[ntp:questions] help - clock jumps up to 5000 ms startup ntpd with usb gps linux

unruh unruh at invalid.ca
Sun Feb 19 16:30:38 UTC 2012

On 2012-02-19, Rob <nomail at example.com> wrote:
> Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
>> *GPS_NMEA(5)     .GPS1.           0 l    4    8   37    0.000  563.733   
> Don't try to use NMEA for timesyncing!  It will only drive you crazy.


> The timestamp in NMEA is a "time of last fix", not the current time.

And you can tell ntp to backdate the timestamp so that it is closer to
the correct time. That it is the timestamp of the end of the message,
rathr than the beginning ( and since the messages at different times
have a different number of bytes in them, the time it takes for the
message to all come through varies) which causes some jitter. 

It is certainly not going to cause 5000ms of offset. 

> Some people are lucky and seem to be able to get a usable result, but
> they really are working outside the specs and can get bitten at any
> firmware change (as was recently seen with some Garmin firmware).

If you mean that you may have to recalibrate the time offset under a
firmware change. Yes. 

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