Timesys note review

[Markus Demleitner]

Hi Steve,

On Wed, Nov 28, 2018 at 08:12:06PM -0800, Steve Allen wrote:
> On Tue 2018-11-27T13:23:50+0100 Markus Demleitner hath writ:
> > A pre-built PDF is available on
> > http://docs.g-vo.org/timesys-draft.pdf, the source is still available
> > from
> > https://volute.g-vo.org/svn/trunk/projects/time-domain/timesysnote.
> 
> Table 1
> 
> I strongly object to the notion that the term UTC shall be used for
> time stamps that were originally labelled as GMT.  The term used for
> that should be just plain UT.

Ok -- I agree UTC was a bad choice for labelling "historical data
taken in GMT".

I'd still like to keep the number of (initial) terms as low as
possible, and I'd expect if we let people choose between UT and UT1,
that'd not help overall, and I'd still like to escape a separate
"GMT" time scale if possible.  

So -- given we already have UT1 would you object to saying
"Historical data given in GMT should be annotated as being in UT1"?

> UTC has always referred to an atomically-regulated time scale.  There
> [...]
> The IAU directed that the catalog used for time determination changed
> from FK3 to FK4 on 1962-01-01, and that caused a shift of 1.5 ms in
> the values of UT that were being provided by various observatories.
> The extrapolated estimates of the value of TAI at that date and before
> have uncertainties and differences of similar size.

It's clear that automatic processing can only go so far, and we
should probably set some (time-dependent) goal as to what precision we
strive for.  I've hoped we could generate some rules of thumb
analogous to what Arnold has suggested for reference positions, where
we recommend (overridably) bumping the systematic errors depending on
time scale and the age of the data.  Your remarks let me doubt it'll
be so simple... ah well.

Thanks,

          Markus