[fitswcs] FITS WCS Time Paper

[Steve Allen]

On Wed 2012-03-21T16:59:10 +0000, patrick.wallace at stfc.ac.uk hath writ:
>   This is potentially misleading to ordinary readers.  It reads as
>   if the author is in denial that GPS time is *the* de facto
>   distribution mechanism for TAI, at least for applications that can
>   make do with 100ns accuracy.

All of that is true.

It is also true that the manufacturers of most GPS receivers are not
going to ply their sales literature full of disclaimers about the
features not implemented in the algorithms of their equipment.  Thus
the text specifically directs the reader to read the GPS Interface
Specification.

In particular it mentions section 20.3.3.3 which describes the
parameters necessary to correct from the SV time scale to the GPS
system time scale as well as the requirement for the receiver to
calculate the relativistic effects, and section 20.3.3.5 which
describes the parameters necessary to correct the GPS system time
scale to get UTC(USNO), including the need to monitor variations in
the ionospheric delay.

The point of the text is "caveat emptor", for a user who wants to use
GPS and get timing to better than a microsecond must exercise
considerable diligence.  It won't just happen with off-the-shelf
GPS hardware.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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