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

Patrick Klos pklos at osmium.mv.net
Thu Dec 1 14:50:45 UTC 2005


In article <dmiven$9sg$1 at nntp.webmaster.com>,
David Schwartz <davids at webmaster.com> wrote:
>    An interesting question is how good a guaranteed relationship for the 
>NMEA data is good enough. I would argue "90% of the time, the NMEA data will 
>follow within 600mS of the corresponding PPS pulse" is good enough. However, 
>you really do want something to be guaranteed by the manufacturer. 
>Otherwise, it's "well, I hope it works, it seems to" with the NMEA data.

We've actually gone to great lengths in our code to "qualify" the NMEA
data when it comes in.  We maintain a history of data until we feel
confident that the data is matching the PPS's and that all are in sync.
This way we can tell when the Garmin has a glitch and we can reject data 
that is bad or comes too late.

Plus, as others have said, it's necessary to limit which NMEA strings are
sent from the GPS so you don't have data from one second constantly crossing
the next second's boundary.

================= GPS based time synchronization solutions =================
    Patrick Klos                           Email: patrick at timegeeks.com
    Klos Technologies, Inc.                Web:   http://www.timegeeks.com/
==================== http://www.loving-long-island.com/ ====================




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