Jim,
Are you sure that the units attribute cannot have "UTC" in it?
CF specifies that a time zone may be represented in a time string like the "-6:00" in:
"seconds since 1992-10-8 15:15:42.5 -6:00"
and CF also says:
Note: if the time zone is omitted the default is UTC, and if both time and time zone are omitted the default is 00:00:00 UTC.
and it seems that the trailing "UTC" string is accepted by udunits and is present in the canonical form:
$ udunits2
You have: microseconds since 2000-01-01 00:00:00 UTC
You want:
(1e-06 s) _at_ 20000101T000000.000000000 UTC
Thanks,
Julien
>Jim Biard jbiard at cicsnc.org
>Fri Apr 24 11:07:15 MDT 2015
>
>Julien,
>
>The units attribute cannot have "UTC" in it. UTC is implied. Other than
>that, it looks great!
>
>Jim
De : Julien Demaria
Envoy? : vendredi 24 avril 2015 18:14
? : 'Manning, Evan M (398B)'; Chris Barker
Cc : Jonathan Gregory; cf-metadata at cgd.ucar.edu
Objet : RE: [CF-metadata] How to define time coordinate in GPS?
Hi,
Thanks a lot for all your accurate answers!
With all these information, for the moment we think the best way to represent our time in CF is to convert the time stamp of the units attribute from GPS to UTC.
We cannot change it for the GPS beginning 1980-01-06 so we must transform the 2000-01-01 from GPS to UTC to be more CF compliant.
We also specify GPS in the long name and in a new non standard time_reference attribute :
int64 time_stamp(rows) ;
time_stamp:standard_name = "time" ;
time_stamp:units = "microseconds since 2000-01-01 00:00:13 UTC" ;
time_stamp:long_name = "Elapsed time since 01 Jan 2000 0h GPS" ;
time_stamp:time_reference = "GPS"
I think with this solution a CF compliant software (which not support leap second) should interpret time "correctly".
Let me know if I missed something.
Thanks,
Julien
De : Manning, Evan M (398B) [mailto:Evan.M.Manning at jpl.nasa.gov]
Envoy? : vendredi 24 avril 2015 14:30
? : Chris Barker; Julien Demaria
Cc : Jonathan Gregory; cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Objet : RE: [CF-metadata] How to define time coordinate in GPS?
It's probably overkill but for Suomi NPP CrIS and ATMS we are planning to provide
timestamps for each obs in both TAI and UTC.
TAI is the true time dimension, but UTC gives users a correct UTC if that's what they want and should minimize the temptation for them to do TAI to UTC conversions using tools which don't understand leap seconds.
-- Evan
________________________________
From: Chris Barker [chris.barker at noaa.gov]
Sent: Thursday, April 23, 2015 4:48 PM
To: Julien Demaria
Cc: Jonathan Gregory; cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] How to define time coordinate in GPS?
On Thu, Apr 23, 2015 at 7:19 AM, Julien Demaria <Julien.Demaria at acri-st.fr<mailto:Julien.Demaria at acri-st.fr>> wrote:
I'm also not an expert on this:
"GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not perturbed by leap seconds GPS is now ahead of UTC by 16 seconds."
It seems to me then, that the "right" way is to express this time as:
time_unit since 1908-01-06T00:00:00Z
since that is technically exactly correct.
clients are likely to translate to year-month-day-hour-minute-second using UTC, but maybe not. And as others have pointed out, most libs don't do leap seconds anyway, so are using "GPS time" whether they specify it or not.
I guess what I am saying is that this isn't really a "how to encode it in CF" question -- if you use that epoch, then it becomes entirely up to the client what time it wants to translate to.
-Chris
http://www.leapsecond.com/java/gpsclock.htm
a more detailed explanation:
https://confluence.qps.nl/display/KBE/UTC+to+GPS+Time+Correction
Thanks in advance,
Julien
>Jonathan Gregory j.m.gregory at reading.ac.uk<http://reading.ac.uk>
>Thu Apr 23 07:58:09 MDT 2015
>
>Dear Julien
>
>Could you explain what the difference is between GPS time and UTC (for a non-
>expert such as me)?
>
>Thanks
>
>Jonathan
De : Julien Demaria
Envoy? : jeudi 23 avril 2015 14:51
? : 'cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>'
Objet : How to define time coordinate in GPS?
Hi,
I need to define a time coordinate variable which use the GPS time referential instead of UTC, but I did not found how to specify this.
For the moment my variable look like this :
int64 time_stamp(rows) ;
time_stamp:standard_name = "time" ;
time_stamp:units = "microseconds since 2000-01-01 00:00:00" ;
time_stamp:_FillValue = -1L ;
Thanks in advance,
Julien
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--
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Oceanographer
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