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

unruh unruh at invalid.ca
Thu Aug 15 21:02:18 UTC 2013

On 2013-08-15, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> On 08/15/2013 10:22 AM, David Taylor wrote:
>> On 15/08/2013 08:34, Rob wrote:
>>> David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>>>> On 14/08/2013 17:44, Rob wrote:
>>>> []
>>>>> How does a "good" receiver know the correct time?  Does it rely on
>>>>> local (backed-up) storage, or is there some way of receiving it via
>>>>> the almanac?  Or are "good" receivers hardwired as well, only with
>>>>> a different valid span?
>>>>> I would not be surprised when "good" receivers turn out to have just
>>>>> a different moment or mode of failure.
>>>> []
>>>> Some receivers have battery backup, in fact all but one of the receiver
>>>> types I use have this.
>>> Ok but what happens when the battery is replaced?
>> []
>> Hope and pray?  Wish for a large capacitor or flash-rom?
>> I had thought that either ephemeris or almanac data might contain the
>> real UTC time, but apparently it does not.  Obviously a system
>> designed too far in advance of the Year2000 fuss and bother!
> They completely avoid it by not numbering it that way. They have their
> own numbering scheme that fit's the system, and the conversion over to
> UTC is an added feature. It's all in ICD-GPS-200 for the current set of
> details, and in the ION red book series for the early stages.
> GPS and GPS problems is best understood if you realize that everything
> is counted in the GPS clock machinery with it's own set of gears.
> Conversion isn't that hard and it is done every second in the GPS receiver.

That is fine, but I think that the question is "what are those internal
geers" and "do those internal geers have a rollover time"? Ie, for how
long a time period is there a unique mapping from the internals of GPS
and the time (UTC or whatever). Obviously the oscillations of the H
atoms in the H laser clocks have a rollover of picoseconds. Somewhere in
those sattelites is some counter with a lot longer period before it
rolls over. 

> Cheers,
> Magnus

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