[ntp:questions] First attempt GPSD/PPS ->NTP time server

Nero Imhard nim at pipe.nl
Fri Jan 25 09:16:43 UTC 2008


David J Taylor schreef:

> The fact that you have a 0.2s offset suggests you are synching to the
> trailing edge of the PPS signal and not the leading edge.

The NMEA output would be my prime suspect. It is not surprising to have
an offset there. The time in an NMEA sentence doesn't tell you what time
it is "now", but instead it specifies the point in time that the rest of
the data (e.g. position) refers to. Also, NMEA sentences lack proper
time markers. It's always tricky business to get clock synchronization
from NMEA alone. I would say it is impossible.

The Garmin GPS 18 LVC manual explicitly mentions that emitted NMEA
sentence describe the when and where of the *previous* rising edge of
the PPS signal. It does not specify how long after that edge it will
give you the NMEA, or even that this time will be constant.

N




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