A Proposal for a TIMESYS Element in VOTable

[Arnold Rots]

TOPOCENTER refers to the location where the measurement is made - that is
not even restricted to earth-based observatories and satellites.
I would favor this definition, as it is the most general.

Whether or not you want to include orbit ephemerides depends on the
question: what timing accuracy is needed?
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.
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.

Cheers,

  - Arnold

-------------------------------------------------------------------------------------------------------------
Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------------------------------------------



On Mon, Nov 5, 2018 at 10:37 AM AdaNebot <ada.nebot at astro.unistra.fr> wrote:

> Hi, Tom, Arnold,
>
> I agree that saying UNKOWN seems to be unfair. The term TOPOCENTER can be
> referred to the location of the instrument as Arnold mentions.
> 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.
> Including ephemerides on the other hand, and in my opinion, would be
> complicating things for this simple annotation.
>
> We are taking into account the comments we are receiving and I propose to
> move the discussion to Apps for the more technical details.
>
> Cheers,
> Ada
>
> On 1 Nov 2018, at 20:20, Arnold Rots <arots at cfa.harvard.edu> wrote:
>
> 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).
> 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.
>
> Cheers,
>
>   - Arnold
>
>
> -------------------------------------------------------------------------------------------------------------
> Arnold H. Rots                                          Chandra X-ray
> Science Center
> Smithsonian Astrophysical Observatory                   tel:  +1 617 496
> 7701
> 60 Garden Street, MS 67                                      fax:  +1 617
> 495 7356
> Cambridge, MA 02138
> arots at cfa.harvard.edu
> USA
> http://hea-www.harvard.edu/~arots/
>
> --------------------------------------------------------------------------------------------------------------
>
>
>
> On Wed, Oct 31, 2018 at 3:55 PM Tom McGlynn <tom.mcglynn at nasa.gov> wrote:
>
>> It seems to me that this makes no accommodation for satellites where the
>> reference position is variable but not
>> unknown.  The definition of topocentric is according to my dictionary:
>> "**relating to, measured from, or as if observed from a particular point
>> on the earth's surface *: *having or relating to such a point as origin"
>> so TOPOCENTER is fine for earth based observations (though there are
>> balloons that move quite a bit during an observation campaign) but it
>> would be a stretch to use it for satellites.  I wouldn't want to say
>> UNKNOWN though.    Maybe SATELLITE or VARIABLE? Lots of HEASARC data
>> have times that are measured at the satellite -- and that includes lots
>> of the GRB detecting satellites for which this kind of feature would be
>> very nice.
>>
>>      Tom McG.
>>
>> AdaNebot wrote:
>> > Dear All,
>> >
>> > A Note entitled “*A Proposal for a TIMESYS Element in VOTable*” can be
>> > found under:
>> >
>> > http://ivoa.net/documents/Notes/TimeSys/
>> >
>> >
>> > Thanks for taking the time to read this document,
>> >
>> > Happy reading!
>> >
>> > See you in College Park,
>> > Ada Nebot
>> > TDIG Chair
>> >
>>
>>
>
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