[time-nuts] Epoch rollover?

Dennis Ferguson dennis.c.ferguson at gmail.com
Wed Oct 19 19:00:33 EDT 2011

On 19 Oct, 2011, at 14:18 , Bruce Lane wrote:
> 	Did we just have another GPS epoch rollover? My trusty old Odetics 425
> seems to believe the date is March 4th, 1992.
> 	I could probably correct it in firmware, if I looked hard and long
> enough, but the ToD is still correct and the frequency standard is
> staying nicely locked. Not sure if recovering the correct Julian date is
> worth the effort.

That's neat.  I understand that the typical heuristics used to determine
the GPS epoch included one or more of:

a) Assume the time must be more recent then the date the firmware was compiled;

b) Assume the time must be more recent than the last time it dumped the
   ephemerides out to flash to speed reacquisition across a reboot (assuming
   it does that); or

c) Take a guess based on the leap second count (i.e. the UTC offset) and some
   expectation of the number of leap seconds per epoch.

If your unit does a) it may be that the units firmware just passed 1024 weeks
of age.  If it does b) it may be your flash memory (or whatever non-volatile
storage it keeps that stuff in) has died.  It it does c) it may be that the
relative dearth of leap seconds in the past dozen years has fooled that
algorithm.  Or it could be something else entirely.

It would be interesting to know which of those it is.

Dennis Ferguson

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