Re: [Orekit Users] Inertial Frame Question

[MAISONOBE Luc <luc.maisonobe@c-s.fr>]

Stephen Ranger <sanosuke001@gmail.com> a écrit :

> Hello,

Hi Stephen,

> I'm attempting to plot the positions of objects based on a current
> visualization time (the time my Inertial Frame should be based on; I
> believe) with each incoming fixed-frame position also having a time but
> potentially different from the visualization time.

When you talk about fixed frame, do you mean fixed with respect to Earth
(i.e. rotating with respect to the rest of the unisverse)?

> My question, if I make a new transform from fixed to inertial (ITRF to
> EME2000) with the current visualization time how do I then get the position
> of my incoming object at it's past or future time?  Is it as simple as
> making a new transform for the object's time from the fixed frame or do I
> need to get a transform from the visualization time's inertial frame to the
> past/future inertial frame?

Yes. The most accurate and more straightforward way is to compute the  
transform
at each date, by the getTransformTo method from Frame. The Transform instance
obtained this way is valid at the time you give and not for  
neighbouring times.

The Transform instance should be considered a temporary object created and
destroyed on the fly during computation.

A lot of work has been put to make the getTransformTo method very efficient,
as there is a lot of caching underneath, so you don't have to attempt to do
caching or interpolation at your use code level. You can rely on the method
being really fast.

> Sample code or a tutorial somewhere would be helpful but an explanation of
> the "right" way to transform position information would be useful, too.

I would suggest something like:


  // These are the things that give you your obects positions
  List<PVCoordinateProvider> myObjectsProvidersInITRF = ...;

  while(stillRunning) {

    java.util.Date now  =  
Calendar.getInstance(TimeZone.getTimeZone("Etc/UTC")).getTime();
    AbsoluteDate absNow = new AbsoluteDate(now, utc);
    Transform itrf2Eme2000 = itrf.getTransformTo(eme2000, absNow);
    for (PVCoordinateProvider provider = myObjectsProvidersInITRF) {
       PVCoordinate pvITRF = provider.getPVCoordinates(absNow, itrf);
       PVCoordinates pvEME2000 = itrf2Eme2000.transformPVCoordinates(pvITRF);
       display(pvEME2000);
       try {
           // wait 100 ms
           Thread.sleep(100);
       } catch (InterruptedException ie) {
         // ignored
       }
    }

  }

In this example, I suppose that your providers naturally provide data  
in ITRF frame, so it is
better to compute itrf2Eme2000 once for all objects, at current time.  
Another approach could
alos be to call provider.getPVCoordinates(absNow, eme2000) directly  
and to not transform the
coordinates by yourself, but in this case the equivalent of  
getTransformTo would be called
under the hood by the provider over and over.

Of course, if instead of a list of PVCoordinateProvider you have your  
own objects (say a simple
ephemeris of already precomputed positions in ITRF), you would just  
replace getPVCoordinates
by your own API.

Hope this helps,
Luc

> Thanks!
>
>
> - Stephen Ranger