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

Patrick Klos pklos at osmium.mv.net
Tue Nov 29 20:03:06 UTC 2005

In article <dmiah2$27i$1 at nntp.webmaster.com>,
David Schwartz <davids at webmaster.com> wrote:
>"Patrick Klos" <pklos at osmium.mv.net> wrote in message 
>news:dmi2vh$pa3$1 at pyrite.mv.net...
>> 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.
>    Ack! I do not recommend using a GPS for a timing application unless the 
>relationship between the NMEA time strings and the actual UTC second 
>boundary is specified by the manufacturer.

You cannot get accurate timing without using the PPS signal.  There's no
way to get accurate timing with JUST the NMEA strings.

The relationship is supposed to be this:  The PPS is aligned with the actual
start of a second, and the NMEA messages that follow it (during that second)
specify the data for that PPS (i.e. the time that PPS represented).  With
GARMIN GPS's (and possibly others), the load on the internal processor will
effect when those NMEA messages actually start being transmitted.  We've
observed that the messages sometimes get delayed by more than a second
which causes confusion.

================= 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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