[time-nuts] Why not TAI? (was: The future of UTC)

[Tom Van Baak]

> My Dual Scale Timekeeper will recover TAI from GPS by adding a constant
> 19 s offset, and it will track and serve out TAI in addition to UTR.

What is usually meant by "TAI" is the single extremely accurate,
post-processed, paper time-scale managed by BIPM. TAI itself
is derived from EAL and other inputs. TAI is the basis of UTC.
No one has copyright on these acronyms and confusion can
result when TAI is used to mean too many things.

You can "recover TAI" from a GPS timing receiver by adding 19
seconds in the same crude way I can recover TAI by looking at
the big clock outside the bank building and adding 7 hours and
34 seconds. Or by taking a PC clock and making my web page:
http://leapsecond.com/java/gpsclock.htm. 

Yes, these look like TAI. But are they really TAI? Or are they all
just another 6-digit hour:minutes:second clock display that tries
to be "close to" what TAI would look like if one had access to it?

A question to ask is how many nanoseconds, or milliseconds,
or even seconds does your TAI clock have to be off before you
can't rightfully call it TAI anymore? I don't have an answer.

Naming ambiguity is even worse with UTC. You can't have a clock
at home that is UTC. What you can have at home is a WWVB
clock that closely follows UTC(NIST) or a GPS display clock that
closely follows UTC(USNO). But how close is left undefined.

If you put the GPS receiver in holdover mode, when does the
display stop being UTC?

Most Windows PC's at home are off by seconds. Does that mean
most of them are running UT1 instead of UTC?

What is missed in many discussions about time scales is intent
or implied accuracy. If I manually adjust my Pacific Daylight
Time wrist-watch ahead by 7 hours does it then become a UTC
watch? If I further adjust it by 0.3 seconds can I now claim it's
showing UT1? Can I even wear a wrist-watch that displays TAI?
Is it possible for any clock with analog hands to display UTC?

What we call a time scale is more than just an integer offset. I'm
working on a paper that explores all these issues.

/tvb