Julien,
The units attribute cannot have "UTC" in it. UTC is implied. Other than
that, it looks great!
Jim
On 4/24/15 12:14 PM, Julien Demaria wrote:
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:[email protected]]
*Envoyé :* vendredi 24 avril 2015 14:30
*À :* Chris Barker; Julien Demaria
*Cc :* Jonathan Gregory; [email protected]
*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 [[email protected]]
*Sent:* Thursday, April 23, 2015 4:48 PM
*To:* Julien Demaria
*Cc:* Jonathan Gregory; [email protected]
<mailto:[email protected]>
*Subject:* Re: [CF-metadata] How to define time coordinate in GPS?
On Thu, Apr 23, 2015 at 7:19 AM, Julien Demaria
<[email protected] <mailto:[email protected]>> 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
*Envoyé :* jeudi 23 avril 2015 14:51
*À :* '[email protected] <mailto:[email protected]>'
*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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