rtcm.c

gpsd, a popular GPS daemon.

/* $Id: rtcm.c 4364 2007-05-31 08:43:23Z esr $ */
/*****************************************************************************

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".  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.

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

#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <stdio.h>
#include <math.h> 		/* for round() */

#include "gpsd_config.h"
#include "gpsd.h"
#include "rtcm.h"

#ifdef RTCM104_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 rtcm_unpack(/*@out@*/struct rtcm_t *tp, char *buf)
/* break out the raw bits into the content fields */
{
    int len;
    unsigned int n, w;
    struct rtcm_msg_t *msg = (struct rtcm_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 b_correction_t    *m = &msg->msg_type.type1.corrections[0];

	    while (len >= 0) {
		if (len >= 2) {
		    tp->msg_data.ranges.sat[n].ident      = m->w3.satident1;
		    tp->msg_data.ranges.sat[n].udre       = m->w3.udre1;
		    tp->msg_data.ranges.sat[n].issuedata  = m->w4.issuedata1;
		    tp->msg_data.ranges.sat[n].rangerr    = m->w3.pc1 * 
			(m->w3.scale1 ? PCLARGE : PCSMALL);
		    tp->msg_data.ranges.sat[n].rangerate  = m->w4.rangerate1 * 
					(m->w3.scale1 ? RRLARGE : RRSMALL);
		    n++;
		}
		if (len >= 4) {
		    tp->msg_data.ranges.sat[n].ident      = m->w4.satident2;
		    tp->msg_data.ranges.sat[n].udre       = m->w4.udre2;
		    tp->msg_data.ranges.sat[n].issuedata  = m->w6.issuedata2;
		    tp->msg_data.ranges.sat[n].rangerr    = m->w5.pc2 * 
			(m->w4.scale2 ? PCLARGE : PCSMALL);
		    tp->msg_data.ranges.sat[n].rangerate  = m->w5.rangerate2 * 
			(m->w4.scale2 ? RRLARGE : RRSMALL);
		    n++;
		}
		if (len >= 5) {
		    tp->msg_data.ranges.sat[n].ident       = m->w6.satident3;
		    tp->msg_data.ranges.sat[n].udre        = m->w6.udre3;
		    tp->msg_data.ranges.sat[n].issuedata   = m->w7.issuedata3;
		    /*@ -shiftimplementation @*/
		    tp->msg_data.ranges.sat[n].rangerr     = ((m->w6.pc3_h<<8)|(m->w7.pc3_l)) *
					(m->w6.scale3 ? PCLARGE : PCSMALL);
		    tp->msg_data.ranges.sat[n].rangerate   = m->w7.rangerate3 * 
					(m->w6.scale3 ? RRLARGE : RRSMALL);
		    /*@ +shiftimplementation @*/
		    n++;
		}
		len -= 5;
		m++;
	    }
	    tp->msg_data.ranges.nentries = n;
	}
	break;
    case 3:
        {
	    struct rtcm_msg3    *m = &msg->msg_type.type3;

	    if ((tp->msg_data.ecef.valid = len >= 4)) {
		tp->msg_data.ecef.x = ((m->w3.x_h<<8)|(m->w4.x_l))*XYZ_SCALE;
		tp->msg_data.ecef.y = ((m->w4.y_h<<16)|(m->w5.y_l))*XYZ_SCALE;
		tp->msg_data.ecef.z = ((m->w5.z_h<<24)|(m->w6.z_l))*XYZ_SCALE;
	    }
	}
	break;
    case 4:
	if ((tp->msg_data.reference.valid = len >= 2)){
	    struct rtcm_msg4    *m = &msg->msg_type.type4;

	    tp->msg_data.reference.system =
		    (m->w3.dgnss==0) ? gps :
			    ((m->w3.dgnss==1) ? glonass : unknown);
	    tp->msg_data.reference.sense = (m->w3.dat != 0) ? global : local;
	    if (m->w3.datum_alpha_char1){
		tp->msg_data.reference.datum[n++] = (char)(m->w3.datum_alpha_char1);
	    }
	    if (m->w3.datum_alpha_char2){
		tp->msg_data.reference.datum[n++] = (char)(m->w3.datum_alpha_char2);
	    }
	    if (m->w4.datum_sub_div_char1){
		tp->msg_data.reference.datum[n++] = (char)(m->w4.datum_sub_div_char1);
	    }
	    if (m->w4.datum_sub_div_char2){
		tp->msg_data.reference.datum[n++] = (char)(m->w4.datum_sub_div_char2);
	    }
	    if (m->w4.datum_sub_div_char3){
		tp->msg_data.reference.datum[n++] = (char)(m->w4.datum_sub_div_char3);
	    }
	    tp->msg_data.reference.datum[n++] = '\0';
	    if (len >= 4) {
		tp->msg_data.reference.dx = m->w5.dx * DXYZ_SCALE;
		tp->msg_data.reference.dy = ((m->w5.dy_h << 8) | m->w6.dy_l) * DXYZ_SCALE;
		tp->msg_data.reference.dz = m->w6.dz * DXYZ_SCALE;
	    } else 
		tp->msg_data.reference.sense = invalid;
	}
	break;
    case 5:
	for (n = 0; n < (unsigned)len; n++) {
	    struct consat_t *csp = &tp->msg_data.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;
	    /*@i@*/csp->snr = (m->cn0?(m->cn0+CNR_OFFSET):SNR_BAD);
	    csp->health_en = m->health_enable;
	    csp->new_data = m->new_nav_data!=0;
	    csp->los_warning = m->loss_warn!=0;
	    csp->tou = m->time_unhealthy*TU_SCALE;
	}
	tp->msg_data.conhealth.nentries = n;
	break;
    case 7:
	for (w = 0; w < (unsigned)len; w++) {
	    struct station_t *np = &tp->msg_data.almanac.station[n];
	    struct b_station_t *mp = &msg->msg_type.type7.almanac[w];

	    np->latitude = mp->w3.lat * LA_SCALE;
	    /*@i@*/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->msg_data.almanac.nentries = (unsigned)(len/3);
	break;
    case 16:
	/*@ -boolops @*/
	for (w = 0; w < (unsigned)len; w++){
	    if (!msg->msg_type.type16.txt[w].byte1) {
		break;
	    }
	    tp->msg_data.message[n++] = (char)(msg->msg_type.type16.txt[w].byte1);
	    if (!msg->msg_type.type16.txt[w].byte2) {
		break;
	    }
	    tp->msg_data.message[n++] = (char)(msg->msg_type.type16.txt[w].byte2);
	    if (!msg->msg_type.type16.txt[w].byte3) {
		break;
	    }
	    tp->msg_data.message[n++] = (char)(msg->msg_type.type16.txt[w].byte3);
	}
	/*@ +boolops @*/
	tp->msg_data.message[n++] = '\0';
	break;

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

bool rtcm_repack(struct rtcm_t *tp, isgps30bits_t *buf)
/* repack the content fields into the raw bits */
{
    int len, sval;
    unsigned int n, w, uval;
    struct rtcm_msg_t  *msg = (struct rtcm_msg_t *)buf;
    struct rtcm_msghw1 *wp  = (struct rtcm_msghw1 *)buf;

    msg->w1.msgtype = tp->type;
    msg->w2.frmlen = tp->length;
    msg->w2.zcnt = (unsigned) round(tp->zcount / ZCOUNT_SCALE);
    msg->w1.refstaid = tp->refstaid;
    msg->w2.sqnum = tp->seqnum;
    msg->w2.stathlth = tp->stathlth;

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

	    while (len >= 0) {
		if (len >= 2) {
		    struct rangesat_t *ssp = &tp->msg_data.ranges.sat[n];
		    m->w3.satident1 = ssp->ident;
		    m->w3.udre1 = ssp->udre;
		    m->w4.issuedata1 = ssp->issuedata;
		    m->w3.scale1 = (unsigned)((ssp->rangerr > MAXPCSMALL) ||
					      (ssp->rangerr < (-MAXPCSMALL)) ||
					      (ssp->rangerate > MAXRRSMALL) ||
					      (ssp->rangerate < (-MAXRRSMALL)));
		    m->w3.pc1 = (int) round(ssp->rangerr / (m->w3.scale1 ? PCLARGE : PCSMALL));
		    m->w4.rangerate1 = (int) round(ssp->rangerate / (m->w3.scale1 ? RRLARGE : RRSMALL));
		    n++;
		}
		if (len >= 4) {
		    struct rangesat_t *ssp = &tp->msg_data.ranges.sat[n];
		    m->w4.satident2 = ssp->ident;
		    m->w4.udre2 = ssp->udre;
		    m->w6.issuedata2 = ssp->issuedata;
		    m->w4.scale2 = (unsigned)((ssp->rangerr > MAXPCSMALL) ||
					      (ssp->rangerr < (-MAXPCSMALL)) ||
					      (ssp->rangerate > MAXRRSMALL) ||
					      (ssp->rangerate < (-MAXRRSMALL)));
		    m->w5.pc2 = (int) round(ssp->rangerr / (m->w4.scale2 ? PCLARGE : PCSMALL));
		    m->w5.rangerate2 = (int) round(ssp->rangerate / (m->w4.scale2 ? RRLARGE : RRSMALL));
		    n++;
		}
		if (len >= 5) {
		    struct rangesat_t *ssp = &tp->msg_data.ranges.sat[n];
		    m->w6.satident3 = ssp->ident;
		    m->w6.udre3 = ssp->udre;
		    m->w7.issuedata3 = ssp->issuedata;
		    m->w6.scale3 = (unsigned)((ssp->rangerr > MAXPCSMALL) ||
					      (ssp->rangerr < (-MAXPCSMALL)) ||
					      (ssp->rangerate > MAXRRSMALL) ||
					      (ssp->rangerate < (-MAXRRSMALL)));
		    sval = (int) round(ssp->rangerr / (m->w6.scale3 ? PCLARGE : PCSMALL));
		    /*@ -shiftimplementation @*/
		    m->w6.pc3_h = sval >> 8;
		    /*@ +shiftimplementation @*/
		    m->w7.pc3_l = (unsigned)sval & 0xff;
		    m->w7.rangerate3 = (int) round(ssp->rangerate / (m->w6.scale3 ? RRLARGE : RRSMALL));
		    n++;
		}
		len -= 5;
		m++;
	    }
	    tp->msg_data.ranges.nentries = n;
	}
	break;
    case 3:	/* R */
	if (tp->msg_data.ecef.valid) {
	    struct rtcm_msg3    *m = &msg->msg_type.type3;
	    unsigned x = (unsigned) round(tp->msg_data.ecef.x / XYZ_SCALE);
	    unsigned y = (unsigned) round(tp->msg_data.ecef.y / XYZ_SCALE);
	    unsigned z = (unsigned) round(tp->msg_data.ecef.z / XYZ_SCALE);

	    m->w4.x_l = x & 0xff;
	    m->w3.x_h = x >> 8;
	    m->w5.y_l = y & 0xffff;
	    m->w4.y_h = y >> 16;
	    m->w6.z_l = z & 0xffffff;
	    m->w5.z_h = z >> 24;
	}
	break;
    case 4:	/* D */
	if (tp->msg_data.reference.valid) {
	    struct rtcm_msg4    *m = &msg->msg_type.type4;

	    m->w3.dgnss = tp->msg_data.reference.system;
	    m->w3.dat = (unsigned)(tp->msg_data.reference.sense == global);
	    /*@ -predboolothers -type @*/
	    if (tp->msg_data.reference.datum[0])
		m->w3.datum_alpha_char1 = tp->msg_data.reference.datum[0];
	    else
		m->w3.datum_alpha_char1 = 0;
	    if (tp->msg_data.reference.datum[1])
		m->w3.datum_alpha_char2 = tp->msg_data.reference.datum[1];
	    else
		m->w3.datum_alpha_char2 = 0;
	    if (tp->msg_data.reference.datum[2])
		m->w4.datum_sub_div_char1 = tp->msg_data.reference.datum[2];
	    else
		m->w4.datum_sub_div_char1 = 0;
	    if (tp->msg_data.reference.datum[3])
		m->w4.datum_sub_div_char2 = tp->msg_data.reference.datum[3];
	    else
		m->w4.datum_sub_div_char2 = 0;
	    if (tp->msg_data.reference.datum[4])
		m->w4.datum_sub_div_char3 = tp->msg_data.reference.datum[4];
	    else
		m->w4.datum_sub_div_char3 = 0;
	    /*@ +predboolothers +type @*/
	    if (tp->msg_data.reference.system != unknown) {
		m->w5.dx = (uint)round(tp->msg_data.reference.dx / DXYZ_SCALE);
		uval = (uint)round(tp->msg_data.reference.dy / DXYZ_SCALE);
		m->w5.dy_h = uval >> 8;
		m->w6.dy_l = uval & 0xff;
		m->w6.dz = (uint)round(tp->msg_data.reference.dz / DXYZ_SCALE);
	    }
	}
	break;
    case 5:	/* C */
	for (n = 0; n < (unsigned)len; n++) {
	    struct consat_t *csp = &tp->msg_data.conhealth.sat[n];
	    struct b_health_t *m = &msg->msg_type.type5.health[n];

	    m->sat_id = csp->ident;
	    m->issue_of_data_link = (unsigned)csp->iodl;
	    m->data_health = csp->health;
	    m->cn0 = (csp->snr == SNR_BAD) ? 0 : (unsigned)csp->snr-CNR_OFFSET;
	    m->health_enable = csp->health_en;
	    m->new_nav_data = (unsigned)csp->new_data;