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

Magnus Danielson magnus at rubidium.dyndns.org
Thu Aug 15 02:49:32 UTC 2013


Hi,

On 08/14/2013 03:54 PM, unruh wrote:
> On 2013-08-14, Mark C. Stephens <marks at non-stop.com.au> wrote:
>> Um Let's see, Datum was bought by Austron, who was bought by ... etc.
>> For collectors such as myself, having this 'mature' equipment still working is great.
>>
>> Looking at Mr Malone's code, he added 2 lines which enabled NTPD compatibility with GPS receivers that would have long ago have been sent to the TIP as waste.
> It is however fragile code. Ie, all kinds of situations could arise in
> which it would give the wrong time. Now, you may say that there are
> situations in which it will give the right time when, without the kludge,
> it would give the wrong time.
This addresses a known feature of the GPS system, common over a large
range of receivers. The differences between them lies in which GPS week
they flip over (GPS week 500, 512 and 729 from the top of my head). The
failure they have is not in their operation, but in their production of
a "human readable date".

This is what I have proposed elsewhere (on time-nuts) and it is a sound
solution considering the situation we have where the ICD-GPS-200 through
it's many revisions have not provided additional bits for the L1 C/A
code signal. For the L2C (and I assume also L1C, but I haven't checked
yet) signal additional bits exists, but very few recievers have that
support.

I recommend reading the time-nuts backlog on this issue.

Among the alternatives you have, it's ditching an otherwise perfectly
operating GPS receiver or use the fact that the 1024 week wrap-around is
bound to happen, is predictable as a systematic effect from how the GPS
C/A data is structured and re-occurs over the fleet of GPS receivers.

Do note that the GPS receivers does compute leap-second info correctly
regardless of this 1024 offset hickup, as that information is structured
modulu 1024 weeks.

Cheers,
Magnus


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