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

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Feb 4 08:07:18 UTC 2009

Dave Hart wrote:
> 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?

The problem is that this only works on GPSs which can be set into 
"Zero-D" mode, i.e. dedicated timing receivers like the Oncore UT+.

Most NMEA-generating GPSs cannot do this, unfortunately.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

More information about the questions mailing list