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Re: [Orekit Users] DihedralFielOfView with a Polygon instead of a PVCoordinatesProvider PVTarget point.

Actually for a large enough area you will be missing a bunch.  A brute force approach I would take would be to see if you have access to any of the boundary points defining the area.  If you had access to any then you had access to the area.  This is a lot easier said than done for cases like:
  1. Fast moving satellite attitudes or sensor pointing profiles
  2. Small sensors which will be between sample points
  3. The case where the sensor is entirely within the boundary.

As long as you are avoiding these sorts of cases, you should be able to come up with a reasonable approximation of accesses to the ground area using the method I mentioned above.

Using the method described by Luc would be preferable, assuming you can get a reasonable sensor surface projection for each step.  This type of calculation is profoundly different from the way these event detectors currently work however.  It is possible to do similar calculations without the new Apache Math library, but it probably wouldn't handle all the edge cases the Apache Math algorithms would.

PS. I'd be more than happy to contribute towards studying this problem further, FYI...

On Thu, Jan 30, 2014 at 9:31 AM, Carlos Krefft <ckrefft@rsmas.miami.edu> wrote:
Thank you for the valuable information.  I'll keep my eyes open for the next release.

In the meantime, I'll try to do something along the lines that Hank suggested. Detect Visibility on the AOI center point given possibly a negative mask as a buffer, record start and end times, generate corridors within those times, and finally intersect them with my AOI. I know, it sounds inefficient, I'm definitely looking forward to the new release.

BTW: Great work on the orekit. We'll be using it for scheduling acquisitions in a production environment soon.


-----Original Message-----
From: orekit-users-request@orekit.org [mailto:orekit-users-request@orekit.org] On Behalf Of MAISONOBE Luc
Sent: Wednesday, January 29, 2014 3:45 PM
To: orekit-users@orekit.org
Subject: Re: [Orekit Users] DihedralFielOfView with a Polygon instead of a PVCoordinatesProvider PVTarget point.

Carlos Krefft <ckrefft@rsmas.miami.edu> a écrit :

> I'm looking for a way to make a DihedralFielOfViewDetector detect
> Visibility against an AOI polygon instead of PVTarget. Is this
> possible?

It will be possible in the next version, that will depend on Apache Commons Math 3.3. This version of Apache Commons Math includes a way to define arbitrary shapes on the sphere and to compute things like distance to the boundary and hence accurate crossing time. This work for simple polygons on the sphere, but also for more complex cases with zones defined in several disconnected parts, non-convex areas, areas with holes, areas including the pole ...

This will be used for two things in Orekit :

  - creating an event handler for spacecrafts flying over zones of
    interest (say a country, even considering separate islands, or
    excluding inner lakes if you want)
  - creating an event handler for arbitrary shaped on-board fields
    of view.

I think what you want is exactly what we are doing right now!

The code for the first event handler is almost ready (it is just quite slow for complex shapes, so I have to improve it). The code for the second event handler does not exist but can be done in a matter of hours. The most important thing to remember is that this induces a dependency to Apache Commons Math 3.3, and this version of the library is not released yet. It is expected to be released soon, but we have no date.

> Any tips or help would be greatly appreciated.

Hope this helps

> Kind regards,
> Carlos Krefft
> University of Miami

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