[CF-metadata] How to define time coordinate in GPS?

From: Jonathan Gregory <j.m.gregory>
Date: Thu, 21 May 2015 18:11:00 +0100

Dear Jim

I don't understand why you think gregorian_nls is not sufficiently specific.
I think it indicates specifically that the conversion between timestamp and
elapsed seconds is done without using leap seconds.

> As an example, the date and time stamps as I'm writing this in the
> different systems are:
>
> * TAI 2015-05-20 20:21:00
> * GPS 2015-05-20 20:21:16
> * UTC 2015-05-20 20:21:35

If you have a timestamp of 2015-05-20 20:21:35 and you encode it with
calendar="gregorian_utc" and units="seconds since 1980-01-06 00:00:00"
you will get the same number (of seconds) as if you encode the timestamp
of 2015-05-20 20:21:16 with calendar="gregorian_nls" and the same units.
Have I got that right? Thus, this would be a way to encode GPS time (the
original question). The same gregorian_nls calendar can be used to encode
TAI time with units="seconds since 1958-01-01 00:00:00". The same software
could do both - it needs to know the Gregorian calendar, and assumes that
all days have 86400 s.

> Redefine 'gregorian' to remove reference to UTC as Jonathan has
> described, and state that presence or absence of leap second
> artifacts in the reference timestamp and elapsed time values is not
> known for a time variable that names this calendar. This is backward
> compatible.

I don't see an advantage in encouraging data-producers to be vague. Deprecating
this calendar, so it gives a warning, might persuade them to specify what they
are doing to encode their times.

Best wishes

Jonathan
Received on Thu May 21 2015 - 11:11:00 BST