[ntp:questions] Garmin GPS18x LVC 5m - Centos 5.5 - Kernel 2.6.18 - gpsd

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Dec 28 19:03:17 UTC 2010

> ntpd timestamps the CR/LF at the end of each NMEA sentence.  According
> to this thread:
> http://www.febo.com/pipermail/time-nuts/2010-December/052859.html
> the latest (3.50) firmware for the Garmin GPS 18x LVC shifts the NMEA
> generation even later in the second.  You might want to try reverting
> to an earlier firmware.
> Cheers,
> Dave Hart

Perhaps a pity that the start of the NMEA sentence isn't time-stamped 
rather than the end, but I expect there's a good reason.

It seems crazy that Garmin would want to increase the delay the generation 
of the NMEA compared to the reference second.  I can only assume that 
there must be more processing going on which requires that delay.  I will 
maybe dust off the 'scope and see what gives on my own unit...

Well, I am very surprised.  OK, it's a scope I haven't used for a while, 
but it reliably triggers so it seems on the PPS signal (200ms wide), but 
the NMEA data seems to be almost arbitrarily timed compared to the PPS, 
sometimes even being in the middle of the PPS signal.  Perhaps someone 
could confirm this!  The unit has firmware 3.50.  With an un-terminated 
NMEA port, the level doesn't look that healthy either.  Better make up a 
better break-out box to check....


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