[fitswcs] Re: [iaufwg] Thoughts about time coordinates

[Steve Allen]

On Mon 2005-07-11T14:28:05 -0400, William Thompson hath writ:
>               TDB     Barycentric Dynamical Time (TDT + periodic (~ 10
>               msec))

TDB doesn't really exist.  In practice it is always a version of T_{eph}.
http://www.ucolick.org/~sla/leapsecs/timescales.html#Teph
That makes it necessary to qualify what ephemeris was implied.

There is also the upcoming matter of refining the meaning of TT when
time transfer becomes precise enough that the diurnal GR variation in
clock rate becomes visible.  At that point it will no longer be
possible to pretend that a single TT or TAI can describe all points on
earth.

Aside from System, Format, and Usage, there is the matter of reference
frame.  The spectral WCS Paper III begins to touch on this,  but it
explicitly disavows spacecraft.

>       Many types of data analysis depend more on relative time than on
>       absolute time

Many types of observation systems have never had requirements for
accurate timing of shutter operations.  On a good day our CCD systems
at Lick and Keck are good to a second, on a bad day the system loading
can make them as bad as 30 seconds.  Nobody demanded anything better
at the time of the design.

>               TIME can be in any of the time units in Paper I.

Paper I does not distinguish SI second from mean solar seconds, nor
does it consider any of the implications of the changes in length of
TAI seconds, nor does it begin to consider reference frame issues.

>               In other words, with the UTC projection, every day is treated
>               as being exactly 86400 seconds long.

Which means they are effectively mean solar seconds from the inception
of UTC until now.  But if the Time Lords succeed in redefining UTC
then they will become TAI seconds at some point in the future.  In a
historical sense UTC would become even more schizophrenic than it is
now.

There is a somewhat serious proposals for putting a GPS-like
constellation of telecom-relay/clock/positioning satellites around
Mars.

In any case, it does not pay to produce the Temporal Coordinates in
FITS paper until after the current ITU-R decision process calms down
with some sort of resolution.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99858
University of California    Voice: +1 831 459 3046           Lng -122.06014
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m