TimeSeries of position (Asteroid)

Arnold Rots arots at cfa.harvard.edu
Fri Nov 23 21:53:48 CET 2018

...  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
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?
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.

  - Arnold

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

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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