Re: [LEAPSECS] Types of time

From: Steve Allen <>
Date: Fri, 8 Sep 2000 11:14:42 -0700

On Fri 2000-09-08T13:55:20 -0400, John Cowan hath writ:
> "Too simple" is too simple. For lots and lots of systems, cost considerations
> preclude the *kinds* of connectivity that would support a time protocol, but they
> need to know what time it is still.
> Embedded systems are the obvious candidates, especially those in hostile
> environments like the ocean floor, the far reaches of the solar system,
> and modern kitchens.

Why wouldn't such a system effectively use TAI?
The systems to which it communicates are certainly complex enough to
do the time conversion.

And I say effectively, for TAI really only has meaning as the average
of all the atomic clocks on the surface of the earth that participate
in defining it. It is a very rare embedded system which has an
internal clock good to better than 1 part in 1e5.

Clocks in the far reaches of the solar system have the additional
special relativistic problems of their relative velocity to the earth,
and the general relativistic problems of being at a different depth in
the gravity well of the sun. They don't keep TAI or UTC, unless the
ground station has a protocol to update the remote clock (e.g., GPS
satellites). Usually the ground station simply keeps track of the
expected clock offsets and drifts based on a model of what the
spacecraft is doing.

Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064      Voice: +1 831 459 3046
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
Received on Fri Sep 08 2000 - 11:14:45 PDT

This archive was generated by hypermail 2.3.0 : Sat Sep 04 2010 - 09:44:54 PDT