On 17/08/2013 09:30, Terje Mathisen wrote:
David Taylor wrote:
[]
Thanks for the pointers to the documents. A pity that they haven't been
able to find two or three spare bits to reduce the 1024 week ambiguity
to nearer a half-century or even 100 years. Oh, well!
That would be even worse:
Exceptional events like the 10-bit week rollover needs to happen often
enough that every programmer is forced to write code to handle them
correctly, or it should not happen at all!
I.e. a fault-tolerant server setup is only fault-tolerant if you are
comfortable doing monthly fire drills where you pull the power cord (or
network cable(s)) from either half.
For GPS 19+ years was probably intended to be long enough that every
given receiver would only live to see a single epoch, meaning that a
simple test against the firmware generation time would suffice, right?
Well, now we've seen a lot of timing receivers that just keep on
working, and that 19+ year range turned out to be not quite long enough.
If this has happened every year or so (i.e. a 64-week rollover), then
every GS would have had some method to enter the current epoch, and a
way to remember it across reboots.
Personally I think the (Trimble?) hack to use the TAI-UTC offset field
as an epoch guess table index is pretty nice:
As long as the offset keeps increasing this will suffice to handle at
least one or two epoch rollovers.
OTOH, the firmware timestamp method I outlined above will work perfectly
as long as (a) somebody is still willing to generate new firmware
versions and (b) you still have some machine with compatible
hardware/software to allow you to load it onto the GPS.
Combined remote antenna/GPS receivers with an RS422 or similar
connection to an NTP server requires that firmware update capability to
be included in the NTP box. :-(
Terje
Thanks for your thoughts, Terje.
Using a 12-bit (or even 16-bit) field to send the current year would be
a preferable solution - at least until they start messing with
leap-seconds and change the whole time scale. But I take your point -
once every 19 years it will be remembered a lot more easily than once
every 76 years.
Having a multiplicity of different GPS sources from different
manufacturers may at least improve the chance of the problem being spotted.
--
Cheers,
David
Web: http://www.satsignal.eu
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