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

Magnus Danielson magnus at rubidium.dyndns.org
Fri Aug 16 07:22:17 UTC 2013

On 08/15/2013 11:02 PM, unruh wrote:
> 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. 

As I just answered to David Taylor, it's all described this document:

You might enjoy reading the earlier revisions as things have been
modified over time, and to understand olrder receivers you need to look
at the older spec, available here:

The 1024 weeks period I have been speaking of comes from interpreting
this document.


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