Re: A lurker surfaces

From: Warner Losh <imp_at_BSDIMP.COM>
Date: Tue, 02 Jan 2007 12:40:01 -0700 (MST)

> > Then let's improve the infrastructure for communicating the best
> > estimation of earth orientation parameters. Then in a world of
> > ubiquitous computing anyone who wants to estimate the current
> > rubber-second-time is free to evaluate the splines or polynomials
> > (or whatever is used) and come up with output devices to display that.
>
> This is fine, but leaves open the question of when 9:00am is here in
> Seattle. And why not transmit rubber-second-time as well, where
> technically feasible (such as over the internet)?

The technical problem is that many timing systems aren't connected to
the internet. They are at secure installations and they only have GPS
almanac data at their disposal. And, no, they aren't likely to ever
be on the internet. Any changes would have to be announced a long
time in advance, and GPS almanac would have to be updated to include
more information.

The second technical problem is that the length of a second is
implicitly encoded in the carrier for many of the longwave time
distribution stations. 10MHz is at SI seconds. For rubber seconds,
the broadcast would drift into adjacent bands reserved for other
things.

Also, GPS would have to remain in SI seconds. The error in GPS time
translates directly to an error in position. Approximately 1m/ns of
error (give of take a factor of 3). Rubber seconds would require that
the rubber timescale be off by as much as .5s. So GPS has to remain
in GPS time (UTC w/o leap seconds, basically). That means that the
rubberness of the seconds would need to be broadcast in the
datastream.

Many GPS receivers put out a PPS. This needs to be in SI seconds, or
a number of other applications break. However, if one defined UTC in
terms of these rubber seconds, then the top of UTC second would be out
of phase with this PPS. That breaks a lot of assumptiosn that rightly
assume that PPS is phase aligned to top of second.

The interval math in UTC that's hard today would be significantly
harder with rubber seconds. But it is just software, eh?

In short, it is an interestingly naive idea that was tried in the
1960's and failed when there were only dozens of high precision time
users rather than the hundreds of thousands there are today.

The earth does not define the second any more. At most, it defines
the day and the year.

> What is a good source of earth orientation parameters, btw?

usno publishes several.

Warner
Received on Tue Jan 02 2007 - 11:43:14 PST