[time-nuts] Four hour cycle in GPS NMEA jitter

Kiwi Geoff geoff36 at gmail.com
Mon Mar 20 19:43:58 EDT 2017


Hi Trent,

> But first things first.  I'm just grabbing the time from NMEA sentences.
> And there's quite a bit of jitter there!  Clearly using the first sentence
> output by the GPS is critical.  I've tried to account for any time delays in
> the software.  I think it's the GPS module that is creating the largest
> source of jitter.  It appears to go in four hour cycles, peaking at 0:00Z,
> 4:00Z, 8:00Z, etc.

>From my observations on various GPS receivers, the 'time" that the
NMEA data is transmitted can be highly variable.

For example, here is a (24 hour) graph from my Garmin 18x (firmware
v3.6) where a plot (thanks to Hal) shows the start time of the NMEA
sentence from the time of the GPS 1PPS edge.

http://www.satsignal.eu/ntp/Garmin-18x-3.7.png

David Taylor explains more about this Garmin firmware variation here:

http://www.satsignal.eu/ntp/Garmin-GSP18x-LVC-firmware-issue.htm

Where an earlier version of the 18x, the latency could be so looooong
that it actually gave the timestamp for the wrong second !

So yes, beware of using the "transmit time" of NMEA sentences, many
GPS receivers appear to have a cuppa tea, a scone and solve a Sudoko
before telling us the time !

Regards, Kiwi Geoff  (Christchurch , New Zealand).



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