[time-nuts] Four hour cycle in GPS NMEA jitter
geoff36 at gmail.com
Mon Mar 20 19:43:58 EDT 2017
> 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.
David Taylor explains more about this Garmin firmware variation here:
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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