[ntp:questions] Garmin GPS18x LVC problem since firmware 3.50

Rob nomail at example.com
Wed Dec 29 13:36:10 UTC 2010

David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> Whilst I understand what you are saying, it seems that this unit /used/ to 
> work as most people expected, with the NMEA output being within the 
> current second rather than outside it, and therefore that /something/ may 
> have changed within the firmware rendering the unit perhaps useless under 
> some circumstances (e.g. stand-alone).
> Garmin say in the technical specifications for the unit:
> "The NMEA 0183 sentences that follow each rising edge of the PPS signal 
> tell you when you were and where you were at that previous rising edge of 
> the PPS signal, beginning with the GPRMC sentence as the lead sentence in 
> any particular NMEA 0183 record"
> This implies to me that there is a sync-lock between the position 
> determination, the PPS signal, and the data in the NMEA output, and I 
> think it may imply that the GPMRC sentence /must/ be started (or even 
> finished?) /before/ the next PPS rising edge.
> I would hope that by bringing this problem to Garmin's attention, they may 
> be able to restore the changes in the firmware which have broken this 
> relationship, and has allowed the NMEA data to drift past the /next/ 
> leading edge.

Yes, it seems that in this case there actually is a spec for the timing
and you should be able to make them change the software so it (again)
adheres to that spec.  The remarks I made were about a receiver (I forget
what make and type) where there was no spec like you wrote above.
But there was a binary protocol and it contained a specific message for
sending the current time, rather than the time-of-fix.  Of course the
content of that message was no longer valid at the time it was received,
but there was a spec that said at which time the content was valid.
(like at the start bit of the first byte sent)

In gpsd, receivers are switched to their binary protocol when possible,
and problems like you have encountered are less likely to occur.  But
still there are offsets caused by the limited serial bitrate and issues
beyond direct control (like the amount of data sent by the receiver,
which may vary depending on the number of satellites in view).

I never felt completely easy with the derivation of time from the messages
received from a GPS.

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