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). _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
