diff options
| -rw-r--r-- | gpsd.h | 29 | ||||
| -rw-r--r-- | isgps.c | 66 | ||||
| -rw-r--r-- | rtcm.c | 93 |
3 files changed, 106 insertions, 82 deletions
@@ -263,8 +263,37 @@ enum isgpsstat_t isgps_decode(struct gps_device_t *session, bool (*preamble_match)(isgps30bits_t *), bool (*length_check)(struct gps_device_t *), unsigned int c); +extern unsigned bitreverse(unsigned x, unsigned w); extern enum isgpsstat_t rtcm_decode(struct gps_device_t *, unsigned int); +/* macros for byte-order-independent bitfield extraction */ +#if WORDS_BIGENDIAN +#define signed16(x) (int16_t)bitreverse(x, 16) +#define signed8(x) (int8_t)bitreverse(x, 8) +#define unsigned2(x) bitreverse(x, 2) +#define unsigned3(x) bitreverse(x, 3) +#define unsigned4(x) bitreverse(x, 4) +#define unsigned5(x) bitreverse(x, 5) +#define unsigned6(x) bitreverse(x, 6) +#define unsigned8(x) bitreverse(x, 8) +#define unsigned10(x) bitreverse(x, 10) +#define unsigned13(x) bitreverse(x, 13) +#define unsigned16(x) bitreverse(x, 16) +#define unsigned24(x) bitreverse(x, 24) +#else +#define signed8(x) x +#define signed16(x) x +#define unsigned2(x) x +#define unsigned3(x) x +#define unsigned4(x) x +#define unsigned5(x) x +#define unsigned6(x) x +#define unsigned8(x) x +#define unsigned10(x) x +#define unsigned13(x) x +#define unsigned16(x) x +#define unsigned24(x) x +#endif extern void rtcm_dump(struct gps_device_t *, /*@out@*/char[], size_t); @@ -1,30 +1,21 @@ /***************************************************************************** -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 <http://www.rtcm.org/> -under "Publications". - -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." +This is a decoder for the unnamed protocol described in IS-GPS-200, +the Navstar GPS Interface Specification, and used as a transport layer +for both GPS satellite downlink transmissions and the RTCM104 format +for briodcasting differential-GPS corrections. The code was originally by Wolfgang Rupprecht. ESR severely hacked it, with Wolfgang's help, in order to separate message analysis from -message dumping (he also added support for big-endian machines and -repacking). You are not expected to understand any of it. +message dumping and separate this lower layer from the upper layer +handing RTCM decoding. You are not expected to understand any of it. -However, in case you think you need to, this decoder is divided into -two layers. The lower one just handles synchronizing with the -incoming bitstream and parity checking, Here are Wolfgang's original -rather cryptic notes on the lower level: +This lower layer just handles synchronizing with the incoming +bitstream and parity checking; all it does is assemble message +packets. It needs an upper layer to analyze the packets into +bitfields and the assemble the bitfields into usable data. + +Here are Wolfgang's original rather cryptic notes on this code: -------------------------------------------------------------------------- 1) trim and bitflip the input. @@ -61,24 +52,6 @@ are used to extract pieces of it (which, if you're on a big-endian machine may need to be swapped end-for-end). Those pieces are copied and (where necessary) reassembled into a struct rtcm_t. -Wolfgang's decoder was loosely based on one written by John Sanger in -1999 (in particular the dump function emits Sanger's dump format). -Here are John Sanger'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. --------------------------------------------------------------------------- - *****************************************************************************/ #include <unistd.h> @@ -332,3 +305,18 @@ enum isgpsstat_t isgps_decode(struct gps_device_t *session, } /*@ +usereleased +compdef @*/ +#if WORDS_BIGENDIAN +unsigned bitreverse(unsigned x, unsigned w) +{ + unsigned char mask = 1 << (w - 1), result = 0; + + while (value) /* skip most significant bits that are zero */ + { + if (value & 1) /* replace mod (machine dependency) */ + result |= mask; + mask >>= 1; + value >>= 1; + } + return result; +} +#endif @@ -1,3 +1,53 @@ +/***************************************************************************** + +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 <http://www.rtcm.org/> +under "Publications". + +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 (which, if you're on a big-endian machine +may need to be swapped end-for-end). 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. + +*****************************************************************************/ + #include <unistd.h> #include <stdlib.h> #include <string.h> @@ -234,49 +284,6 @@ struct rtcm_msg_t { static unsigned int tx_speed[] = { 25, 50, 100, 110, 150, 200, 250, 300 }; -#if WORDS_BIGENDIAN -#define signed16(x) (int16_t)bitreverse(x, 16) -#define signed8(x) (int8_t)bitreverse(x, 8) -#define unsigned2(x) bitreverse(x, 2) -#define unsigned3(x) bitreverse(x, 3) -#define unsigned4(x) bitreverse(x, 4) -#define unsigned5(x) bitreverse(x, 5) -#define unsigned6(x) bitreverse(x, 6) -#define unsigned8(x) bitreverse(x, 8) -#define unsigned10(x) bitreverse(x, 10) -#define unsigned13(x) bitreverse(x, 13) -#define unsigned16(x) bitreverse(x, 16) -#define unsigned24(x) bitreverse(x, 24) - -static unsigned bitreverse(unsigned x, unsigned w) -{ - unsigned char mask = 1 << (w - 1), result = 0; - - while (value) /* skip most significant bits that are zero */ - { - if (value & 1) /* replace mod (machine dependency) */ - result |= mask; - mask >>= 1; - value >>= 1; - } - return result; -} -#else -/* placeholders for field inversion macros */ -#define signed8(x) x -#define signed16(x) x -#define unsigned2(x) x -#define unsigned3(x) x -#define unsigned4(x) x -#define unsigned5(x) x -#define unsigned6(x) x -#define unsigned8(x) x -#define unsigned10(x) x -#define unsigned13(x) x -#define unsigned16(x) x -#define unsigned24(x) x -#endif - static void unpack(struct gps_device_t *session) /* break out the raw bits into the content fields */ { |