<div dir="ltr"><div>TOPOCENTER refers to the location where the measurement is made - that is not even restricted to earth-based observatories and satellites.</div><div>I would favor this definition, as it is the most general.</div><div><br></div><div>Whether or not you want to include orbit ephemerides depends on the question: what timing accuracy is needed?</div><div>If a dataset specifies TOPOCENTER and requires, say, a microsecond accuracy, one will have to provide the location of the instrument, whether it is earth-bound, in low-earth orbit, in L3, or wherever. There is no escaping this.</div><div>On the other hand, if second-accuracy is required, the timestamps can simply be converted to GEOCENTER by changing the systematic time error parameter to (roughly) 25 ms for earth-based, 30 ms for LEO, 400 ms for Chandra, etc., without touching the actual timestamps values.</div><div><br></div><div>Cheers,</div><div><br></div><div> - Arnold</div><div><br></div><div><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr">On Mon, Nov 5, 2018 at 10:37 AM AdaNebot <<a href="mailto:ada.nebot@astro.unistra.fr">ada.nebot@astro.unistra.fr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space">Hi, Tom, Arnold, <div><br></div><div>I agree that saying UNKOWN seems to be unfair. The term TOPOCENTER can be referred to the location of the instrument as Arnold mentions. </div><div>Perhaps the best way to avoid any possible ambiguity is to write some more examples in the document and explicitly mention that TOPOCENTER includes position of satellites too. </div><div>Including ephemerides on the other hand, and in my opinion, would be complicating things for this simple annotation. </div><div><br></div><div>We are taking into account the comments we are receiving and I propose to move the discussion to Apps for the more technical details. </div><div><br></div><div>Cheers,</div><div>Ada</div><div><br><blockquote type="cite"><div>On 1 Nov 2018, at 20:20, Arnold Rots <<a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a>> wrote:</div><br class="m_-8512648881258051473Apple-interchange-newline"><div><div dir="ltr"><div>The term TOPOCENTER has been used for several decades now to indicate the location where the time is actually measured, i.e., at the detector/telescope/observatory, regardless of whether the instrument is located on the earth's surface or in space (or wherever else).</div><div>In both cases information is needed that specifies where that location is, whether it is expressed as a fixed location wrt to the GEOCENTER (in any type of spatial coordinate system) or as an (orbit) ephemeris.</div><div><br></div><div>Cheers,</div><div><br></div><div> - Arnold</div><div><br></div><div><div><div dir="ltr" class="m_-8512648881258051473gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr">On Wed, Oct 31, 2018 at 3:55 PM Tom McGlynn <<a href="mailto:tom.mcglynn@nasa.gov" target="_blank">tom.mcglynn@nasa.gov</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">It seems to me that this makes no accommodation for satellites where the <br>
reference position is variable but not<br>
unknown. The definition of topocentric is according to my dictionary: <br>
"**relating to, measured from, or as if observed from a particular point <br>
on the earth's surface *: *having or relating to such a point as origin" <br>
so TOPOCENTER is fine for earth based observations (though there are <br>
balloons that move quite a bit during an observation campaign) but it <br>
would be a stretch to use it for satellites. I wouldn't want to say <br>
UNKNOWN though. Maybe SATELLITE or VARIABLE? Lots of HEASARC data <br>
have times that are measured at the satellite -- and that includes lots <br>
of the GRB detecting satellites for which this kind of feature would be <br>
very nice.<br>
<br>
Tom McG.<br>
<br>
AdaNebot wrote:<br>
> Dear All,<br>
><br>
> A Note entitled “*A Proposal for a TIMESYS Element in VOTable*” can be <br>
> found under:<br>
><br>
> <a href="http://ivoa.net/documents/Notes/TimeSys/" rel="noreferrer" target="_blank">http://ivoa.net/documents/Notes/TimeSys/</a><br>
><br>
><br>
> Thanks for taking the time to read this document,<br>
><br>
> Happy reading!<br>
><br>
> See you in College Park,<br>
> Ada Nebot<br>
> TDIG Chair<br>
><br>
<br>
</blockquote></div>
</div></blockquote></div><br></div></blockquote></div>