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

Jim Biard jbiard at cicsnc.org
Tue Apr 28 15:41:42 MDT 2015


Jonathan,

As I've mentioned in other emails, I don't think TAI and GPS are 
calendars, but I understand where you are coming from.

When you are working in the model domain you are generating your own 
timestamps, so you are self-consistent and have no problems. As far as 
that goes, there are models that use no_leap or all_leap calendars, and 
they are again self-consistent and have no problems. In the "real" 
world, we often start with UTC timestamps that have leap seconds 
accounted for, yet convert them to elapsed times using calculators that 
don't account for leap seconds. This can actually lead to elapsed time 
values that encode a time discontinuity and cannot be counted on to 
produce accurate differences between every pair of values. I'm not sure 
what we should call that, but I don't like the idea of naming it with a 
calendar.

It is true that a large majority of files "in the wild" probably either 
use time at a resolution that is coarse enough that none of this matters 
or managed to avoid having any leap seconds creep in to their elapsed 
time values.

I'm not suggesting that we add anything to the units attribute. I am 
suggesting that we need to do two things. One is to more precisely 
define what sorts of times can be used in the time reference part of the 
units attribute. I just reread section 4.4, and it actually says that 
the time is UTC or a time zone offset from it. I think it should stay 
that way and the wording strengthened to make it clearer. Since there 
are no leap seconds defined before the TAI epoch date, I think there 
would be no problem with saying that times in years before Jan 1, 1958 
are assumed to have no leap seconds. (Call it proleptic UTC if you like.)

The other thing I think we need to do is provide a way to indicate that 
the elapsed times in a time variable are true elapsed times that are 
certain to be free of leap second discontinuities, or are possibly 
contaminated with leap second discontinuities. In connection with this 
we would need to add clarifying language in the CF conventions to 
educate people on the importance of using time calculators that are 
aware of leap seconds when moving between UTC timestamps and elapsed 
time values. This could be handled by adding a modifier to a calendar 
name in the calendar attribute, or it could be handled by adding a new 
attribute to hold this information. I think that coming up with one or 
more new calendar names is a more confusing and less useful way to 
accomplish this.

It seems to me that the historical default for existing datasets is that 
the leap second handling has to be "unknown". For most (maybe all) of 
the model datasets, the actual situation is "no leap second 
discontinuities". (Do you actually have any models where time in the 
output has a resolution fine enough and span wide enough for it to 
matter anyway?) For many "real world" datasets, the actual situation is 
"possibly contaminated with leap second discontinuities", since the 
times started as UTC timestamps and were converted to elapsed times 
using calculators that didn't consider leap seconds. (But as I 
mentioned, most of them probably aren't affected by the error.) We have 
two communities that have different defaults, so we shouldn't pick one 
over the other.

Grace and peace,

Jim

On 4/28/15 12:52 PM, Jonathan Gregory wrote:
> Dear Jim
>
> I agree that TAI and GPS aren't distinct CF calendars if they differ only
> because of the epoch. In CF and udunits, the reference time is part of the
> the units, as you know; it's not a property of the calendar. I referred to
> GPS calendar just to contrast it with UTC.
>
> Unlike you, I don't live only in the real world. :-) Many models use the
> default CF calendar (called standard or gregorian) as an approximation to
> the real world. In these models there are no leap seconds, and it would not
> be correct to call this UTC, or to include the leap seconds in the encoding
> and decoding of time cooordinates. In any case, as we understand, most software
> doesn't allow for leap seconds. For those two reasons, I propose that we
> more precisely define the default (standard, gregorian) calendar to say
> explicitly that it does not include leap seconds i.e. every day is 86400 s
> long exactly. This is probably correct for most of the data which has been
> written with this calendar.
>
> I call UTC a calendar because it affects the encoding, since it implies leap
> seconds.  You suggest, I think, that the treatment of leap seconds should be
> indicated in the reference time in the units string. This doesn't seem quite
> right to me because the units doesn't say anything about the encoding. We
> use the same format of units string for all calendars. udunits does mention
> UTC in respect of the format of this string because of wanting to talk about
> time zones, not because of leap seconds. Time zones could be used in models
> too (though I don't know of a case). Hence I still propose that we should
> regard UTC as a calendar, if it's correct that the leap seconds in use in the
> real world are part of the definition of UTC. This means we should define a
> a new calendar, which is not the default, in which time units have just the
> same format as usual i.e. udunits, but the encoding and decoding of values
> is done including leap seconds according to UTC. The same value, with the
> same units string, may translate into a different date-time (by a number of
> seconds) according to whether the calendar is default (standard, gregorian)
> or utc. It wouldn't make sense to use this calendar for dates before the
> introduction of UTC, so it should be illegal to do so. We could do this by
> prohibiting reference times earlier than the start of UTC and negative values
> of time in this calendar.
>
> Maybe I haven't grasped your point properly?
>
> Best wishes
>
> Jonathan
>
> ----- Forwarded message from Jim Biard <jbiard at cicsnc.org-----
>
> Date: Tue, 28 Apr 2015 11:35:16 -0400
> From: Jim Biard <jbiard at cicsnc.org>
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] How to define time coordinate in GPS?
> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0)
> 	Gecko/20100101 Thunderbird/31.6.0
>
> Jonathan,
>
> I see where you are coming from, and there's validity in that line
> of thought. Leap seconds represent a finer-grained adjustment to the
> overall date/time system being used. I still think it makes good
> sense to add a new attribute to declare whether or not leap second
> handling was used or strengthen our standards for time variables so
> that problems are averted.
>
>  From a human understandability perspective, a calendar attribute of
> "GPS" or "UTC" will be somewhat confusing. In my experience, people
> don't speak of the UTC calendar, it's UTC time. Further, TAI and GPS
> time don't really concern themselves with anything but counts of
> seconds since an epoch date & time. People convert the counts to
> time stamps for convenience, but they are actually more equivalent
> to the Julian Day Number (JDN) than they are to the Gregorian or
> Julian calendar. TAI and GPS time have different epochs, and TAI is
> more accurate, but they both are running counts of seconds that
> aren't tied to the motions of the Earth. As a result, I think that
> it's improper to talk about a GPS calendar or TAI calendar.
>
> What is being exposed by this discussion is the reality that any of
> us (myself included) have often ignored or been unaware of the fact
> that the time calculators (time handling software) we used when
> filling our time variables with elapsed times weren't giving us true
> counts of seconds since the epochs we wrote into our units
> attributes. If you are working at a resolution of days or hours,
> this will probably never cause a problem. If you are working at a
> resolution of minutes or less and working over a time span of
> greater than two years, it may well have caused at least occasional
> small problems. If you are working with full-resolution
> polar-orbiting satellite data, one second represents ~7 km of
> satellite motion, so such errors can produce significant geolocation
> errors.
>
> A set of elapsed seconds since a Gregorian/UTC epoch that were
> calculated from Gregorian/UTC time stamps without regard for leap
> seconds and which crossed a leap second boundary are not "GPS"
> seconds. Nor are they "UTC" seconds. They are, strictly speaking,
> elapsed times into which one or more step errors have been
> introduced. As I mentioned in a previous email, as long as you use
> the same time calculator to extract time stamps as you did to get
> elapsed times from input time stamps, you won't notice anything. You
> may notice a problem if you are taking differences between elapsed
> times and a leap second boundary gets involved.
>
> As I've considered all of this more, I'm tending to favor the second
> option I suggested.
>
>     We could also be more strict, and say the epoch time stamp in the units
>     attribute must always be in UTC. The question would then be reduced to
>     whether or not leap seconds were counted into the elapsed times stored
>     in the time variable. In this case, we could add a "leap_seconds"
>     attribute which would have a value of "UTC" if UTC leap seconds were
>     counted into the elapsed times, and "none" if not. This would also allow
>     for some other system of leap seconds to be used. (I don't know if there
>     are others.) For backward compatibility, considering history, the
>     default value for this attribute would probably be "UTC".
>
> Clearly, having epoch time stamps with time zone offsets from UTC,
> as described in the CF conventions, would be OK as well. I'm also
> open to other namings for the new attribute and for its possible
> values. The leap seconds only become an issue in certain rather
> specific instances, so I think that such an attribute, along with a
> bit of discussion in the document, would likely be sufficient to
> warn those people that may find themselves negatively affected by
> improper leap second handling.
>
> Grace and peace,
>
> Jim
>
>
> On 4/27/15 1:07 PM, Jonathan Gregory wrote:
>> Dear Jim
>>
>>> I don't think calendars are the right place to encode this. We could
>>> add a new "time_system" attribute where you would declare whether
>>> your time stamps and elapsed times were based on UTC, GPS, TAI, etc.
>>> If we take this route, we should require the elapsed times to encode
>>> leap seconds if the time system is UTC, and state that the default
>>> time system is UTC.
>> I think this is a calendar issue because the calendar is the set of rules
>> which translate between components of time (YMDhms) and elapsed time (in
>> fixed time units) since the reference time. Your later email seems to me
>> to be consistent with that. In the real world, the elapsed interval (expressed
>> e.g. as the number of seconds) between the ref YMDhms and the actual YMDhs
>> depends on whether your calendar includes leap seconds (UTC) or not (GPS).
>> It seems that GPS is the calendar likely to have been assumed in existing
>> CF datasets, so it would be logical to say that the default is the real-
>> world calendar without leap seconds. Have I misunderstood something? If we
>> regard this as a property of the calendar, we don't need a new attribute.
>>
>> Best wishes
>>
>> Jonathan
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata at cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata

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