[ntp:questions] Start of new GPS 1024 week epoch

David Malone dwmalone at walton.maths.tcd.ie
Tue Aug 13 19:29:43 UTC 2013

Magnus Danielson <magnus at rubidium.dyndns.org> writes:

>Remember that any Sunday, it is likely that a GPS reciever have slipped
>a multiple of 1024 weeks. NTP drivers should be able to recognice it and
>compensate for it, as it is a re-occuring bug in many recievers.

>This issue have been discussed over and over again at time-nuts.

It seems my ancient GPSclock 200 has recently slipped back to
December 1993 too. Resetting it hasn't helped and I doubt I will
be able to do a firmware update, so I've made a hack to refclock_nmea.c
(version ntp-4.2.6p5), by replacing:

        reftime.l_ui += caltontp(&date);


        reftime.l_ui += caltontp(&date);
	while (reftime.l_i + 512*7*86400 < rd_timestamp.l_i)
		reftime.l_i += 1024*7*86400;

I'm trying to adjust the timestamp given by NMEA might be slow by
some multiple of 1024 weeks, and so tries to adjust it so that it
is reasonably close to the system time associated with the read of
the NMEA data.  I'm not sure if I've got the code exactly bang-on,
but it has got ntp running with the unit again.


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