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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Wed Dec 29 11:28:38 UTC 2010

"Rob" <nomail at example.com> wrote in message 
news:slrnihlurn.9vm.nomail at xs8.xs4all.nl...
> Technically, the timestamps in the NMEA messages are the "time of fix",
> not the "current time".  So a receiver can rightfully keep a calculated
> fix in memory and send it at some later time, with the timestamp of the
> original fix included in the message.
> This makes NMEA unusable for time synchronization.
> I have seen the behaviour you describe from other receivers, from other
> manufacturers, as well.  For example, I have seen it happen that the
> offset between PPS and NMEA output varies every time the receiver is
> restarted, and slowly wanders while the receiver is running.  Apparently
> there is a software timer that sends the messages every second or other
> interval, but is not locked to the PPS signal.
> I think by expecting the NMEA to be locked to PPS, you are in fact
> relying on an implementation detail that often is not covered in the
> receiver spec, and that can be changed at will (accidentally or for
> some reason).
> It is better to switch to the binary protocol, which is manufacturer
> specific.  There usually are messages in the binary protocol to 
> communicate
> the current time, as opposed to the time of fix, and there are specs
> that define the relationship between the time and the moment the message
> is sent.


Thanks for your input.

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.


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