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

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 29 Jun 2015 17:21:43 +0100

Dear Tim and Nan

If I have understood correctly, I think your two emails suggest that we do need
a distinction of the precise and imprecise cases. As usual, I believe that CF
should not prescribe to users what they should do; its aim is to allow them to
describe what they have done. Different levels of precision are needed for
different datasets.

Following the emails that Jim and I exchanged, we could distinguish:

gregorian: Real world-times, but without specifying whether UTC or GPS
timestamps are intended, nor whether the encoding was done with or without leap
seconds. The decoded times could differ by several seconds from UTC. I think
this is Nan's use-case.

gregorian_nls: UTC timestamps were encoded without leap seconds, with a
reference UTC timestamp. I think this is Tim's use-case. This is not accurate
according to UTC but it can be decoded precisely as intended. Jim points out
that it's not a real-world calendar, but it's not far off.

Have I correctly described these as your cases?

In addition, we propose two other calendars:

gregorian_utc: The encoded and reference timestamps are UTC, and the encoding
is done with leap seconds allowed for. Hence the time coord is an accurate
elapsed time.

gregorian_gps: The encoded and reference timestamps are GPS, and the encoding
is done without leap seconds. Again, the time coord is accurately elapsed time.
I think this is the use-case which originally started this thread!

Best wishes

Jonathan
Received on Mon Jun 29 2015 - 10:21:43 BST