<div dir="ltr">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 <a href="http://tycho.usno.navy.mil/gpstt.html">http://tycho.usno.navy.mil/gpstt.html</a>. <div>
<br></div><div> -- Paul</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Dec 9, 2013 at 6:37 PM, Aldcroft, Thomas <span dir="ltr"><<a href="mailto:aldcroft@head.cfa.harvard.edu" target="_blank">aldcroft@head.cfa.harvard.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Based on the discussion here I added an astropy issue to consider adding a GPS time scale:<div><br></div>
<div><a href="https://github.com/astropy/astropy/issues/1879" target="_blank">https://github.com/astropy/astropy/issues/1879</a><br>
</div><div><br></div><div>Cheers,</div><div>Tom</div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Dec 9, 2013 at 3:58 PM, Paul Hirst <span dir="ltr"><<a href="mailto:phirst@gemini.edu" target="_blank">phirst@gemini.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="font-size:14px;font-family:Calibri,sans-serif;word-wrap:break-word"><div>
<div>On 9/12/13, 09:48 , "Aldcroft, Thomas" <<a href="mailto:aldcroft@head.cfa.harvard.edu" target="_blank">aldcroft@head.cfa.harvard.edu</a>> wrote:</div>
</div><span>
<blockquote style="BORDER-LEFT:#b5c4df 5 solid;PADDING:0 0 0 5;MARGIN:0 0 0 5">
<div>
<div>
<div dir="ltr"><br>
<div class="gmail_extra">
<div class="gmail_quote"><div>On Mon, Dec 9, 2013 at 12:22 PM, David Berry <span dir="ltr">
<<a href="mailto:d.berry@jach.hawaii.edu" target="_blank">d.berry@jach.hawaii.edu</a>></span> wrote:<br>
</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
On 9 December 2013 16:57, Aldcroft, Thomas<br>
</div><div><div><<a href="mailto:aldcroft@head.cfa.harvard.edu" target="_blank">aldcroft@head.cfa.harvard.edu</a>> wrote:<br>
> This basically creates a Time object with the GPS format representation. By<br>
> default this object is in TAI scale, so you first convert to UTC and then<br>
> ask for the ISO format representation. As I understand GPS is not a time<br>
> scale, but a simple difference of time in the TAI scale from the given<br>
> epoch.<br>
<br>
</div>
Doesn't that make it a timescale? After all TT (Terrestrial Time) is<br>
also just a constant offset from TAI.<br>
</div></blockquote><div>
<div><br>
</div>
<div>This is a good question and one for which I'm definitely interested in input.</div>
<div><br>
</div>
<div>From what I can see there are two ways to view "GPS time":</div>
<div><br>
</div>
<div>1. As a time scale which is behind TAI by 19 seconds.</div>
<div>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.</div>
</div></div>
</div>
</div>
</div>
</div>
</blockquote>
</span>
<div><br>
</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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 <a href="ftp://tai.bipm.org/TFG/TT(BIPM)/TTBIPM.12" target="_blank">ftp://tai.bipm.org/TFG/TT(BIPM)/TTBIPM.12</a></div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Cheers,</div>
<div> Paul.</div>
<div><br>
</div>
</div>
</blockquote></div><br></div>
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