Re: [Orekit Users] Question on using getIntersectionPoint

[Luc Maisonobe <Luc.Maisonobe@c-s.fr>]

Le 16/04/2012 22:59, Petrus Hyvönen a écrit :
> Hi,

Hi Petrus,

> 
> I'm having a question if this is the right way to use getIntersectionPoint.
> I'm trying to simulate the coverage of a relay satellite in
> geostationary orbit.
> 
> The geostationary satellite I have modelled as a GroundPoint with 6
> earth radius altitude, and then transformed to an inertialFrame EME2000
> frame.
> 
> I have my two positions as Vector3D coordinates in inertialFrame.
> geo_pos 
> 
> ref_pos 


I assume ref_pos is a point on ground, is this right ? If not, see the
last remark at the end of this message.

With such a model, you can use an Earth frame for both points instead of
an inertial frame. It does not really matter, though as conversions will
be done under the hood as needed.

> 
> 
> Create a Line with 
> los = Line(ref_pos, geo_pos)
> 
> ip = mysc.ore_earth.getIntersectionPoint(los, geo_pos, inertialFrame, time)
> 
> 
> and if this is == None (Null) then it has a clear view I would think.

No. If the result is null, then the line that joins your two points does
not intersect the ore_earth BodyShape. This should not happen if your
ref point is on Earth (except perhaps at limb as seen from geo_pos).

If the result is non null, then the line enters Earth surface at the
returned point (and exits at some other point further away).

> 
> 
> The results are not what I expect them to be, no contacts in what I
> would expect the coverage region and some at other locations. Would this
> be the/a correct way to use getIntersectionPoint?

It is a strange way, but it should work.
As you already have a ground point ref_pos, a simpler way could be to
build a TopocentricFrame from it and to use its getElevation method to
check is it is above observation threshold.


> Is it ok that the Line
> is in a different frame than the OneAxisEllipsoid, as long as it is
> specified in the call?

Yes, it's OK. Both los and geo_pos must be in the same specified frame,
but this frame can be anything. These elements are converted to the
BodyShape internal frame at the start of the computation.

> 
> 
> I do not understand the Line object really, los.contains(geo_pos) gives
> False, as well as los.contains(ref_pos). The los.origin gives a
> different value than given in the constructor?

This is strange. Could you send some numerical values with coordinates
of the various points in all the involved frames (body shape and
inertial) ? There was also a Line class up to version 5.x of the Orekit
library, but now we use directly the class from Apache Commons Math
(which has different and incompatible meanings in the construction
parameters). What version do you use, Apache Commons Math 2.x/Orekit 5.x
or Apache commons Math 3.0/Orekit development ? Note that you cannot mix
these versions.

best regards,
Luc

> 
> 
> Any comments appreciated,
> 
> 
> /Petrus
> 
> 
> -- 
> _____________________________________________
> Petrus Hyvönen, Uppsala, Sweden
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