[fitswcs] FITS WCS Time Paper

[patrick.wallace at stfc.ac.uk]

A few more comments on the paper.

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Section 2, penultimate paragraph:

   In addition to “proper” time axes, we provide accommodation for
   three types of time-related coordinates: Phase, Timelag, and
   Frequency; see Section 5.5.
 
I don't think proper time is meant here.  I would write

   In addition to absolute time axes, i.e. proper or coordinate
   time, we provide accommodation for three types of time-related
   coordinates: Phase, Timelag, and Frequency; see Section 5.5.

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Table 2:

You might want to update the "as of 2009-01-01" information to
"as of 2012-07-01".

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Table 3:

I'd simply omit Pluto now that its membership of the golf club has
been revoked.  Later on, when the possibility of including to minor
planets is mentioned, you could also say dwarf planets.

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Section 4.2.2, second sentence:

There should be a comma after "TREFPOS".

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Section 4.2.2, on p6 just after Table 4:

   "with respect to Earth"

This should be "the reference position" (which may not be the
Earth).

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Section 4.2.2:

I am uncomfortable about the whole "reference position" concept.  A
system of spacetime coordinates has a spatial origin, and a proper
time is associated with a location in space.  But to think of, say,
TDB as being "the time read by a clock at the SSB" is completely
wrong.  Indeed, the whole purpose of a coordinate time system, which
*all* of those listed except LOCAL are, is to provide a consistent
system of time reckoning for widely separated spatial locations.
You can label any event with any of them, and the choice depends
simply on what makes the observed phenomenon easiest to describe.

I suggest taking the statement "The reference position...specifies
the spatial location at which the time is valid" and trying to
decide what is actually meant.  The only two meanings I can come up
with are (i) the spatial origin for a system of spacetime
coordinates and (ii) the topocentre, and these are different things.

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