In message: <43D3774C.3000807_at_edavies.nildram.co.uk>
Ed Davies <ls_at_edavies.nildram.co.uk> writes:
: Earlier, I wrote:
: > We all know that it (and any other world-wide timescale) is
: > "postal" at the level of the time it takes light to cross a
: > moderately small room but for microsecond precision and looser
: > this is not an issue.
:
: I ought to qualify that. There are, of course, time scales which
: are synchronized across the world with much better than microsecond
: precision (e.g., GPS time) but it's my understanding that they are
: not anything like as uniform as TAI. Is this right? Can anybody
: quote accuracy information for GPS time? At what level of precision
: do time transfers using GPS time need out-of-band ("postal")
: correction for uniformity?
GPS is synchronized to UTC(USNO) which agrees with UTC(NIST) to within
a few nanoseconds. GPS receivers can recover UTC(USNO) to within
about a few nanoseconds (as measured by a direct two way time exchange
with NIST), although not all of them are that quality. The rms of a
good timing receiver is on the order of nanoseconds. NIST and USNO
coordinate with each other to make sure there's close agreement in the
two minor variations of UTC. They also participate in the computation
of TAI.
TAI is computed after the fact based on the observed time of different
frequency standards. Phase offsets between different time sources are
measured, the data is decimated and placed into a particular format
and sent to BIPM. These measurements are sent in batches and used to
measure interesting effects.
Here are some reports on how the various labs measure time relative to
a known source:
http://www.bipm.fr/cc/CCTF/Allowed/16/cctf04-18.pdf
http://www.bipm.org/en/publications/scientif/tai.html
Warner
Received on Sun Jan 22 2006 - 08:40:16 PST