driver_rtcm2.c

/*****************************************************************************

This is a decoder for RTCM-104 2.x, 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 GNSS (GLOBAL NAVIGATION
SATELLITE) SERVICE, VERSION 2.3 (RTCM PAPER 136-2001/SC104-STD)

Ordering instructions are accessible from <http://www.rtcm.org/>
under "Publications".  This describes version 2.3 of the RTCM specification.
RTCM-104 was later completely redesigned as level 3.0.

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 2.x 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 30-bit words of clean parity-checked data.  At this point this upper layer
takes over.  struct rtcm2_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 rtcm2_t.

This code and the contents of isgps.c are evolved from code by
Wolfgang Rupprecht.  Wolfgang's decoder was loosely based on one
written by John Sager in 1999.  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.

This code has been checked against ASCII dumps made by a proprietary
decoder running under Windows and is known to be consistent with it
with respect to message types 1, 3, 9, 14, 16, and 31.  Decoding of
message types 4, 5, 6, 7, and 13 has not been checked. Message types
8, 10-12, 15-27, 28-30 (undefined), 31-37, 38-58 (undefined), and
60-63 are not yet supported.

This file is Copyright (c) 2010 by the GPSD project
SPDX-License-Identifier: BSD-2-clause

*****************************************************************************/

#include <stdio.h>
#include <string.h>

#include "gpsd.h"

/*
  __BYTE_ORDER__, __ORDER_BIG_ENDIAN__ and __ORDER_LITTLE_ENDIAN__ are
  defined in some gcc versions only, probably depending on the
  architecture. Try to use endian.h if the gcc way fails - endian.h also
  does not seem to be available on all platforms.
*/

#if HAVE_BUILTIN_ENDIANNESS
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
#define WORDS_BIGENDIAN 1
#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
#undef WORDS_BIGENDIAN
#else
#error Unknown endianness!
#endif

#else /* HAVE_BUILTIN_ENDIANNESS */

#if defined(HAVE_ENDIAN_H)
#include <endian.h>
#elif defined(HAVE_SYS_ENDIAN_H)
#include <sys/endian.h>
#elif defined(HAVE_MACHINE_ENDIAN_H)
#include <machine/endian.h>
#endif

/*
 * Darwin (Mac OS X) uses special defines.
 * This must precede the BSD case, since _BIG_ENDIAN may be incorrectly defined
 */
#if !defined( __BYTE_ORDER) && defined(__DARWIN_BYTE_ORDER)
#define __BYTE_ORDER __DARWIN_BYTE_ORDER
#endif
#if !defined( __BIG_ENDIAN) && defined(__DARWIN_BIG_ENDIAN)
#define __BIG_ENDIAN __DARWIN_BIG_ENDIAN
#endif
#if !defined( __LITTLE_ENDIAN) && defined(__DARWIN_LITTLE_ENDIAN)
#define __LITTLE_ENDIAN __DARWIN_LITTLE_ENDIAN
#endif

/*
 * BSD uses _BYTE_ORDER, and Linux uses __BYTE_ORDER.
 */
#if !defined( __BYTE_ORDER) && defined(_BYTE_ORDER)
#define __BYTE_ORDER _BYTE_ORDER
#endif
#if !defined( __BIG_ENDIAN) && defined(_BIG_ENDIAN)
#define __BIG_ENDIAN _BIG_ENDIAN
#endif
#if !defined( __LITTLE_ENDIAN) && defined(_LITTLE_ENDIAN)
#define __LITTLE_ENDIAN _LITTLE_ENDIAN
#endif

#if !defined(__BYTE_ORDER) || !defined(__BIG_ENDIAN) || !defined(__LITTLE_ENDIAN)
#error endianness macros are not defined
#endif

#if __BYTE_ORDER == __BIG_ENDIAN
#define WORDS_BIGENDIAN 1
#elif __BYTE_ORDER == __LITTLE_ENDIAN
#undef WORDS_BIGENDIAN
#else
#error Unknown endianness!
#endif /* __BYTE_ORDER */

#endif /* HAVE_BUILTIN_ENDIANNESS */

/*
 * Structures for interpreting words in an RTCM-104 2.x message (after
 * parity checking and removing inversion).  Note, these structures
 * are overlayed on the raw data in order to decode them into
 * bitfields; this will fail horribly if your C compiler introduces
 * padding between or before bit fields, or between 8-bit-aligned
 * bitfields and character arrays despite #pragma pack(1).  The right
 * things happen under gcc 4.x on amd64, i386, ia64, all arm and mips
 * variants, m68k, and powerpc)
 *
 * (In practice, the only class of machines on which this is likely
 * to fail are word-aligned architectures without barrel shifters.
 * Very few of these are left in 2012. By test, we know of s390, s390x,
 * and sparc.)
 *
 * The RTCM 2.1 standard is less explicit than it should be about
 * signed-integer representations.  Two's complement is specified for
 * some but not all.
 */

#define	ZCOUNT_SCALE	0.6	/* sec */
#define	PRCSMALL	0.02	/* meters */
#define	PRCLARGE	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 */

#define LATLON_SCALE	0.01	/* degrees */
#define RANGE_SCALE	4	/* kilometers */

#pragma pack(1)

/*
 * Reminder: Emacs reverse-region is useful...
 */

#ifndef WORDS_BIGENDIAN	/* little-endian, like x86 */

struct rtcm2_msg_t {
    struct rtcm2_msghw1 {			/* header word 1 */
	unsigned int            parity:6;
	unsigned int            refstaid:10;	/* reference station ID */
	unsigned int            msgtype:6;	/* RTCM message type */
	unsigned int            preamble:8;	/* fixed at 01100110 */
	unsigned int            _pad:2;
    } w1;

    struct rtcm2_msghw2 {			/* header word 2 */
	unsigned int            parity:6;
	unsigned int            stathlth:3;	/* station health */
	unsigned int            frmlen:5;
	unsigned int            sqnum:3;
	unsigned int            zcnt:13;
	unsigned int            _pad:2;
    } w2;

    union {
	/* msg 1 - differential gps corrections */
	struct rtcm2_msg1 {
	    struct gps_correction_t {
		struct {			/* msg 1 word 3 */
		    unsigned int    parity:6;
		    int             prc1:16;
		    unsigned int    satident1:5;	/* satellite ID */
		    unsigned int    udre1:2;
		    unsigned int    scale1:1;
		    unsigned int    _pad:2;
		} w3;

		struct {			/* msg 1 word 4 */
		    unsigned int    parity:6;
		    unsigned int    satident2:5;	/* satellite ID */
		    unsigned int    udre2:2;
		    unsigned int    scale2:1;
		    unsigned int    iod1:8;
		    int             rrc1:8;
		    unsigned int    _pad:2;
		} w4;

		struct {			/* msg 1 word 5 */
		    unsigned int    parity:6;
		    int             rrc2:8;
		    int             prc2:16;
		    unsigned int    _pad:2;
		} w5;

		struct {			/* msg 1 word 6 */
		    unsigned int    parity:6;
		    int             prc3_h:8;
		    unsigned int    satident3:5;	/* satellite ID */
		    unsigned int    udre3:2;
		    unsigned int    scale3:1;
		    unsigned int    iod2:8;
		    unsigned int    _pad:2;
		} w6;

		struct {			/* msg 1 word 7 */
		    unsigned int    parity:6;
		    unsigned int    iod3:8;
		    int             rrc3:8;
		    unsigned int    prc3_l:8;		/* NOTE: unsigned int for low byte */
		    unsigned int    _pad:2;
		} w7;
	    } corrections[(RTCM2_WORDS_MAX - 2) / 5];
	} type1;

	/* msg 3 - reference station parameters */
	struct rtcm2_msg3 {
	    struct {
		unsigned int        parity:6;
		unsigned int	    x_h:24;
		unsigned int        _pad:2;
	    } w3;
	    struct {
		unsigned int        parity:6;
		unsigned int	    y_h:16;
		unsigned int	    x_l:8;
		unsigned int        _pad:2;
	    } w4;
	    struct {
		unsigned int        parity:6;
		unsigned int	    z_h:8;
		unsigned int	    y_l:16;
		unsigned int        _pad:2;
	    } w5;

	    struct {
		unsigned int        parity:6;
		unsigned int	    z_l:24;
		unsigned int        _pad:2;
	    } w6;
	} type3;

	/* msg 4 - reference station datum */
	struct rtcm2_msg4 {
	    struct {
		unsigned int        parity:6;
		unsigned int	    datum_alpha_char2:8;
		unsigned int	    datum_alpha_char1:8;
		unsigned int	    spare:4;
		unsigned int	    dat:1;
		unsigned int	    dgnss:3;
		unsigned int        _pad:2;
	    } w3;
	    struct {
		unsigned int        parity:6;
		unsigned int	    datum_sub_div_char2:8;
		unsigned int	    datum_sub_div_char1:8;
		unsigned int	    datum_sub_div_char3:8;
		unsigned int        _pad:2;
	    } w4;
	    struct {
		unsigned int        parity:6;
		unsigned int	    dy_h:8;
		unsigned int	    dx:16;
		unsigned int        _pad:2;
	    } w5;
	    struct {
		unsigned int        parity:6;
		unsigned int	    dz:24;
		unsigned int	    dy_l:8;
		unsigned int        _pad:2;
	    } w6;
	} type4;

	/* msg 5 - constellation health */
	struct rtcm2_msg5 {
	    struct b_health_t {
		unsigned int        parity:6;
		unsigned int	    unassigned:2;
		unsigned int	    time_unhealthy:4;
		unsigned int	    loss_warn:1;
		unsigned int	    new_nav_data:1;
		unsigned int	    health_enable:1;
		unsigned int	    cn0:5;
		unsigned int	    data_health:3;
		unsigned int	    issue_of_data_link:1;
		unsigned int	    sat_id:5;
		unsigned int	    reserved:1;
		unsigned int        _pad:2;
	    } health[MAXHEALTH];
	} type5;

	/* msg 6 - null message */

	/* msg 7 - beacon almanac */
	struct rtcm2_msg7 {
	    struct b_station_t {
		struct {
		    unsigned int    parity:6;
		    int	    	    lon_h:8;
		    int	            lat:16;
		    unsigned int    _pad:2;
		} w3;
		struct {
		    unsigned int    parity:6;
		    unsigned int    freq_h:6;
		    unsigned int    range:10;
		    unsigned int    lon_l:8;
		    unsigned int    _pad:2;
		} w4;
		struct {
		    unsigned int    parity:6;
		    unsigned int    encoding:1;
		    unsigned int    sync_type:1;
		    unsigned int    mod_mode:1;
		    unsigned int    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
		     */
		    unsigned int    station_id:10;
		    unsigned int    health:2;
		    unsigned int    freq_l:6;
		    unsigned int    _pad:2;
		} w5;
	    } almanac[(RTCM2_WORDS_MAX - 2)/3];
	} type7;

	/* msg 13 - Ground Transmitter Parameters (RTCM2.3 only) */
	struct rtcm2_msg13 {
	    struct {
		unsigned int        parity:6;
		int         	    lat:16;
		unsigned int        reserved:6;
		unsigned int        rangeflag:1;
		unsigned int        status:1;
		unsigned int        _pad:2;
	    } w1;
	    struct {
		unsigned int        parity:6;
		unsigned int        range:8;
		int                 lon:16;
		unsigned int        _pad:2;
	    } w2;
	} type13;

	/* msg 14 - GPS Time of Week (RTCM2.3 only) */
	struct rtcm2_msg14 {
	    struct {
		unsigned int        parity:6;
		unsigned int        leapsecs:6;
		unsigned int        hour:8;
		unsigned int        week:10;
		unsigned int        _pad:2;
	    } w1;
	} type14;

	/* msg 16 - text msg */
	struct rtcm2_msg16 {
	    struct {
		unsigned int        parity:6;
		unsigned int	    byte3:8;
		unsigned int	    byte2:8;
		unsigned int	    byte1:8;
		unsigned int        _pad:2;
	    } txt[RTCM2_WORDS_MAX-2];
	} type16;

	/* msg 31 - differential GLONASS corrections */
	struct rtcm2_msg31 {
	    struct glonass_correction_t {
		struct {			/* msg 1 word 3 */
		    unsigned int    parity:6;
		    int             prc1:16;
		    unsigned int    satident1:5;	/* satellite ID */
		    unsigned int    udre1:2;
		    unsigned int    scale1:1;
		    unsigned int    _pad:2;
		} w3;

		struct {			/* msg 1 word 4 */
		    unsigned int    parity:6;
		    unsigned int    satident2:5;	/* satellite ID */
		    unsigned int    udre2:2;
		    unsigned int    scale2:1;
		    unsigned int    tod1:7;
		    unsigned int    change1:1;
		    int             rrc1:8;
		    unsigned int    _pad:2;
		} w4;

		struct {			/* msg 1 word 5 */
		    unsigned int    parity:6;
		    int             rrc2:8;
		    int             prc2:16;
		    unsigned int    _pad:2;
		} w5;

		struct {			/* msg 1 word 6 */
		    unsigned int    parity:6;
		    int             prc3_h:8;
		    unsigned int    satident3:5;	/* satellite ID */
		    unsigned int    udre3:2;
		    unsigned int    scale3:1;
		    unsigned int    tod2:7;
		    unsigned int    change2:1;
		    unsigned int    _pad:2;
		} w6;

		struct {			/* msg 1 word 7 */
		    unsigned int    parity:6;
		    unsigned int    tod3:7;
		    unsigned int    change3:1;
		    int             rrc3:8;
		    unsigned int    prc3_l:8;	/* NOTE: unsigned int for low byte */
		    unsigned int    _pad:2;
		} w7;
	    } corrections[(RTCM2_WORDS_MAX - 2) / 5];
	} type31;

	/* unknown message */
	isgps30bits_t	rtcm2_msgunk[RTCM2_WORDS_MAX-2];
    } msg_type;
} __attribute__((__packed__));

#endif /* LITTLE_ENDIAN */

#ifdef WORDS_BIGENDIAN

struct rtcm2_msg_t {
    struct rtcm2_msghw1 {			/* header word 1 */
	unsigned int            _pad:2;
	unsigned int            preamble:8;	/* fixed at 01100110 */
	unsigned int            msgtype:6;	/* RTCM message type */
	unsigned int            refstaid:10;	/* reference station ID */
	unsigned int            parity:6;
    } w1;

    struct rtcm2_msghw2 {			/* header word 2 */
	unsigned int            _pad:2;
	unsigned int            zcnt:13;
	unsigned int            sqnum:3;
	unsigned int            frmlen:5;
	unsigned int            stathlth:3;	/* station health */
	unsigned int            parity:6;
    } w2;

    union {
	/* msg 1 - differential GPS corrections */
	struct rtcm2_msg1 {
	    struct gps_correction_t {
		struct {			/* msg 1 word 3 */
		    unsigned int    _pad:2;
		    unsigned int    scale1:1;
		    unsigned int    udre1:2;
		    unsigned int    satident1:5;  /* satellite ID */
		    int             prc1:16;
		    unsigned int    parity:6;
		} w3;

		struct {			/* msg 1 word 4 */
		    unsigned int    _pad:2;
		    int             rrc1:8;
		    unsigned int    iod1:8;
		    unsigned int    scale2:1;
		    unsigned int    udre2:2;
		    unsigned int    satident2:5;	/* satellite ID */
		    unsigned int    parity:6;
		} w4;

		struct {			/* msg 1 word 5 */
		    unsigned int    _pad:2;
		    int             prc2:16;
		    int             rrc2:8;
		    unsigned int    parity:6;
		} w5;

		struct {			/* msg 1 word 6 */
		    unsigned int    _pad:2;
		    unsigned int    iod2:8;
		    unsigned int    scale3:1;
		    unsigned int    udre3:2;
		    unsigned int    satident3:5;	/* satellite ID */
		    int             prc3_h:8;
		    unsigned int    parity:6;
		} w6;

		struct {			/* msg 1 word 7 */
		    unsigned int    _pad:2;
		    unsigned int    prc3_l:8;   /* NOTE: unsigned for low byte */
		    int             rrc3:8;
		    unsigned int    iod3:8;
		    unsigned int    parity:6;
		} w7;
	    } corrections[(RTCM2_WORDS_MAX - 2) / 5];
	} type1;

	/* msg 3 - reference station parameters */
	struct rtcm2_msg3 {
	    struct {
		unsigned int        _pad:2;
		unsigned int	    x_h:24;
		unsigned int        parity:6;
	    } w3;
	    struct {
		unsigned int        _pad:2;
		unsigned int	    x_l:8;
		unsigned int	    y_h:16;
		unsigned int        parity:6;
	    } w4;
	    struct {
		unsigned int        _pad:2;
		unsigned int	    y_l:16;
		unsigned int	    z_h:8;
		unsigned int        parity:6;
	    } w5;

	    struct {
		unsigned int        _pad:2;
		unsigned int	    z_l:24;
		unsigned int        parity:6;
	    } w6;
	} type3;

	/* msg 4 - reference station datum */
	struct rtcm2_msg4 {
	    struct {
		unsigned int        _pad:2;
		unsigned int	    dgnss:3;
		unsigned int	    dat:1;
		unsigned int	    spare:4;
		unsigned int	    datum_alpha_char1:8;
		unsigned int	    datum_alpha_char2:8;
		unsigned int        parity:6;
	    } w3;
	    struct {
		unsigned int        _pad:2;
		unsigned int	    datum_sub_div_char3:8;
		unsigned int	    datum_sub_div_char1:8;
		unsigned int	    datum_sub_div_char2:8;
		unsigned int        parity:6;
	    } w4;
	    struct {
		unsigned int        _pad:2;
		unsigned int	    dx:16;
		unsigned int	    dy_h:8;
		unsigned int        parity:6;
	    } w5;
	    struct {
		unsigned int        _pad:2;
		unsigned int	    dy_l:8;
		unsigned int	    dz:24;
		unsigned int        parity:6;
	    } w6;
	} type4;

	/* msg 5 - constellation health */
	struct rtcm2_msg5 {
	    struct b_health_t {
		unsigned int        _pad:2;
		unsigned int	    reserved:1;
		unsigned int	    sat_id:5;
		unsigned int	    issue_of_data_link:1;
		unsigned int	    data_health:3;
		unsigned int	    cn0:5;
		unsigned int	    health_enable:1;
		unsigned int	    new_nav_data:1;
		unsigned int	    loss_warn:1;
		unsigned int	    time_unhealthy:4;
		unsigned int	    unassigned:2;
		unsigned int        parity:6;
	    } health[MAXHEALTH];
	} type5;

	/* msg 6 - null message */

	/* msg 7 - beacon almanac */
	struct rtcm2_msg7 {
	    struct b_station_t {
		struct {
		    unsigned int    _pad:2;
		    int	            lat:16;
		    int	    	    lon_h:8;
		    unsigned int    parity:6;
		} w3;
		struct {
		    unsigned int    _pad:2;
		    unsigned int    lon_l:8;
		    unsigned int    range:10;
		    unsigned int    freq_h:6;
		    unsigned int    parity:6;
		} w4;
		struct {
		    unsigned int    _pad:2;
		    unsigned int    freq_l:6;
		    unsigned int    health:2;
		    unsigned int    station_id:10;
			     /* see comments in LE struct above. */
		    unsigned int    bit_rate:3;
		    unsigned int    mod_mode:1;
		    unsigned int    sync_type:1;
		    unsigned int    encoding:1;
		    unsigned int    parity:6;
		} w5;
	    } almanac[(RTCM2_WORDS_MAX - 2)/3];
	} type7;

	/* msg 13 - Ground Transmitter Parameters (RTCM2.3 only) */
	struct rtcm2_msg13 {
	    struct {
		unsigned int        _pad:2;
		unsigned int        status:1;
		unsigned int        rangeflag:1;
		unsigned int        reserved:6;
		int         	    lat:16;
		unsigned int        parity:6;
	    } w1;
	    struct {
		unsigned int        _pad:2;
		int                 lon:16;
		unsigned int        range:8;
		unsigned int        parity:6;
	    } w2;
	} type13;

	/* msg 14 - GPS Time of Week (RTCM2.3 only) */
	struct rtcm2_msg14 {
	    struct {
		unsigned int        _pad:2;
		unsigned int        week:10;
		unsigned int        hour:8;
		unsigned int        leapsecs:6;
		unsigned int        parity:6;
	    } w1;
	} type14;

	/* msg 16 - text msg */
	struct rtcm2_msg16 {
	    struct {
		unsigned int        _pad:2;
		unsigned int	    byte1:8;
		unsigned int	    byte2:8;
		unsigned int	    byte3:8;
		unsigned int        parity:6;
	    } txt[RTCM2_WORDS_MAX-2];
	} type16;

	/* msg 31 - differential GLONASS corrections */
	struct rtcm2_msg31 {
	    struct glonass_correction_t {
		struct {			/* msg 1 word 3 */
		    unsigned int    _pad:2;
		    unsigned int    scale1:1;
		    unsigned int    udre1:2;
		    unsigned int    satident1:5;	/* satellite ID */
		    int             prc1:16;
		    unsigned int    parity:6;
		} w3;

		struct {			/* msg 1 word 4 */
		    unsigned int    _pad:2;
		    int             rrc1:8;
		    unsigned int    change1:1;
		    unsigned int    tod1:7;
		    unsigned int    scale2:1;
		    unsigned int    udre2:2;
		    unsigned int    satident2:5;	/* satellite ID */
		    unsigned int    parity:6;
		} w4;

		struct {			/* msg 1 word 5 */
		    unsigned int    _pad:2;
		    int             prc2:16;
		    int             rrc2:8;
		    unsigned int    parity:6;
		} w5;

		struct {			/* msg 1 word 6 */
		    unsigned int    _pad:2;
		    unsigned int    change2:1;
		    unsigned int    tod2:7;
		    unsigned int    scale3:1;
		    unsigned int    udre3:2;
		    unsigned int    satident3:5;	/* satellite ID */
		    int             prc3_h:8;
		    unsigned int    parity:6;
		} w6;

		struct {			/* msg 1 word 7 */
		    unsigned int    _pad:2;
		    unsigned int    prc3_l:8;	/* NOTE: unsigned for low byte */
		    int             rrc3:8;
		    unsigned int    change3:1;
		    unsigned int    tod3:7;
		    unsigned int    parity:6;
		} w7;
	    } corrections[(RTCM2_WORDS_MAX - 2) / 5];
	} type31;

	/* unknown message */
	isgps30bits_t	rtcm2_msgunk[RTCM2_WORDS_MAX-2];
    } msg_type;
} __attribute__((__packed__));

#endif /* BIG ENDIAN */

#ifdef RTCM104V2_ENABLE

#define PREAMBLE_PATTERN 0x66

static unsigned int tx_speed[] = { 25, 50, 100, 110, 150, 200, 250, 300 };

#define DIMENSION(a) (unsigned)(sizeof(a)/sizeof(a[0]))

void rtcm2_unpack(struct rtcm2_t *tp, char *buf)
/* break out the raw bits into the content fields */
{
    int len;
    unsigned int n, w;
    struct rtcm2_msg_t *msg = (struct rtcm2_msg_t *)buf;

    tp->type = msg->w1.msgtype;
    tp->length = msg->w2.frmlen;
    tp->zcount = msg->w2.zcnt * ZCOUNT_SCALE;
    tp->refstaid = msg->w1.refstaid;
    tp->seqnum = msg->w2.sqnum;
    tp->stathlth = msg->w2.stathlth;

    len = (int)tp->length;
    n = 0;
    switch (tp->type) {
    case 1:
    case 9:
    {
	struct gps_correction_t *m = &msg->msg_type.type1.corrections[0];

	while (len >= 0) {
	    if (len >= 2) {
		tp->gps_ranges.sat[n].ident = m->w3.satident1;
		tp->gps_ranges.sat[n].udre = m->w3.udre1;
		tp->gps_ranges.sat[n].iod = m->w4.iod1;
		tp->gps_ranges.sat[n].prc = m->w3.prc1 *
		    (m->w3.scale1 ? PRCLARGE : PRCSMALL);
		tp->gps_ranges.sat[n].rrc = m->w4.rrc1 *
		    (m->w3.scale1 ? RRLARGE : RRSMALL);
		n++;
	    }
	    if (len >= 4) {
		tp->gps_ranges.sat[n].ident = m->w4.satident2;
		tp->gps_ranges.sat[n].udre = m->w4.udre2;
		tp->gps_ranges.sat[n].iod = m->w6.iod2;
		tp->gps_ranges.sat[n].prc = m->w5.prc2 *
		    (m->w4.scale2 ? PRCLARGE : PRCSMALL);
		tp->gps_ranges.sat[n].rrc = m->w5.rrc2 *
		    (m->w4.scale2 ? RRLARGE : RRSMALL);
		n++;
	    }
	    if (len >= 5) {
		tp->gps_ranges.sat[n].ident = m->w6.satident3;
		tp->gps_ranges.sat[n].udre = m->w6.udre3;
		tp->gps_ranges.sat[n].iod = m->w7.iod3;
		tp->gps_ranges.sat[n].prc =
		    ((m->w6.prc3_h << 8) | (m->w7.prc3_l)) *
		    (m->w6.scale3 ? PRCLARGE : PRCSMALL);
		tp->gps_ranges.sat[n].rrc =
		    m->w7.rrc3 * (m->w6.scale3 ? RRLARGE : RRSMALL);
		n++;
	    }
	    len -= 5;
	    m++;
	}
	tp->gps_ranges.nentries = n;
    }
	break;
    case 3:
    {
	struct rtcm2_msg3 *m = &msg->msg_type.type3;

	if ((tp->ecef.valid = len >= 4)) {
	    tp->ecef.x = ((m->w3.x_h << 8) | (m->w4.x_l)) * XYZ_SCALE;
	    tp->ecef.y = ((m->w4.y_h << 16) | (m->w5.y_l)) * XYZ_SCALE;
	    tp->ecef.z = ((m->w5.z_h << 24) | (m->w6.z_l)) * XYZ_SCALE;
	}
    }
	break;
    case 4:
	if ((tp->reference.valid = len >= 2)) {
	    struct rtcm2_msg4 *m = &msg->msg_type.type4;

	    tp->reference.system =
		(m->w3.dgnss == 0) ? NAVSYSTEM_GPS :
		((m->w3.dgnss == 1) ? NAVSYSTEM_GLONASS : NAVSYSTEM_UNKNOWN);
	    tp->reference.sense =
		(m->w3.dat != 0) ? SENSE_GLOBAL : SENSE_LOCAL;
	    if (m->w3.datum_alpha_char1) {
		tp->reference.datum[n++] = (char)(m->w3.datum_alpha_char1);
	    }
	    if (m->w3.datum_alpha_char2) {
		tp->reference.datum[n++] = (char)(m->w3.datum_alpha_char2);
	    }
	    if (m->w4.datum_sub_div_char1) {
		tp->reference.datum[n++] = (char)(m->w4.datum_sub_div_char1);
	    }
	    if (m->w4.datum_sub_div_char2) {
		tp->reference.datum[n++] = (char)(m->w4.datum_sub_div_char2);
	    }
	    if (m->w4.datum_sub_div_char3) {
		tp->reference.datum[n++] = (char)(m->w4.datum_sub_div_char3);
	    }
	    /* we used to say n++ here, but scan-build complains */
	    tp->reference.datum[n] = '\0';
	    if (len >= 4) {
		tp->reference.dx = m->w5.dx * DXYZ_SCALE;
		tp->reference.dy =
		    ((m->w5.dy_h << 8) | m->w6.dy_l) * DXYZ_SCALE;
		tp->reference.dz = m->w6.dz * DXYZ_SCALE;
	    } else
		tp->reference.sense = SENSE_INVALID;
	}
	break;
    case 5:
	for (n = 0; n < (unsigned)len; n++) {
	    struct consat_t *csp = &tp->conhealth.sat[n];
	    struct b_health_t *m = &msg->msg_type.type5.health[n];

	    csp->ident = m->sat_id;
	    csp->iodl = m->issue_of_data_link != 0;
	    csp->health = m->data_health;
	    csp->snr = (int)(m->cn0 ? (m->cn0 + CNR_OFFSET) : SNR_BAD);
	    csp->health_en = m->health_enable != 0;
	    csp->new_data = m->new_nav_data != 0;
	    csp->los_warning = m->loss_warn != 0;
	    csp->tou = m->time_unhealthy * TU_SCALE;
	}
	tp->conhealth.nentries = n;
	break;
    case 7:
	for (w = 0; w < (unsigned)len; w++) {
	    struct station_t *np = &tp->almanac.station[n];
	    struct b_station_t *mp = &msg->msg_type.type7.almanac[w];

	    np->latitude = mp->w3.lat * LA_SCALE;
	    np->longitude = ((mp->w3.lon_h << 8) | mp->w4.lon_l) * LO_SCALE;
	    np->range = mp->w4.range;
	    np->frequency =
		(((mp->w4.freq_h << 6) | mp->w5.freq_l) * FREQ_SCALE) +
		FREQ_OFFSET;
	    np->health = mp->w5.health;
	    np->station_id = mp->w5.station_id,
		np->bitrate = tx_speed[mp->w5.bit_rate];
	    n++;
	}
	tp->almanac.nentries = (unsigned)(len / 3);
	break;
    case 13:
	tp->xmitter.status = (bool)msg->msg_type.type13.w1.status;
	tp->xmitter.rangeflag = (bool)msg->msg_type.type13.w1.rangeflag;
	tp->xmitter.lat = msg->msg_type.type13.w1.lat * LATLON_SCALE;
	tp->xmitter.lon = msg->msg_type.type13.w2.lon * LATLON_SCALE;
	tp->xmitter.range = msg->msg_type.type13.w2.range * RANGE_SCALE;
	if (tp->xmitter.range == 0)
	    tp->xmitter.range = 1024;
	break;
    case 14:
	tp->gpstime.week = msg->msg_type.type14.w1.week;
	tp->gpstime.hour = msg->msg_type.type14.w1.hour;
	tp->gpstime.leapsecs = msg->msg_type.type14.w1.leapsecs;
	break;
    case 16:
	for (w = 0; w < (unsigned)len; w++) {
	    if (!msg->msg_type.type16.txt[w].byte1) {
		break;
	    }
	    tp->message[n++] = (char)(msg->msg_type.type16.txt[w].byte1);
	    if (!msg->msg_type.type16.txt[w].byte2) {
		break;
	    }
	    tp->message[n++] = (char)(msg->msg_type.type16.txt[w].byte2);
	    if (!msg->msg_type.type16.txt[w].byte3) {
		break;
	    }
	    tp->message[n++] = (char)(msg->msg_type.type16.txt[w].byte3);
	}
	tp->message[n] = '\0';
	break;

    case 31:
    {
	struct glonass_correction_t *m = &msg->msg_type.type31.corrections[0];

	while (len >= 0) {
	    if (len >= 2) {
		tp->glonass_ranges.sat[n].ident = m->w3.satident1;
		tp->glonass_ranges.sat[n].udre = m->w3.udre1;
		tp->glonass_ranges.sat[n].change = (bool)m->w4.change1;
		tp->glonass_ranges.sat[n].tod = m->w4.tod1;
		tp->glonass_ranges.sat[n].prc = m->w3.prc1 *
		    (m->w3.scale1 ? PRCLARGE : PRCSMALL);
		tp->glonass_ranges.sat[n].rrc = m->w4.rrc1 *
		    (m->w3.scale1 ? RRLARGE : RRSMALL);
		n++;
	    }
	    if (len >= 4) {
		tp->glonass_ranges.sat[n].ident = m->w4.satident2;
		tp->glonass_ranges.sat[n].udre = m->w4.udre2;
		tp->glonass_ranges.sat[n].change = (bool)m->w6.change2;
		tp->glonass_ranges.sat[n].tod = m->w6.tod2;
		tp->glonass_ranges.sat[n].prc = m->w5.prc2 *
		    (m->w4.scale2 ? PRCLARGE : PRCSMALL);
		tp->glonass_ranges.sat[n].rrc = m->w5.rrc2 *
		    (m->w4.scale2 ? RRLARGE : RRSMALL);
		n++;
	    }
	    if (len >= 5) {
		tp->glonass_ranges.sat[n].ident = m->w6.satident3;
		tp->glonass_ranges.sat[n].udre = m->w6.udre3;
		tp->glonass_ranges.sat[n].change = (bool)m->w7.change3;
		tp->glonass_ranges.sat[n].tod = m->w7.tod3;
		tp->glonass_ranges.sat[n].prc =
		    ((m->w6.prc3_h << 8) | (m->w7.prc3_l)) *
		    (m->w6.scale3 ? PRCLARGE : PRCSMALL);
		tp->glonass_ranges.sat[n].rrc =
		    m->w7.rrc3 * (m->w6.scale3 ? RRLARGE : RRSMALL);
		n++;
	    }
	    len -= 5;
	    m++;
	}
	tp->glonass_ranges.nentries = n;
    }
	break;

    default:
	memcpy(tp->words, msg->msg_type.rtcm2_msgunk,
	       (RTCM2_WORDS_MAX - 2) * sizeof(isgps30bits_t));
	break;
    }
}

static bool preamble_match(isgps30bits_t * w)
{
    return (((struct rtcm2_msghw1 *)w)->preamble == PREAMBLE_PATTERN);
}

static bool length_check(struct gps_lexer_t *lexer)
{
    return lexer->isgps.bufindex >= 2
	&& lexer->isgps.bufindex >=
	((struct rtcm2_msg_t *)lexer->isgps.buf)->w2.frmlen + 2u;
}

enum isgpsstat_t rtcm2_decode(struct gps_lexer_t *lexer, unsigned int c)
{
    return isgps_decode(lexer,
			preamble_match, length_check, RTCM2_WORDS_MAX, c);
}

#endif /* RTCM104V2_ENABLE */