Get current second from the last GPS timestamp.

ntpshm_pps was guessing the current second for the PPS from the system
clock, yet we have zero trust in the system clock.  So instead grab
the current second from the last GPS serial timestamp.

A radical change for the better, but someone will surely find improvements
with experience on the code.

1 files changed, 24 insertions, 13 deletions

diff --git a/ntpshm.c b/ntpshm.c
index cb61c185..9102bde9 100644
--- a/ntpshm.c
+++ b/ntpshm.c
@@ -184,20 +184,30 @@ int ntpshm_pps(struct gps_device_t *session, struct timeval *tv)
 	(shmTimeP = session->context->shmTime[session->shmTimeP]) == NULL)
 	return 0;
 
-    /* check if received time messages are within locking range */
-
-    if (tv->tv_usec < PPS_MAX_OFFSET) {
-	seconds = (time_t)tv->tv_sec;
-	offset = (double)tv->tv_usec / 1000000.0;
-    } else if (tv->tv_usec > (1000000 - PPS_MAX_OFFSET)) {
-	seconds = (time_t)(tv->tv_sec + 1);
-	offset = 1 - ((double)tv->tv_usec / 1000000.0);
-    } else {
-	shmTimeP->precision = -1;	/* lost lock */
-	gpsd_report(LOG_INF, "PPS ntpshm_pps: lost PPS lock\n");
+    /* PPS has no seconds attached to it.
+     * check to see if we have a fresh timestamp from the
+     * GPS serial input then use that */
+
+    /* FIXME, does not handle 5Hz yet */
+
+#ifdef S_SPLINT_S      /* avoids an internal error in splint 3.1.1 */
+    l_offset = 0;
+#else
+    l_offset = tv->tv_sec - shmTime->receiveTimeStampSec;
+#endif
+    /*@ -ignorequals @*/
+    l_offset *= 1000000;
+    l_offset += tv->tv_usec - shmTime->receiveTimeStampUSec;
+    if ( 0 > l_offset || 1000000 < l_offset ) {
+	gpsd_report(LOG_RAW, "PPS ntpshm_pps: no current GPS seconds: %ld\n"
+	    , (long)l_offset);
 	return -1;
     }
 
+    seconds = shmTime->clockTimeStampSec + 1;
+    offset = fabs(((tv->tv_sec - seconds) + (tv->tv_usec - 0) / 1000000.0));
+
+
     /* we use the shmTime mode 1 protocol
      *
      * ntpd does this:
@@ -218,13 +228,14 @@ int ntpshm_pps(struct gps_device_t *session, struct timeval *tv)
     shmTimeP->clockTimeStampUSec = microseconds;
     shmTimeP->receiveTimeStampSec = (time_t)tv->tv_sec;
     shmTimeP->receiveTimeStampUSec = (int)tv->tv_usec;
-    /* this is more a jitter/dispersion than precision, but still useful */
+    /* this is more an offset jitter/dispersion than precision, 
+     * but still useful */
     shmTimeP->precision = offset != 0 ? (int)(ceil(log(offset) / M_LN2)) : -20;
     shmTimeP->count++;
     shmTimeP->valid = 1;
 
     gpsd_report(LOG_RAW
-        , "PPS ntpshm_pps: clock: %lu.%03lu @ %lu.%06lu, precision %d\n"
+        , "PPS ntpshm_pps %lu.%03lu @ %lu.%06lu, precision %d\n"
 	, (unsigned long)seconds, (unsigned long)microseconds
 	, (unsigned long)tv->tv_sec, (unsigned long)tv->tv_usec
 	, shmTimeP->precision);