/* $Id$ */ /***************************************************************************** This is a decoder for RTCM-104, an obscure and complicated serial protocol used for broadcasting pseudorange corrections from differential-GPS reference stations. The applicable standard is RTCM RECOMMENDED STANDARDS FOR DIFFERENTIAL NAVSTAR GPS SERVICE, RTCM PAPER 194-93/SC 104-STD Ordering instructions are accessible from under "Publications". This describes version 2.1 of the RTCM specification. Also applicable is ITU-R M.823: "Technical characteristics of differential transmissions for global navigation satellite systems from maritime radio beacons in the frequency band 283.5 - 315 kHz in region 1 and 285 - 325 kHz in regions 2 & 3." The RTCM protocol uses as a transport layer the GPS satellite downlink protocol described in IS-GPS-200, the Navstar GPS Interface Specification. This code relies on the lower-level packet-assembly code for that protocol in isgps.c. The lower layer's job is done when it has assembled a message of up to 33 words of clean parity-checked data. At this point this upper layer takes over. struct rtcm_msg_t is overlaid on the buffer and the bitfields are used to extract pieces of it. Those pieces are copied and (where necessary) reassembled into a struct rtcm_t. This code and the contents of isgps.c are evolved from code by Wolgang Rupprecht. Wolfgang's decoder was loosely based on one written by John Sager in 1999 (in particular the dump function emits a close descendant of Sager's dump format). Here are John Sager's original notes: The RTCM decoder prints a legible representation of the input data. The RTCM SC-104 specification is copyrighted, so I cannot quote it - in fact, I have never read it! Most of the information used to develop the decoder came from publication ITU-R M.823. This is a specification of the data transmitted from LF DGPS beacons in the 300kHz band. M.823 contains most of those parts of RTCM SC-104 directly relevant to the air interface (there are one or two annoying and vital omissions!). Information about the serial interface format was gleaned from studying the output of a beacon receiver test program made available on Starlink's website. *****************************************************************************/ /* * Structures for interpreting words in an RTCM-104 message (after * parity checking and removing inversion). * * The RTCM standard is less explicit than it should be about signed-integer * representations. Two's compliment is specified for prc and rrc (msg1wX), * but not everywhere. */ #define ZCOUNT_SCALE 0.6 /* sec */ #define PCSMALL 0.02 /* meters */ #define PCLARGE 0.32 /* meters */ #define RRSMALL 0.002 /* meters/sec */ #define RRLARGE 0.032 /* meters/sec */ #define MAXPCSMALL (0x7FFF * PCSMALL) /* 16-bits signed */ #define MAXRRSMALL (0x7F * RRSMALL) /* 8-bits signed */ #define XYZ_SCALE 0.01 /* meters */ #define DXYZ_SCALE 0.1 /* meters */ #define LA_SCALE (90.0/32767.0) /* degrees */ #define LO_SCALE (180.0/32767.0) /* degrees */ #define FREQ_SCALE 0.1 /* kHz */ #define FREQ_OFFSET 190.0 /* kHz */ #define CNR_OFFSET 24 /* dB */ #define TU_SCALE 5 /* minutes */ #pragma pack(1) #ifndef WORDS_BIGENDIAN /* little-endian, like x86 */ struct rtcm_msg_t { struct rtcm_msghw1 { /* header word 1 */ uint parity:6; uint refstaid:10; /* reference station ID */ uint msgtype:6; /* RTCM message type */ uint preamble:8; /* fixed at 01100110 */ uint _pad:2; } w1; struct rtcm_msghw2 { /* header word 2 */ uint parity:6; uint stathlth:3; /* station health */ uint frmlen:5; uint sqnum:3; uint zcnt:13; uint _pad:2; } w2; union { /* msg 1 - differential gps corrections */ struct rtcm_msg1 { struct b_correction_t { struct { /* msg 1 word 3 */ uint parity:6; int pc1:16; uint satident1:5; /* satellite ID */ uint udre1:2; uint scale1:1; uint _pad:2; } w3; struct { /* msg 1 word 4 */ uint parity:6; uint satident2:5; /* satellite ID */ uint udre2:2; uint scale2:1; uint issuedata1:8; int rangerate1:8; uint _pad:2; } w4; struct { /* msg 1 word 5 */ uint parity:6; int rangerate2:8; int pc2:16; uint _pad:2; } w5; struct { /* msg 1 word 6 */ uint parity:6; int pc3_h:8; uint satident3:5; /* satellite ID */ uint udre3:2; uint scale3:1; uint issuedata2:8; uint _pad:2; } w6; struct { /* msg 1 word 7 */ uint parity:6; uint issuedata3:8; int rangerate3:8; uint pc3_l:8; /* NOTE: uint for low byte */ uint _pad:2; } w7; } corrections[(RTCM_WORDS_MAX - 2) / 5]; } type1; /* msg 3 - reference station parameters */ struct rtcm_msg3 { struct { uint parity:6; uint x_h:24; uint _pad:2; } w3; struct { uint parity:6; uint y_h:16; uint x_l:8; uint _pad:2; } w4; struct { uint parity:6; uint z_h:8; uint y_l:16; uint _pad:2; } w5; struct { uint parity:6; uint z_l:24; uint _pad:2; } w6; } type3; /* msg 4 - reference station datum */ struct rtcm_msg4 { struct { uint parity:6; uint datum_alpha_char2:8; uint datum_alpha_char1:8; uint spare:4; uint dat:1; uint dgnss:3; uint _pad:2; } w3; struct { uint parity:6; uint datum_sub_div_char2:8; uint datum_sub_div_char1:8; uint datum_sub_div_char3:8; uint _pad:2; } w4; struct { uint parity:6; uint dy_h:8; uint dx:16; uint _pad:2; } w5; struct { uint parity:6; uint dz:24; uint dy_l:8; uint _pad:2; } w6; } type4; /* msg 5 - constellation health */ struct rtcm_msg5 { struct b_health_t { uint parity:6; uint unassigned:2; uint time_unhealthy:4; uint loss_warn:1; uint new_nav_data:1; uint health_enable:1; uint cn0:5; uint data_health:3; uint issue_of_data_link:1; uint sat_id:5; uint reserved:1; uint _pad:2; } health[MAXHEALTH]; } type5; /* msg 6 - null message */ /* msg 7 - beacon almanac */ struct rtcm_msg7 { struct b_station_t { struct { uint parity:6; int lon_h:8; int lat:16; uint _pad:2; } w3; struct { uint parity:6; uint freq_h:6; uint range:10; uint lon_l:8; uint _pad:2; } w4; struct { uint parity:6; uint encoding:1; uint sync_type:1; uint mod_mode:1; uint bit_rate:3; /* * ITU-R M.823-2 page 9 and RTCM-SC104 v2.1 pages * 4-21 and 4-22 are in conflict over the next two * field sizes. ITU says 9+3, RTCM says 10+2. * The latter correctly decodes the USCG station * id's so I'll use that one here. -wsr */ uint station_id:10; uint health:2; uint freq_l:6; uint _pad:2; } w5; } almanac[(RTCM_WORDS_MAX - 2)/3]; } type7; /* msg 16 - text msg */ struct rtcm_msg16 { struct { uint parity:6; uint byte3:8; uint byte2:8; uint byte1:8; uint _pad:2; } txt[RTCM_WORDS_MAX-2]; } type16; /* unknown message */ isgps30bits_t rtcm_msgunk[RTCM_WORDS_MAX-2]; } msg_type; }; #endif /* LITTLE_ENDIAN */ #ifdef WORDS_BIGENDIAN /* This struct was generated from the above using invert-bitfields.pl */ #ifndef S_SPLINT_S /* splint thinks it's a duplicate definition */ struct rtcm_msg_t { struct rtcm_msghw1 { /* header word 1 */ uint _pad:2; uint preamble:8; /* fixed at 01100110 */ uint msgtype:6; /* RTCM message type */ uint refstaid:10; /* reference station ID */ uint parity:6; } w1; struct rtcm_msghw2 { /* header word 2 */ uint _pad:2; uint zcnt:13; uint sqnum:3; uint frmlen:5; uint stathlth:3; /* station health */ uint parity:6; } w2; union { /* msg 1 - differential gps corrections */ struct rtcm_msg1 { struct b_correction_t { struct { /* msg 1 word 3 */ uint _pad:2; uint scale1:1; uint udre1:2; uint satident1:5; /* satellite ID */ int pc1:16; uint parity:6; } w3; struct { /* msg 1 word 4 */ uint _pad:2; int rangerate1:8; uint issuedata1:8; uint scale2:1; uint udre2:2; uint satident2:5; /* satellite ID */ uint parity:6; } w4; struct { /* msg 1 word 5 */ uint _pad:2; int pc2:16; int rangerate2:8; uint parity:6; } w5; struct { /* msg 1 word 6 */ uint _pad:2; uint issuedata2:8; uint scale3:1; uint udre3:2; uint satident3:5; /* satellite ID */ int pc3_h:8; uint parity:6; } w6; struct { /* msg 1 word 7 */ uint _pad:2; uint pc3_l:8; /* NOTE: uint for low byte */ int rangerate3:8; uint issuedata3:8; uint parity:6; } w7; } corrections[(RTCM_WORDS_MAX - 2) / 5]; } type1; /* msg 3 - reference station parameters */ struct rtcm_msg3 { struct { uint _pad:2; uint x_h:24; uint parity:6; } w3; struct { uint _pad:2; uint x_l:8; uint y_h:16; uint parity:6; } w4; struct { uint _pad:2; uint y_l:16; uint z_h:8; uint parity:6; } w5; struct { uint _pad:2; uint z_l:24; uint parity:6; } w6; } type3; /* msg 4 - reference station datum */ struct rtcm_msg4 { struct { uint _pad:2; uint dgnss:3; uint dat:1; uint spare:4; uint datum_alpha_char1:8; uint datum_alpha_char2:8; uint parity:6; } w3; struct { uint _pad:2; uint datum_sub_div_char3:8; uint datum_sub_div_char1:8; uint datum_sub_div_char2:8; uint parity:6; } w4; struct { uint _pad:2; uint dx:16; uint dy_h:8; uint parity:6; } w5; struct { uint _pad:2; uint dy_l:8; uint dz:24; uint parity:6; } w6; } type4; /* msg 5 - constellation health */ struct rtcm_msg5 { struct b_health_t { uint _pad:2; uint reserved:1; uint sat_id:5; uint issue_of_data_link:1; uint data_health:3; uint cn0:5; uint health_enable:1; uint new_nav_data:1; uint loss_warn:1; uint time_unhealthy:4; uint unassigned:2; uint parity:6; } health[MAXHEALTH]; } type5; /* msg 6 - null message */ /* msg 7 - beacon almanac */ struct rtcm_msg7 { struct b_station_t { struct { uint _pad:2; int lat:16; int lon_h:8; uint parity:6; } w3; struct { uint _pad:2; uint lon_l:8; uint range:10; uint freq_h:6; uint parity:6; } w4; struct { uint _pad:2; uint freq_l:6; uint health:2; uint station_id:10; /* see comments in LE struct above. */ uint bit_rate:3; uint mod_mode:1; uint sync_type:1; uint encoding:1; uint parity:6; } w5; } almanac[(RTCM_WORDS_MAX - 2)/3]; } type7; /* msg 16 - text msg */ struct rtcm_msg16 { struct { uint _pad:2; uint byte1:8; uint byte2:8; uint byte3:8; uint parity:6; } txt[RTCM_WORDS_MAX-2]; } type16; /* unknown message */ isgps30bits_t rtcm_msgunk[RTCM_WORDS_MAX-2]; } msg_type; }; #endif /* S_SPLINT_S */ #endif /* BIG ENDIAN */