[time-nuts] Z3801A gps week rollover

Bob Camp kb8tq at n1k.org
Fri Sep 9 07:17:36 EDT 2016


Hi


> On Sep 9, 2016, at 3:06 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> Hi,
> 
> On 09/08/2016 11:53 PM, Hal Murray wrote:
>> 
>> petervince1952 at gmail.com said:
>>> Can I just ask why the Z3801As are  having week roll-over problems now - I
>>> didn't think it was 2048 weeks since GPS "zero-hour" until late on the 6th
>>> of April 2019?
>> 
>> It's probably 1024 weeks since a date was built into the firmware.
>> 
>> It's like the year 2000 problem.  If you aren't worried about old people and
>> I tell you somebody was born in 03, you can assume that's 2003 rather than
>> 1903.  For GPS, a handy value for the cutoff is the date the firmware was
>> built.  Any date that looks like it is older than the firmware is probably
>> off by a rollover.
> 
> We had this discussion in NTP context, and people where saying "We won't fix receiver problems" and where not helpful. When I explained how GPS time actually worked and just showing how the receivers was attempting to correct the GPS signal behaviors they realized that 1024 week wrap-around is more of a GPS generic problem and accepting time modulus 1024 weeks was not too hard. I don't know if that ever made it into the code thought.

The bigger problem for NTP is when the leap second correction process is thrown off by the “time warp”. When leap seconds get fixed in 
mid-August rather than the end of June … not a good thing.

Bob

> 
> Cheers,
> Magnus
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