[ntp:questions] $GPGGA fix indicator 6 estimated/dead reckoning

Dave Hart davehart at gmail.com
Tue Feb 3 19:38:45 UTC 2009

$GPGGA fix indicator field 6 has at least the following possible

0  no fix
1  GPS fix
2  DGPS fix

One reference [1] mentioned

3  GPS PPS fix

and several reference

6  estimated or dead reckoning

ntpd/refclock_nmea.c when using $GPGGA tests for 0 and declares the
clock not in sync.  When I read "estimated" or "dead reckoning" wonder
if ntpd should consider the clock not of sync when it sees fix
indicator 6.  The gotcha that comes to mind is what if there are too
few satellites in view for a good position fix but still enough for
the time to be stable and accurate?

I'm just reading code and inferring from what little I've found
regarding fix indicator 6.  GPS wisdom/reality check solicited.

Dave Hart


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