refclock_nmea.c   [plain text]

/*
 * refclock_nmea.c - clock driver for an NMEA GPS CLOCK
 *		Michael Petry Jun 20, 1994
 *		 based on refclock_heathn.c
 *
 * Updated to add support for Accord GPS Clock
 * 		Venu Gopal Dec 05, 2007
 * 		neo.venu@gmail.com, venugopal_d@pgad.gov.in
 *
 * Updated to process 'time1' fudge factor
 *		Venu Gopal May 05, 2008
 *
 * Converted to common PPSAPI code, separate PPS fudge time1
 * from serial timecode fudge time2.
 *		Dave Hart July 1, 2009
 *		hart@ntp.org, davehart@davehart.com
 */
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#if defined(REFCLOCK) && defined(CLOCK_NMEA)

#include <sys/stat.h>
#include <stdio.h>
#include <ctype.h>

#include "ntpd.h"
#include "ntp_io.h"
#include "ntp_unixtime.h"
#include "ntp_refclock.h"
#include "ntp_stdlib.h"

#ifdef HAVE_PPSAPI
# include "ppsapi_timepps.h"
#include "refclock_atom.h"
#endif /* HAVE_PPSAPI */

#ifdef SYS_WINNT
#undef write	/* ports/winnt/include/config.h: #define write _write */
extern int async_write(int, const void *, unsigned int);
#define write(fd, data, octets)	async_write(fd, data, octets)
#endif

/*
 * This driver supports NMEA-compatible GPS receivers
 *
 * Prototype was refclock_trak.c, Thanks a lot.
 *
 * The receiver used spits out the NMEA sentences for boat navigation.
 * And you thought it was an information superhighway.  Try a raging river
 * filled with rapids and whirlpools that rip away your data and warp time.
 *
 * If HAVE_PPSAPI is defined code to use the PPSAPI will be compiled in.
 * On startup if initialization of the PPSAPI fails, it will fall back
 * to the "normal" timestamps.
 *
 * The PPSAPI part of the driver understands fudge flag2 and flag3. If
 * flag2 is set, it will use the clear edge of the pulse. If flag3 is
 * set, kernel hardpps is enabled.
 *
 * GPS sentences other than RMC (the default) may be enabled by setting
 * the relevent bits of 'mode' in the server configuration line
 * server 127.127.20.x mode X
 * 
 * bit 0 - enables RMC (1)
 * bit 1 - enables GGA (2)
 * bit 2 - enables GLL (4)
 * bit 3 - enables ZDA (8) - Standard Time & Date
 * bit 3 - enables ZDG (8) - Accord GPS Clock's custom sentence with GPS time 
 *			     very close to standard ZDA
 * 
 * Multiple sentences may be selected except when ZDG/ZDA is selected.
 *
 * bit 4/5/6 - selects the baudrate for serial port :
 *		0 for 4800 (default) 
 *		1 for 9600 
 *		2 for 19200 
 *		3 for 38400 
 *		4 for 57600 
 *		5 for 115200 
 */
#define NMEA_MESSAGE_MASK_OLD    0x07
#define NMEA_MESSAGE_MASK_SINGLE 0x08
#define NMEA_MESSAGE_MASK        (NMEA_MESSAGE_MASK_OLD | NMEA_MESSAGE_MASK_SINGLE)

#define NMEA_BAUDRATE_MASK       0x70
#define NMEA_BAUDRATE_SHIFT      4

/*
 * Definitions
 */
#define	DEVICE		"/dev/gps%d"	/* GPS serial device */
#define	PPSDEV		"/dev/gpspps%d"	/* PPSAPI device override */
#define	SPEED232	B4800	/* uart speed (4800 bps) */
#define	PRECISION	(-9)	/* precision assumed (about 2 ms) */
#define	PPS_PRECISION	(-20)	/* precision assumed (about 1 us) */
#define	REFID		"GPS\0"	/* reference id */
#define	DESCRIPTION	"NMEA GPS Clock" /* who we are */
#define NANOSECOND	1000000000 /* one second (ns) */
#define RANGEGATE	500000	/* range gate (ns) */
#ifndef O_NOCTTY
#define M_NOCTTY	0
#else
#define M_NOCTTY	O_NOCTTY
#endif
#ifndef O_NONBLOCK
#define M_NONBLOCK	0
#else
#define M_NONBLOCK	O_NONBLOCK
#endif
#define PPSOPENMODE	(O_RDWR | M_NOCTTY | M_NONBLOCK)

/*
 * Unit control structure
 */
struct nmeaunit {
#ifdef HAVE_PPSAPI
	struct refclock_atom atom; /* PPSAPI structure */
	int	ppsapi_tried;	/* attempt PPSAPI once */
	int	ppsapi_lit;	/* time_pps_create() worked */
	int	ppsapi_fd;	/* fd used with PPSAPI */
	int	tcount;		/* timecode sample counter */
	int	pcount;		/* PPS sample counter */
#endif /* HAVE_PPSAPI */
	l_fp	tstamp;		/* timestamp of last poll */
	int	gps_time;	/* 0 UTC, 1 GPS time */
};

/*
 * Function prototypes
 */
static	int	nmea_start	(int, struct peer *);
static	void	nmea_shutdown	(int, struct peer *);
static	void	nmea_receive	(struct recvbuf *);
static	void	nmea_poll	(int, struct peer *);
#ifdef HAVE_PPSAPI
static	void	nmea_control	(int, struct refclockstat *,
				 struct refclockstat *, struct peer *);
static  void	nmea_timer	(int, struct peer *);
#define		NMEA_CONTROL	nmea_control
#define		NMEA_TIMER	nmea_timer
#else
#define		NMEA_CONTROL	noentry
#define		NMEA_TIMER	noentry
#endif /* HAVE_PPSAPI */
static	void	gps_send	(int, const char *, struct peer *);
static	char *	field_parse	(char *, int);
static	int	nmea_checksum_ok(const char *);

/*
 * Transfer vector
 */
struct	refclock refclock_nmea = {
	nmea_start,		/* start up driver */
	nmea_shutdown,		/* shut down driver */
	nmea_poll,		/* transmit poll message */
	NMEA_CONTROL,		/* fudge control */
	noentry,		/* initialize driver */
	noentry,		/* buginfo */
	NMEA_TIMER		/* called once per second */
};

/*
 * nmea_start - open the GPS devices and initialize data for processing
 */
static int
nmea_start(
	int unit,
	struct peer *peer
	)
{
	register struct nmeaunit *up;
	struct refclockproc *pp;
	int fd;
	char device[20];
	int baudrate;
	char *baudtext;

	pp = peer->procptr;

	/*
	 * Open serial port. Use CLK line discipline, if available.
	 */
	snprintf(device, sizeof(device), DEVICE, unit);
	
	/*
	 * Opening the serial port with appropriate baudrate
	 * based on the value of bit 4/5/6
	 */
	switch ((peer->ttl & NMEA_BAUDRATE_MASK) >> NMEA_BAUDRATE_SHIFT) {
	    case 0:
	    case 6:
	    case 7:
	    default:
		baudrate = SPEED232;
		baudtext = "4800";
		break;
	    case 1:
		baudrate = B9600;
		baudtext = "9600";
		break;
	    case 2:
		baudrate = B19200;
		baudtext = "19200";
		break;
	    case 3:
		baudrate = B38400;
		baudtext = "38400";
		break;
#ifdef B57600
	    case 4:
		baudrate = B57600;
		baudtext = "57600";
		break;
#endif
#ifdef B115200
	    case 5:
		baudrate = B115200;
		baudtext = "115200";
		break;
#endif
	}

	fd = refclock_open(device, baudrate, LDISC_CLK);
	
	if (fd <= 0) {
#ifdef HAVE_READLINK
		/* nmead support added by Jon Miner (cp_n18@yahoo.com)
		 *
		 * See http://home.hiwaay.net/~taylorc/gps/nmea-server/
		 * for information about nmead
		 *
		 * To use this, you need to create a link from /dev/gpsX to
		 * the server:port where nmead is running.  Something like this:
		 *
		 * ln -s server:port /dev/gps1
		 */
		char buffer[80];
		char *nmea_host, *nmea_tail;
		int   nmea_port;
		int   len;
		struct hostent *he;
		struct protoent *p;
		struct sockaddr_in so_addr;

		if ((len = readlink(device,buffer,sizeof(buffer))) == -1)
			return(0);
		buffer[len] = 0;

		if ((nmea_host = strtok(buffer,":")) == NULL)
			return(0);
		if ((nmea_tail = strtok(NULL,":")) == NULL)
			return(0);

		nmea_port = atoi(nmea_tail);

		if ((he = gethostbyname(nmea_host)) == NULL)
			return(0);
		if ((p = getprotobyname("ip")) == NULL)
			return(0);
		memset(&so_addr, 0, sizeof(so_addr));
		so_addr.sin_family = AF_INET;
		so_addr.sin_port = htons(nmea_port);
		so_addr.sin_addr = *((struct in_addr *) he->h_addr);

		if ((fd = socket(PF_INET,SOCK_STREAM,p->p_proto)) == -1)
			return(0);
		if (connect(fd,(struct sockaddr *)&so_addr, sizeof(so_addr)) == -1) {
			close(fd);
			return (0);
		}
#else
		pp->io.fd = -1;
		return (0);
#endif
	}

	msyslog(LOG_NOTICE, "%s serial %s open at %s bps",
		refnumtoa(&peer->srcadr), device, baudtext);

	/*
	 * Allocate and initialize unit structure
	 */
	up = emalloc(sizeof(*up));
	memset(up, 0, sizeof(*up));
	pp->io.clock_recv = nmea_receive;
	pp->io.srcclock = (caddr_t)peer;
	pp->io.datalen = 0;
	pp->io.fd = fd;
	if (!io_addclock(&pp->io)) {
		pp->io.fd = -1;
		close(fd);
		free(up);
		return (0);
	}
	pp->unitptr = (caddr_t)up;

	/*
	 * Initialize miscellaneous variables
	 */
	peer->precision = PRECISION;
	pp->clockdesc = DESCRIPTION;
	memcpy(&pp->refid, REFID, 4);

	gps_send(fd,"$PMOTG,RMC,0000*1D\r\n", peer);

	return (1);
}


/*
 * nmea_shutdown - shut down a GPS clock
 * 
 * NOTE this routine is called after nmea_start() returns failure,
 * as well as during a normal shutdown due to ntpq :config unpeer.
 */
static void
nmea_shutdown(
	int unit,
	struct peer *peer
	)
{
	register struct nmeaunit *up;
	struct refclockproc *pp;

	UNUSED_ARG(unit);

	pp = peer->procptr;
	up = (struct nmeaunit *)pp->unitptr;
	if (up != NULL) {
#ifdef HAVE_PPSAPI
		if (up->ppsapi_lit) {
			time_pps_destroy(up->atom.handle);
			if (up->ppsapi_fd != pp->io.fd)
				close(up->ppsapi_fd);
		}
#endif
		free(up);
	}
	if (-1 != pp->io.fd)
		io_closeclock(&pp->io);
}

/*
 * nmea_control - configure fudge params
 */
#ifdef HAVE_PPSAPI
static void
nmea_control(
	int unit,
	struct refclockstat *in_st,
	struct refclockstat *out_st,
	struct peer *peer
	)
{
	char device[32];
	register struct nmeaunit *up;
	struct refclockproc *pp;
	int pps_fd;
	
	UNUSED_ARG(in_st);
	UNUSED_ARG(out_st);

	pp = peer->procptr;
	up = (struct nmeaunit *)pp->unitptr;

	if (!(CLK_FLAG1 & pp->sloppyclockflag)) {
		if (!up->ppsapi_tried)
			return;
		up->ppsapi_tried = 0;
		if (!up->ppsapi_lit)
			return;
		peer->flags &= ~FLAG_PPS;
		peer->precision = PRECISION;
		time_pps_destroy(up->atom.handle);
		if (up->ppsapi_fd != pp->io.fd)
			close(up->ppsapi_fd);
		up->atom.handle = 0;
		up->ppsapi_lit = 0;
		up->ppsapi_fd = -1;
		return;
	}

	if (up->ppsapi_tried)
		return;
	/*
	 * Light up the PPSAPI interface.
	 */
	up->ppsapi_tried = 1;

	/*
	 * if /dev/gpspps$UNIT can be opened that will be used for
	 * PPSAPI.  Otherwise, the GPS serial device /dev/gps$UNIT
	 * already opened is used for PPSAPI as well.
	 */
	snprintf(device, sizeof(device), PPSDEV, unit);

	pps_fd = open(device, PPSOPENMODE, S_IRUSR | S_IWUSR);

	if (-1 == pps_fd)
		pps_fd = pp->io.fd;
	
	if (refclock_ppsapi(pps_fd, &up->atom)) {
		up->ppsapi_lit = 1;
		up->ppsapi_fd = pps_fd;
		return;
	}

	NLOG(NLOG_CLOCKINFO)
		msyslog(LOG_WARNING, "%s flag1 1 but PPSAPI fails",
			refnumtoa(&peer->srcadr));
}
#endif	/* HAVE_PPSAPI */


/*
 * nmea_timer - called once per second, fetches PPS
 *		timestamp and stuffs in median filter.
 */
#ifdef HAVE_PPSAPI
static void
nmea_timer(
	int		unit,
	struct peer *	peer
	)
{
	struct nmeaunit *up;
	struct refclockproc *pp;

	UNUSED_ARG(unit);

	pp = peer->procptr;
	up = (struct nmeaunit *)pp->unitptr;

	if (up->ppsapi_lit &&
	    refclock_pps(peer, &up->atom, pp->sloppyclockflag) > 0) {
		up->pcount++,
		peer->flags |= FLAG_PPS;
		peer->precision = PPS_PRECISION;
	}
}
#endif	/* HAVE_PPSAPI */


/*
 * nmea_receive - receive data from the serial interface
 */
static void
nmea_receive(
	struct recvbuf *rbufp
	)
{
	register struct nmeaunit *up;
	struct refclockproc *pp;
	struct peer *peer;
	int month, day;
	char *cp, *dp, *msg;
	int cmdtype;
	int cmdtypezdg = 0;
	/* Use these variables to hold data until we decide its worth keeping */
	char	rd_lastcode[BMAX];
	l_fp	rd_timestamp;
	int	rd_lencode;

	/*
	 * Initialize pointers and read the timecode and timestamp
	 */
	peer = rbufp->recv_peer;
	pp = peer->procptr;
	up = (struct nmeaunit *)pp->unitptr;

	rd_lencode = refclock_gtlin(
			rbufp, 
			rd_lastcode, 
			sizeof(rd_lastcode), 
			&rd_timestamp);

	/*
	 * There is a case that a <CR><LF> gives back a "blank" line
	 */
	if (rd_lencode == 0)
		return;

	DPRINTF(1, ("nmea: gpsread %d %s\n", rd_lencode, rd_lastcode));

	/*
	 * We check the timecode format and decode its contents. The
	 * we only care about a few of them.  The most important being
	 * the $GPRMC format
	 * $GPRMC,hhmmss,a,fddmm.xx,n,dddmmm.xx,w,zz.z,yyy.,ddmmyy,dd,v*CC
	 * mode (0,1,2,3) selects sentence ANY/ALL, RMC, GGA, GLL, ZDA
	 * $GPGLL,3513.8385,S,14900.7851,E,232420.594,A*21
	 * $GPGGA,232420.59,3513.8385,S,14900.7851,E,1,05,3.4,00519,M,,,,*3F
	 * $GPRMC,232418.19,A,3513.8386,S,14900.7853,E,00.0,000.0,121199,12.,E*77
	 *
	 * Defining GPZDA to support Standard Time & Date
	 * sentence. The sentence has the following format 
	 *  
	 *  $--ZDA,HHMMSS.SS,DD,MM,YYYY,TH,TM,*CS<CR><LF>
	 *
	 *  Apart from the familiar fields, 
	 *  'TH'    Time zone Hours
	 *  'TM'    Time zone Minutes
	 *
	 * Defining GPZDG to support Accord GPS Clock's custom NMEA 
	 * sentence. The sentence has the following format 
	 *  
	 *  $GPZDG,HHMMSS.S,DD,MM,YYYY,AA.BB,V*CS<CR><LF>
	 *
	 *  It contains the GPS timestamp valid for next PPS pulse.
	 *  Apart from the familiar fields, 
	 *  'AA.BB' denotes the signal strength( should be < 05.00 ) 
	 *  'V'     denotes the GPS sync status : 
	 *	   '0' indicates INVALID time, 
	 *	   '1' indicates accuracy of +/-20 ms
	 *	   '2' indicates accuracy of +/-100 ns
	 */
#define GPXXX		0	/* any/all */
#define GPRMC		1
#define GPGGA		2
#define GPGLL		4
#define GPZDG_ZDA	8

	cp = rd_lastcode;
	cmdtype=0;
	if (cp[0] == '$') {
		/* Allow for GLGGA and GPGGA etc. */
		msg = cp + 3;

		if (strncmp(msg, "RMC", 3) == 0)
			cmdtype = GPRMC;
		else if (strncmp(msg, "GGA", 3) == 0)
			cmdtype = GPGGA;
		else if (strncmp(msg, "GLL", 3) == 0)
			cmdtype = GPGLL;
		else if (strncmp(msg, "ZD", 2) == 0) {
			cmdtype = GPZDG_ZDA;
			if ('G' == msg[2])
				cmdtypezdg = 1;
			else if ('A' != msg[2])
				return;
		} else
			return;
	} else
		return;

	/* See if I want to process this message type */
	if (peer->ttl && !(cmdtype & (peer->ttl & NMEA_MESSAGE_MASK)))
		return;

	/* 
	 * $GPZDG provides GPS time not UTC, and the two mix poorly.
	 * Once have processed a $GPZDG, do not process any further
	 * UTC sentences (all but $GPZDG currently).
	 */
	if (up->gps_time && !cmdtypezdg)
		return;

	/* make sure it came in clean */
	if (!nmea_checksum_ok(rd_lastcode)) {
		refclock_report(peer, CEVNT_BADREPLY);
		return;
	}

	pp->lencode = (u_short) rd_lencode;
	memcpy(pp->a_lastcode, rd_lastcode, pp->lencode + 1);
	cp = pp->a_lastcode;

	up->tstamp = rd_timestamp;
	pp->lastrec = up->tstamp;

	DPRINTF(1, ("nmea: timecode %d %s\n", pp->lencode, pp->a_lastcode));

	/* Grab field depending on clock string type */
	switch (cmdtype) {

	case GPRMC:
		/*
		 * Test for synchronization.  Check for quality byte.
		 */
		dp = field_parse(cp, 2);
		if (dp[0] != 'A')
			pp->leap = LEAP_NOTINSYNC;
		else
			pp->leap = LEAP_NOWARNING;

		/* Now point at the time field */
		dp = field_parse(cp, 1);
		break;

	case GPGGA:
		/*
		 * Test for synchronization.  Check for quality byte.
		 */
		dp = field_parse(cp, 6);
		if (dp[0] == '0')
			pp->leap = LEAP_NOTINSYNC;
		else
			pp->leap = LEAP_NOWARNING;

		/* Now point at the time field */
		dp = field_parse(cp, 1);
		break;

	case GPGLL:
		/*
		 * Test for synchronization.  Check for quality byte.
		 */
		dp = field_parse(cp, 6);
		if (dp[0] != 'A')
			pp->leap = LEAP_NOTINSYNC;
		else
			pp->leap = LEAP_NOWARNING;

		/* Now point at the time field */
		dp = field_parse(cp, 5);
		break;
	
	case GPZDG_ZDA:
		/*
		 * Test for synchronization.  For $GPZDG check for validity of GPS time.
		 */
		if (cmdtypezdg) {
			dp = field_parse(cp, 6);
			if (dp[0] == '0') 
				pp->leap = LEAP_NOTINSYNC;
			else 
				pp->leap = LEAP_NOWARNING;
		} else
			pp->leap = LEAP_NOWARNING;

		/* Now point at the time field */
		dp = field_parse(cp, 1);
		break;

	default:
		return;
	}

	/*
	 * Check time code format of NMEA
	 */
	if (!isdigit((int)dp[0]) ||
	    !isdigit((int)dp[1]) ||
	    !isdigit((int)dp[2]) ||
	    !isdigit((int)dp[3]) ||
	    !isdigit((int)dp[4]) ||
	    !isdigit((int)dp[5])) {

		DPRINTF(1, ("NMEA time code %c%c%c%c%c%c non-numeric",
			    dp[0], dp[1], dp[2], dp[3], dp[4], dp[5]));
		refclock_report(peer, CEVNT_BADTIME);
		return;
	}

	/*
	 * Convert time and check values.
	 */
	pp->hour = ((dp[0] - '0') * 10) + dp[1] - '0';
	pp->minute = ((dp[2] - '0') * 10) + dp[3] -  '0';
	pp->second = ((dp[4] - '0') * 10) + dp[5] - '0';
	/* 
	 * Default to 0 milliseconds, if decimal convert milliseconds in
	 * one, two or three digits
	 */
	pp->nsec = 0; 
	if (dp[6] == '.') {
		if (isdigit((int)dp[7])) {
			pp->nsec = (dp[7] - '0') * 100000000;
			if (isdigit((int)dp[8])) {
				pp->nsec += (dp[8] - '0') * 10000000;
				if (isdigit((int)dp[9])) {
					pp->nsec += (dp[9] - '0') * 1000000;
				}
			}
		}
	}

	/*
	 * Manipulating GPS timestamp in GPZDG as the seconds field
	 * is valid for next PPS tick. Just rolling back the second,
	 * minute and hour fields appopriately
	 */
	if (cmdtypezdg) {
		if (pp->second == 0) {
			pp->second = 59;
			if (pp->minute == 0) {
				pp->minute = 59;
				if (pp->hour == 0)
					pp->hour = 23;
			}
		} else
			pp->second -= 1;
	}

	if (pp->hour > 23 || pp->minute > 59 || 
	    pp->second > 59 || pp->nsec > 1000000000) {

		DPRINTF(1, ("NMEA hour/min/sec/nsec range %02d:%02d:%02d.%09ld\n",
			    pp->hour, pp->minute, pp->second, pp->nsec));
		refclock_report(peer, CEVNT_BADTIME);
		return;
	}

	/*
	 * Convert date and check values.
	 */
	if (GPRMC == cmdtype) {

		dp = field_parse(cp,9);
		day = dp[0] - '0';
		day = (day * 10) + dp[1] - '0';
		month = dp[2] - '0';
		month = (month * 10) + dp[3] - '0';
		pp->year = dp[4] - '0';
		pp->year = (pp->year * 10) + dp[5] - '0';

	} else if (GPZDG_ZDA == cmdtype) {

		dp = field_parse(cp, 2);
		day = 10 * (dp[0] - '0') + (dp[1] - '0');
		dp = field_parse(cp, 3);
		month = 10 * (dp[0] - '0') + (dp[1] - '0');
		dp = field_parse(cp, 4);
		pp->year = /* 1000 * (dp[0] - '0') + 100 * (dp[1] - '0') + */ 10 * (dp[2] - '0') + (dp[3] - '0');

	} else {
		/* only time */
		time_t tt = time(NULL);
		struct tm * t = gmtime(&tt);
		day = t->tm_mday;
		month = t->tm_mon + 1;
		pp->year= t->tm_year + 1900;
	}

	if (month < 1 || month > 12 || day < 1) {
		refclock_report(peer, CEVNT_BADDATE);
		return;
	}

	/* pp->year will be 2 or 4 digits if read from GPS, 4 from gmtime */
	if (pp->year < 100) {
		if (pp->year < 9)	/* year of our line of code is 2009 */
			pp->year += 2100;
		else
			pp->year += 2000;
	}

	/* pp->year now 4 digits as ymd2yd requires */
	day = ymd2yd(pp->year, month, day);
	if (-1 == day) {
		refclock_report(peer, CEVNT_BADDATE);
		return;
	}
	pp->day = day;

	/*
	 * If "fudge 127.127.20.__ flag4 1" is configured in ntp.conf,
	 * remove the location and checksum from the NMEA sentence
	 * recorded as the last timecode and visible to remote users
	 * with:
	 *
	 * ntpq -c clockvar <server>
	 *
	 * Note that this also removes the location from the clockstats
	 * log (if it is enabled).  Some NTP operators monitor their
	 * NMEA GPS using the change in location in clockstats over
	 * time as as a proxy for the quality of GPS reception and
	 * thereby time reported.
	 */
	if (CLK_FLAG4 & pp->sloppyclockflag) {
		/*
		 * Start by pointing cp and dp at the fields with 
		 * longitude and latitude in the last timecode.
		 */
		switch (cmdtype) {

		case GPGLL:
			cp = field_parse(pp->a_lastcode, 1);
			dp = field_parse(cp, 2);
			break;

		case GPGGA:
			cp = field_parse(pp->a_lastcode, 2);
			dp = field_parse(cp, 2);
			break;

		case GPRMC:
			cp = field_parse(pp->a_lastcode, 3);
			dp = field_parse(cp, 2);
			break;

		case GPZDG_ZDA:
		default:
			cp = dp = NULL;
		}

		/* Blank the entire latitude & longitude. */
		while (cp) {
			while (',' != *cp) {
				if ('.' != *cp)
					*cp = '_';
				cp++;
			}

			/* Longitude at cp then latitude at dp */
			if (cp < dp)
				cp = dp;
			else
				cp = NULL;
		}

		/* Blank the checksum, the last two characters */
		if (dp) {
			cp = pp->a_lastcode + pp->lencode - 2;
			if (0 == cp[2])
				cp[0] = cp[1] = '_';
		}

	}

	/*
	 * Note if we're only using GPS timescale from now on.
	 */
	if (cmdtypezdg && !up->gps_time) {
		up->gps_time = 1;
		NLOG(NLOG_CLOCKINFO)
			msyslog(LOG_INFO, "%s using only $GPZDG",
				refnumtoa(&peer->srcadr));
	}

	/*
	 * Process the new sample in the median filter and determine the
	 * timecode timestamp, but only if the PPS is not in control.
	 */
#ifdef HAVE_PPSAPI
	up->tcount++;
	if (peer->flags & FLAG_PPS)
		return;
#endif /* HAVE_PPSAPI */
	if (!refclock_process_f(pp, pp->fudgetime2))
		refclock_report(peer, CEVNT_BADTIME);
}


/*
 * nmea_poll - called by the transmit procedure
 *
 * We go to great pains to avoid changing state here, since there may be
 * more than one eavesdropper receiving the same timecode.
 */
static void
nmea_poll(
	int unit,
	struct peer *peer
	)
{
	register struct nmeaunit *up;
	struct refclockproc *pp;

	pp = peer->procptr;
	up = (struct nmeaunit *)pp->unitptr;

	/*
	 * Process median filter samples. If none received, declare a
	 * timeout and keep going.
	 */
#ifdef HAVE_PPSAPI
	if (up->pcount == 0) {
		peer->flags &= ~FLAG_PPS;
		peer->precision = PRECISION;
	}
	if (up->tcount == 0) {
		pp->coderecv = pp->codeproc;
		refclock_report(peer, CEVNT_TIMEOUT);
		return;
	}
	up->pcount = up->tcount = 0;
#else /* HAVE_PPSAPI */
	if (pp->coderecv == pp->codeproc) {
		refclock_report(peer, CEVNT_TIMEOUT);
		return;
	}
#endif /* HAVE_PPSAPI */

	pp->polls++;
	pp->lastref = pp->lastrec;
	refclock_receive(peer);
	record_clock_stats(&peer->srcadr, pp->a_lastcode);

	/*
	 * usually nmea_receive can get a timestamp every second, 
	 * but at least one Motorola unit needs prompting each
	 * time.
	 */

	gps_send(pp->io.fd,"$PMOTG,RMC,0000*1D\r\n", peer);
}


/*
 *
 *	gps_send(fd,cmd, peer)  Sends a command to the GPS receiver.
 *	 as	gps_send(fd,"rqts,u\r", peer);
 *
 *	We don't currently send any data, but would like to send
 *	RTCM SC104 messages for differential positioning. It should
 *	also give us better time. Without a PPS output, we're
 *	Just fooling ourselves because of the serial code paths
 *
 */
static void
gps_send(
	int fd,
	const char *cmd,
	struct peer *peer
	)
{
	if (write(fd, cmd, strlen(cmd)) == -1) {
		refclock_report(peer, CEVNT_FAULT);
	}
}


static char *
field_parse(
	char *cp,
	int fn
	)
{
	char *tp;
	int i = fn;

	for (tp = cp; i && *tp; tp++)
		if (*tp == ',')
			i--;

	return tp;
}


/*
 * nmea_checksum_ok verifies 8-bit XOR checksum is correct then returns 1
 *
 * format is $XXXXX,1,2,3,4*ML
 *
 * 8-bit XOR of characters between $ and * noninclusive is transmitted
 * in last two chars M and L holding most and least significant nibbles
 * in hex representation such as:
 *
 *   $GPGLL,5057.970,N,00146.110,E,142451,A*27
 *   $GPVTG,089.0,T,,,15.2,N,,*7F
 */
int
nmea_checksum_ok(
	const char *sentence
	)
{
	u_char my_cs;
	u_long input_cs;
	const char *p;

	my_cs = 0;
	p = sentence;

	if ('$' != *p++)
		return 0;

	for ( ; *p && '*' != *p; p++) {

		my_cs ^= *p;
	}

	if ('*' != *p++)
		return 0;

	if (0 == p[0] || 0 == p[1] || 0 != p[2])
		return 0;

	if (0 == hextoint(p, &input_cs))
		return 0;

	if (my_cs != input_cs)
		return 0;

	return 1;
}
#else
int refclock_nmea_bs;
#endif /* REFCLOCK && CLOCK_NMEA */