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

Rob nomail at example.com
Thu Aug 15 08:47:15 UTC 2013


David Taylor <david-taylor at blueyonder.co.uk.invalid> 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!

It was designed in the seventies, when it was still commonplace to use
"6 bits for this, 10 bits for that" in data fields.  Apparently the first
design did not even have a weeknumber and the system had no notion of
date, only of time and day-of-week.

MS-DOS and the BIOS interface is full of those bitfields as well.
There they even crammed a time-of-day in 16 bits, giving it a 2-second
resolution.  Who would ever think of that?

I read somewhere that there has been an updated specification where some
more weeknumber bits are jammed in some other place in the almanac.



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