[ntp:questions] Re: ntp use, was Re: Garmin GPS 18 firmware update -> 200ms offset

Terje Mathisen terje.mathisen at hda.hydro.com
Wed Nov 30 09:59:23 UTC 2005

rtxo wrote:

> Patrick Klos wrote:
>> I'm sure you can't rely on the NMEA strings coming out with any specific
>> relationship to the pulse.  We've seen Garmin GPS's get very busy inside
>> and skew the NMEA strings all over the place, including after the NEXT
>> PPS.
> I'd suggest turning off all output of the GPS, except for the
> $GPGGA or $GPRMC strings, and turning on the PPS. This should
> provide good operation with the Dave Mills NTP software.
> I've run a Garmin GPS35 this way and have had consistent
> 1 microsecond PPS performance with NTP.

The NMEA driver defaults to using only the RMC message, so unless you
use a mode mask to also look for the others, nothing but RMC should be
needed or transmitted.

Re. the GPS 18:

In NMEA mode the default selection of messages is so large that it takes
more than a second to send them all, meaning that you lose every other
second update!

Limiting the number of messages is therefore a required step to get
optimal timing.


- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

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