[fitswcs] FITS WCS Time Paper

[patrick.wallace at stfc.ac.uk]

Steve Allen wrote:

> Thus the text specifically directs the reader to read the GPS
> Interface Specification.

Yes, but hardly anyone will:  lay readers won't, and it is old
hat to time experts.  Even I became inattentive by the time I
reached p94 or so.

The point I was trying to make is that the paper can afford to be
more helpful to readers.  It has already mentioned that GPS is
more or less TAI-19s, and it can be said again here.  And the
statement "GPS time is only required to match UTC to within a
microsecond" would not be harmed by adding a preamble "The
fractional part of" or something of the kind.  Moreover, the
statement "That is to emphasize that GPS time is *not* defined in
terms of TAI" seems redundant, given that UTC is itself defined in
terms of TAI.  I would write this:

  GPS time is defined by the document "ICD-GPS-200, Revision C with
  IRNs 1 through 5". Note that GPS time is aligned to a specific
  UTC(USNO) epoch, 19s behind TAI, with the fractional part
  matching UTC(USNO) to within a microsecond.

Incidentally the introduction to the appendix contains the typo:

  "there are a more subtle issues"


Patrick Wallace
____________________________________________________________________
Space Science & Technology Department                +44-1235-531198
STFC Rutherford Appleton Laboratory
Harwell Oxford
Didcot, Oxfordshire, OX11 0QX, UK         Patrick.Wallace at stfc.ac.uk
____________________________________________________________________