s/s 2 * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. The system time computation is as follows: The user is able to compute BDT at time of signal transmission as: t = t sv Δt sv where, t is BDT in seconds at time of signal transmission; t sv is the effective satellite ranging code phase time in seconds at time of signal transmission; Δt sv is the offset of satellite ranging code phase time in seconds and is given by the equation: Δt sv = a 0 + a (t t oc ) + a 2 (t t oc ) 2 + Δt r Where, t can be replaced by t sv regardless of its sensitivity. Δt r is the correction term to relativistic effect with value of t Fe A sin r E e is the orbit eccentricity, which is given in ephemeris of the broadcasting satellite; A is the square root of semi-major axis of satellite orbit, which is given in ephemeris of the broadcasting satellite; E is eccentric anomaly of satellite orbit, which is given in ephemeris of the broadcasting satellite; 29

34 203 China Satellite Navigation Office F = -2μ /2 /C 2 ; μ = m 3 /s 2, is the value of earth s universal gravitational constant; C = m/s, is the light speed. The BI user should mae a further correction as follows: (Δt sv ) BI = Δt sv -T GD The B2I user should mae a further correction as follows: (Δt sv ) B2I = Δt sv -T GD Age of Data, Ephemeris (AODE) Age of data, ephemeris (AODE) is updated at the start of each hour in BDT, and it is 5 bits long with definitions as follows: Table 5-8 AODE definitions AODE Definition < 25 Age of the satellite ephemeris parameters in hours 25 Age of the satellite ephemeris parameters is two days 26 Age of the satellite ephemeris parameters is three days 27 Age of the satellite ephemeris parameters is four days 28 Age of the satellite ephemeris parameters is five days 29 Age of the satellite ephemeris parameters is six days 30 Age of the satellite ephemeris parameters is seven days 3 Age of the satellite ephemeris parameters is over seven days Ephemeris arameters (t oe, A, e, ω, Δn, M 0, Ω 0,, i 0, IDOT, C uc, C us, C rc, C rs, C ic, C is ) The ephemeris parameters describe the satellite orbit during the curve fit interval, including 5 orbit parameters and an ephemeris 30

35 203 China Satellite Navigation Office reference time. The update rate of ephemeris parameters is one hour. The definitions of ephemeris parameters are listed in Table 5-9. Table 5-9 Ephemeris arameters definitions arameter Definition t oe Ephemeris reference time A e ω Square root of semi-major axis Eccentricity Argument of perigee Δn Mean motion difference from computed value M 0 Ω 0 Mean anomaly at reference time Longitude of ascending node of orbital of plane computed according to reference time Rate of right ascension i 0 Inclination angle at reference time IDOT Rate of inclination angle C uc Amplitude of cosine harmonic correction term to the argument of latitude C us Amplitude of sine harmonic correction term to the argument of latitude C rc Amplitude of cosine harmonic correction term to the orbit radius C rs Amplitude of sine harmonic correction term to the orbit radius C ic Amplitude of cosine harmonic correction term to the angle of inclination C is Amplitude of sine harmonic correction term to the angle of inclination Characteristics of ephemeris parameters are shown in Table

36 203 China Satellite Navigation Office Table 5-0 Ephemeris parameters characteristics arameter of Bits Scale factor () Effective Range Units t oe s A m /2 e ω 32 * 2-3 π Δn 6 * π/s M 0 32 * 2-3 π Ω 0 32 * 2-3 π 24 * π/s i 0 32 * 2-3 π IDOT 4 * π/s C uc 8 * rad C us 8 * rad C rc 8 * m C rs 8 * m C ic 8 * rad C is 8 * rad * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. The user receiver shall compute the satellite antenna phase center position in coordinate system CGCS2000 according to the received ephemeris parameters. The algorithms are listed in Table

37 203 China Satellite Navigation Office Table 5- Computation μ = m 3 /s 2 e π = rad/s Ephemeris algorithm for user Description Value of the earth s universal gravitational constant of CGCS2000 Value of the earth s rotation rate of CGCS2000 Ratio of a circle s circumference to its diameter A A 2 Computed semi-major axis n t 0 Computed mean motion (radians/sec) 3 A t t oe * Computed time from ephemeris reference epoch n n 0 n Corrected mean motion M M nt Computed mean anomaly 0 M E sin v cos v esin E 2 e sin E ecos E cos E e ecos E Kepler s Equation for Eccentric anomaly solved by iteration (radians) Computed true anomaly v Computed argument of latitude u C us sin r C rs sin i Cis sin 2 C uc cos2 2 C rc cos2 2 C cos2 ic Argument of latitude correction Radius correction Inclination correction u u Corrected Argument of latitude parameters r i ecose r A Corrected radius i IDOT t i Corrected inclination 0 x y r cosu r sin u Computed satellite positions in orbital plane 33

38 203 China Satellite Navigation Office X Y Z X Y Z X Y Z 0 x x y GK GK GK 0 x x y R Where, R X Z Computation e t et oe cos sin sin i t cos sin sin i ( t e y y e t )R X y cosi cosi oe y ( 5 cosi cosi X ) Y Z GK GK GK sin cos 0 0 ( ) 0 cos sin 0 sin cos sin cos Description Corrected longitude of ascending node in CGCS2000; MEO/IGSO satellite coordinates in CGCS2000 Corrected longitude of ascending node in inertial coordinate system; GEO satellite coordinates in user-defined inertial system; GEO satellite coordinates in CGCS2000 R Z cos ( ) sin 0 sin cos * In the equations, t is the time of signal transmission in BDT. t is the total time difference between t and ephemeris reference time t oe after taing account of beginning or end of a wee crossovers. That is, subtract seconds from t if t is greater than , add seconds to t if t is less than seconds age number (num) The bits 44 through 50, 7 bits altogether of subframe 4 and subframe 5 are for page numbers (num). subframe 4 and subframe 5 are subcommutated 24 times via pages through 24. num identifies the page number of the subframe. The almanac information of SV ID through 24 is arranged in pages through 24 of subframe 4. The almanac information of SV ID 25 through 30 is arranged in pages through 6 of subframe 5. The page number corresponds to the SV ID one by one. 34

39 203 China Satellite Navigation Office Almanac arameters (t oa, A, e, ω, M 0, Ω 0,, δi, a 0, a ) Almanac parameters are updated within every 7 days. Definitions, characteristics and user algorithms of almanac parameters are listed in Tables 5-2, 5-3 and 5-4 respectively. Table 5-2 Almanac parameters definitions arameter t oa A e ω M 0 Ω 0 δ i a 0 a Almanac reference time Square root of semi-major axis Eccentricity Argument of erigee Mean anomaly at reference time Definition Longitude of ascending node of orbital plane computed according to reference time Rate of right ascension Correction of orbit reference inclination at reference time Satellite cloc bias Satellite cloc rate Table 5-3 Almanac parameters characteristics arameter of Bits Scale factor () Effective range Units t oa s A m /2 e ω 24 * 2-23 π M 0 24 * 2-23 π Ω 0 24 * 2-23 π 7 * 2-38 π/s δ i 6 * 2-9 π a 0 * 2-20 s a * 2-38 s/s * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. 35

40 203 China Satellite Navigation Office Computation Table 5-4 μ = m 3 /s 2 Almanac algorithms for users Description Earth s universal gravitational constant of CGCS rad/s Value of the earth s rotation rate of CGCS2000 e 2 A ( A) Computed semi-major axis n 0 Computed mean motion (rad/sec) 3 A t t * t oa Computed time from Almanac reference epoch M M n t Computed mean anomaly 0 0 M E sin v cos v esin E 2 e sin E ecos E cos E e ecos E Kepler s equation for eccentric anomaly by iteration (radians) Computed true anomaly v Computed argument of latitude r A( ecose ) Corrected radius x y i r r i 0 0 cos sin ( )t t ** i e e oa Computed satellite positions in orbital plane Corrected longitude of ascending node in CGCS2000 Orbit inclination at reference time X Y Z x x y cos sin sin i y y cosisin cosi cos Computed GEO/MEO/IGSO satellite coordinates in CGCS2000 * In the equations, t is the time of signal transmission in BDT. t is the total time offset between time t and Almanac reference time t oa taing account of beginning or end of a wee crossover. That is, subtract seconds from t if t is greater than , add seconds to t if t is less than ** For MEO/IGSO satellites, i 0 =0.30 semi-circles; for GEO satellites, i 0 =

41 203 China Satellite Navigation Office Almanac time computation is as follows: t = t sv Δt sv where t is BDT in seconds at time of signal transmission; t sv is the effective satellite ranging code phase time in seconds at time of signal transmission; Δt sv is the offset of satellite ranging code phase time in seconds and is given by the equation: Δt sv = a 0 + a (t t oa ) Where t can be replaced by t sv regardless of its sensitivity. The almanac reference time t oa is counted from the starting time of almanac wee number (WN a ) Almanac Wee Number (WN a ) Almanac wee number (WN a ) of 8 bits is the BDT integer wee count (Modulo 256) with effective range of 0 to Satellite Health Information (Hea i, i=~30) The satellite health information (Hea i ) occupies 9 bits. The 9th bit indicates the satellite cloc health flag, while the 8 th bit indicates the BI signal health status. The 7th bit indicates the B2I signal health status,and the 2th bit indicates the information health status. The definitions are in Table

42 203 China Satellite Navigation Office Table 5-5 Satellite health information definitions Bit allocation Information code Health information definition Bit 9 () Bit 8 Bit 7 Bit 6~3 Bit 2 Bit () 0 Satellite cloc OK * 0 BI Signal OK BI Signal Wea ** 0 B2I Signal OK B2I Signal Wea ** 0 Reserved Reserved 0 NAV Message OK NAV Message Bad (IOD over limit) 0 Reserved Reserved * the satellite cloc is unavailable if the other 8 bits are all 0 ; the satellite is in failure or permanently shut off if the last are all ; the definition is reserved if the other 8 bits are in other values. ** The signal power is 0 db lower than nominal value Time arameters relative to UTC (A 0UTC, A UTC, Δt LS, WN LSF, DN, Δt LSF ) These parameters indicate the relationship between BDT and UTC. Definition of the parameters are listed in Table 5-6. Table 5-6 arameters relative to UTC arameter of bits Scale factor() Effective range Units A 0UTC 32 * 2-30 s A UTC 24 * 2-50 s/s Δt LS 8 * s WN LSF 8 wee DN 8 6 day Δt LSF 8 * s * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. 38

43 effective; effective; 203 China Satellite Navigation Office A 0UTC : BDT cloc bias relative to UTC; A UTC : BDT cloc rate relative to UTC; Δt LS : Delta time due to leap seconds before the new leap second WN LSF : Wee number of the new leap second; DN: Day number of wee of the new leap second; Δt LSF : Delta time due to leap seconds after the new leap second Conversion from BDT into UTC: The broadcast UTC parameters, the WN LSF and DN values mae users compute UTC with error not greater than microsecond. Depending upon the relationship of the effectivity time of leap second event and user s current BDT, the following three different cases of UTC/BDT conversion exist. ) Whenever the effectivity time indicated by the WN LSF and the DN values is not in the past (relative to the user s present time), and the user s current time t E is prior to DN+2/3, the UTC/BDT relationship is given by: t UTC = (t E Δt UTC )[modulo 86400], seconds Δt UTC = Δt LS + A 0UTC + A UTC t E, seconds Where, t E is the in BDT computed by user. 2) Whenever the user s current time t E falls within the time span of DN+2/3 to DN+5/4, proper accommodation of leap second event with possible wee number transition is provided by the following equation for UTC: where, t UTC =W[modulo( Δt LSF Δt LS )], seconds W=( t E Δt UTC 43200)[modulo 86400] , seconds Δt UTC = Δt LS + A 0OUT + A UTC t E, seconds 39

44 203 China Satellite Navigation Office 3) Whenever the effectivity time of leap second event, as indicated by the WN LSF and DN values, is in the past (relative to the user s current time), and the user s current time t E is after DN+5/4, the UTC/BDT relationship is given by: t UTC = (t E Δt UTC )[modulo86400], seconds where, Δt UTC = Δt LSF + A 0UTC + A UTC t E, seconds The parameter definitions are the same with those in case ) Time arameters relative to GS time (A 0GS, A GS ) These parameters indicate the relationship between BDT and GS time as in Table 5-7. (Not broadcast temporarily) Table 5-7 Time parameters relative to GS time arameter of Bits Scale factor () Units A 0GS 4 * 0. ns A GS 6 * 0. ns/s * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. A 0GS : BDT cloc bias relative to GS time; A GS : BDT cloc rate relative to GS time. The relationship between BDT and GS time is as follows: t GS = t E Δt GS where, Δt GS = A 0GS + A GS t E ; t E is the in BDT computed by user Time arameters relative to Galileo time(a 0Gal, A Gal ) These parameters indicate the relationship between BDT and Galileo time as in Table 5-8. (Not broadcast temporarily) 40

45 203 China Satellite Navigation Office Table 5-8 Time parameters relative to Galileo time arameter of Bits Scale factor () Units A 0Gal 4 * 0. ns A Gal 6 * 0. ns/s * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. A 0Gal : BDT cloc bias relative to Galileo system time; A Gal : BDT cloc rate relative to Galileo system time. Relationship between BDT and Galileo system time is as follows: where Δt Gal = A 0Gal + A Gal t E ; t Gal = t E Δt Gal t E is the in BDT computed by user Time arameters relative to GLONASS time (A 0GLO, A GLO ) These parameters indicate the relationship between BDT and GLONASS time as in Table 5-9. (Not broadcast temporarily) Table 5-9 Time parameters relative to GLONASS time arameter of Bits Scale factor () Units A 0GLO 4 * 0. ns A GLO 6 * 0. ns/s * arameters so indicated are two s complement, with the sign bit (+ or ) occupying the. A 0GLO : BDT cloc bias relative to GLONASS time; A GLO : BDT cloc rate relative to GLONASS time. Relationship between BDT and GLONASS time is as follows: t GLO = t E Δt GLO where Δt GLO = A 0GLO + A GLO t E ; t E is the in BDT computed by user. 4

46 203 China Satellite Navigation Office 5.3 D2 NAV Message 5.3. D2 NAV Message Frame Structure The NAV message in format D2 is structured with superframe, frame and subframe. Every superframe is bits long, lasting 6 minutes. Every superframe is composed of 20 frames each with 500 bits and lasting 3 seconds. Every frame is composed of 5 subframes, each with 300 bits and lasting 0.6 second. Every subframe is composed of 0 words, each with 30 bits and lasting 0.06 second. Every word is composed of NAV message data and parity bits. The first 5 bits in word of every subframe is not encoded, and the last bits is encoded in BCH(5,,) for error correction. For the other 9 words of the subframe both BCH(5,,) encoding and interleaving are involved. Thus there are 22 information bits and 8 parity bits in each word. See Figure 5-2 for the detailed structure. Superframe of bits, 6 min Frame Frame 2 Frame n Frame20 Frame of 500 bits, 3 sec of 300bits, 0.6 sec Word Word 2 Word 0 Word, 30 bits, 0.06 sec Word 2~0, 30 bits, 0.06 sec NAV message data, 26 bits 4 arity bits NAV message data, 22 bits 8 arity bits Fig 5-2 Structure of NAV message in format D2 42

47 203 China Satellite Navigation Office D2 NAV Message Detailed structure Information in format D2 includes: the basic NAV information of the broadcasting satellite, almanac, time offset from other systems, integrity and differential correction information of BDS and ionospheric grid information as shown in Figure 5-3. The subframe shall be subcommutated 0 times via 0 pages. The subframe 2, subframe 3 and subframe 4 shall be subcommutated 6 times each via 6 pages. The subframe 5 shall be subcommutated 20 times via 20 pages Basic NAV information of the broadcating satellite Integrity and differential correction information of BDS Almanac, ionospheric grid points and time offsets from other systems Fig 5-3 Frame structure and information contents of NAV message in format D2 The bit allocation of each subframe in format D2 is shown in Figures 5-4 through 5-8. The 50 s of pages through 0 in subframe, pages through 6 of subframe 4, pages 4 through 34, pages 74 through 94 pages and 03 through 20 of subframe 5 in format D2 are to be reserved. 43

48 203 China Satellite Navigation Office first Word 50 s of (300bits) age 2 6 re num SatH bit AODC 5bits 6 URAI WN t oc 5bits 9 t oc T GD 0bits 2 T GD2 0bits 2s 5s 2s Fig 5-4- Bits allocation of 50 s of page in subframe of format D2 first 50 s of (300bits) Word age re num α 0 6 α 0 α α 2 α 3 9 α 3 β 0 β β 2 2 β 2 β 3 2s 6s 2s 4s 4s 2s 6s Fig Bits allocation of 50 s of page 2 in subframe of format D2 44

49 203 China Satellite Navigation Office first 50 bits s of (300bits) Word age re num bits a 0 2 a 0 a 2s 2s 2s 4s Fig Bits allocation of 50 s of page 3 in subframe of format D2 first 50 bits s of (300bits) Word age re num a 6 a a 2 0bits 9 a 2 bit AODE 5bits Δn 2 C uc 2s * 2s 0s 4s * These are data bits next to s and before s. Fig Bits allocation of 50 s of page 4 in subframe of format D2 45