Re: [LEAPSECS] RAS hits the news

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Mon, 26 Sep 2005 07:39:06 -0700

M. Warner Losh replies to Steve Allen:

>> In my understanding the GPS system itself handles leap seconds
>> pretty well, almost optimally.
>
> One could say that GPS handles them perfectly, in that they do not
> exist at all in the GPS time scale. However, GPS' propigation of
> the GPS UTC offset leaves much to be desired. That data is sent in
> the alminac, which takes at least 20 minutes to down when a
> reciever is started "cold"[*].

Um - the quote you are replying to was actually:

>> In my understanding the GPS system itself handles leap seconds
>> pretty well, almost optimally. It's some receivers which have
>> difficulties.

The question is whether "at least 20 minutes" (presuming this to be
accurate) is intrinsic to the system design or is rather a result of
poor implementation by some receiver manufacturers.

> Although you know the GPS time to within a few tens of nanos as
> soon as you have 4 satellites, you have to wait another 20 minutes
> after that to know UTC time if you are coming up cold.
>
> One can debate the meaning of 'almost optimally' til the cows come
> home, but my views lean away from such a characterization...

A debate of the meaning of "almost optimally" is at the heart of any
design effort. One might assert that this list could debate such,
but the fact is that has only extremely rarely been what has been
debated over the past six years. Is GPS close to, or far from,
optimal? I suspect it is closer to optimal than the complete
abandonment of the process that is being pushed by the ITU.

A typical design pattern for conveying crucial metadata that is only
rarely updated is to also convey a timestamp or expiration date.
Either the eggs are expired or they aren't. There certainly are ways
for a GPS receiver to store metadata - even over a period of a year.
Having cached the leap second table, the only question is whether it
is expired for which a 4 or 8 byte timestamp would be quite sufficient.

The conceptual fault here - as with so much of this discussion - is a
design that assumes that the receivers or other clocks will not be
restarted "frequently" (another word whose definition we could debate).

Rob Seaman
National Optical Astronomy Observatory
Received on Mon Sep 26 2005 - 07:39:49 PDT