[AstroPy] Convert GPS time to UTC time

[Paul Barrett]

GPS time is its own time scale, since it is defined as a Composite Clock.
 It just happens to be steered to UTC by USNO, but if necessary can be run
autonomously. For more details, see http://tycho.usno.navy.mil/gpstt.html.

 -- Paul


On Mon, Dec 9, 2013 at 6:37 PM, Aldcroft, Thomas <
aldcroft at head.cfa.harvard.edu> wrote:

> Based on the discussion here I added an astropy issue to consider adding a
> GPS time scale:
>
> https://github.com/astropy/astropy/issues/1879
>
> Cheers,
> Tom
>
>
> On Mon, Dec 9, 2013 at 3:58 PM, Paul Hirst <phirst at gemini.edu> wrote:
>
>>  On 9/12/13, 09:48 , "Aldcroft, Thomas" <aldcroft at head.cfa.harvard.edu>
>> wrote:
>>
>>
>>  On Mon, Dec 9, 2013 at 12:22 PM, David Berry <d.berry at jach.hawaii.edu>wrote:
>>
>>> On 9 December 2013 16:57, Aldcroft, Thomas
>>> <aldcroft at head.cfa.harvard.edu> wrote:
>>> > This basically creates a Time object with the GPS format
>>> representation.  By
>>> > default this object is in TAI scale, so you first convert to UTC and
>>> then
>>> > ask for the ISO format representation.  As I understand GPS is not a
>>> time
>>> > scale, but a simple difference of time in the TAI scale from the given
>>> > epoch.
>>>
>>>  Doesn't that make it a timescale? After all TT (Terrestrial Time) is
>>> also just a constant offset from TAI.
>>>
>>
>>  This is a good question and one for which I'm definitely interested in
>> input.
>>
>>  From what I can see there are two ways to view "GPS time":
>>
>>  1.  As a time scale which is behind TAI by 19 seconds.
>> 2.  As a time format which is the floating point number giving the number
>> of TAI seconds since UTC 1980-01-06 00:00:00.  This is roughly similar to
>> the "unix" format or "cxcsec" format.
>>
>>
>>
>>  I would have thought (1) was actually a more appropriate
>> representation. I'm not sure of the formal definition, but I always
>> considered GPS time to be another timescale, which just happens to have a
>> nice and simple transformation to and from TAI - I understand it's by
>> definition just the simple and exact 19.0 second offset.
>>
>>  Depending on the level of accuracy required, I don't think that's true
>> for TT as suggested above, the TT = TAI - 32.184s is an approximation; a
>> more accurate transformation is given at eg
>> ftp://tai.bipm.org/TFG/TT(BIPM)/TTBIPM.12
>>
>>  Now, I'm not suggesting that this is a priority for implementation
>> right now, but in general I'm thinking that having things as separate time
>> scales would make it easier in the future for someone to contribute a more
>> advanced transformation - as opposed to figuring out how to make it a
>> separate timescale first.
>>
>>  Cheers,
>>  Paul.
>>
>>
>
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