TimeSeries of position (Asteroid)

[Arnold Rots]

...  also known as an ephemeris.

We use a standard FITS file format for orbit and solar system body
ephemerides, with 7 columns - time and state vector:
Time (TT), X, Y, Z, Vx, Vy, Vz
The state vector is in ITRS, its units m and m/s. All at GEOCENTER.
The solar system body ephemerides are derived from an ephemeris format that
contains:
Time (TDB), Xs, Ys, Zs, Xe, Ye, Ze
The position vector for earth (e) and SS object (s) are in ICRS at origin
BARYCENTER, their units AU.

The three points I want to make are:

1. 3-D Cartesian coordinates are preferable, since you are working in the
near field
ICRS RA and Dec at BARYCENTER, without a solar distance, are useless for
conversion to some specific location

2. State vectors (i.e., adding the spatial time derivatives) are extremely
helpful, especially for fast-moving objects

3. There is one ambiguity that is not addressed here:
If you provide time T and location S of an object with a common coordinate
origin, does S represent:
a. the location of the object at time T at the coordinate origin?
or:
b. the apparent location of the object as seen from the origin at time T?
This is especially relevant if spherical coordinates are given - the
location on the sky where the object IS at time T or where we SEE it at
time T.

Just some points to ponder - we need to be explicit about these matters.
And the example needs some serious work.
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/
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On Thu, Nov 22, 2018 at 3:51 PM François Bonnarel <
francois.bonnarel at astro.unistra.fr> wrote:

> Der Dave, all,*
>
> Before interop Dave asked me to provide how a TimeSeries of Asteroid
> could look like with the current ts model proposal and Utypes serialisation.
>
>
>
> Someone recently described to me a use case that I haven't seen in our
>  discussions.
>
>  To help me understand how to use the new model, could you send me a
>  simple example of how a time series of position over time would look ?
>
>  For a single moving object we would get multiple ra/dec or Point
>  values plotted against time.
>
>  For a mode complex example, multiple position and magnitude values
>  plotted against time.
>
> You can find a "dummy example" of this at this URL :
> http://volute.g-vo.org/svn/trunk/projects/time-domain/time-series/note/DATA/Proposed_Serializations/UTYPES/AsteroidSimple.xml
> Could be from gaia for an Asteroid. Then Dave gave more details for the
> second more complex example
>
> Two use cases from the Gaia meeting.
>
>  Exoplanets, the star will change both brightness and position by tiny
> amounts linked to the planet's orbit. Very small and slow changes in
>  both position and brightness, hard to detect above the noise.
>
>  Near earth asteroid will be travelling very fast, and change
>  brightness in response to distance from the sun, position relative to
>  the sun, and rotation and shape of the asteroid itself. Very large and
>  rapid changes in both position and brightness.
>
>  Both are valid cases for time series of positions and magnitudes.
>
> This is an attempt for a Near Earth Asteroid
> http://volute.g-vo.org/svn/trunk/projects/time-domain/time-series/note/DATA/Proposed_Serializations/UTYPES/AsteroidFull.xml
> Data are dummy of course. but with about 1 magnitude amplitude and 1deg
> position change in 10 days !!! *Changes for the Exoplanets use case can
> be easilly done :* There will be no real "look and feel" change for the
> magnitudes. For position, the best is probably to set the mid position of
> the star in the refPOsition of COOSYS (see note below) and to have the lon
> and lat columns giving the deviation from this  position along the ra and
> dec axes (and no more ecliptic longitude and latitude). The unit for these
> two columns will probbaly be "mas" instead of "deg".   Cheers François
> COOSYS note : currently there is no "refPosition" attribute in COOSYS. We
> propose to add one , taking the opportunity of the change in VOTable schema
> required to introduce TIMESYS. this attribute allows accurate distinctions
> of positions computed from BARYCENTER, TOPOCENTER, GEOCENTER, as well as
> definitions of standalone "local" spatial frames to code for  very accurate
> measurements.
>
>
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