RE: [Orekit Users] Howto: Moving LLA Point Transformation?

["Schatzman, James" <Schatzman-James@n-ask.com>]

You may be right, that a matrix inversion (or solving a linear system via LU) is the way to go. I will see about doing that for my immediate problem.

Thanks for the explanation about ECEF. What you say makes sense. That state of affairs is unfortunate in this case. If anyone is interested, I can supply code to go back and forth between Geodetic pos/vel and ECEF pos/vel. I did not implement acceleration but that would be easy enough to do. Conversion between ECEF and the other coordinate systems (ITRF, GCRF, EME2000, etc.) does not exist, as you say, except by going back to Geodetic.

Jim


________________________________________
From: orekit-users-request@orekit.org [orekit-users-request@orekit.org] on behalf of Luc Maisonobe [Luc.Maisonobe@c-s.fr]
Sent: Friday, August 21, 2015 3:00 AM
To: orekit-users@orekit.org
Subject: Re: [Orekit Users] Howto: Moving LLA Point Transformation?

Le 21/08/2015 04:36, schatzman-james@n-ask.com a écrit :
> Likewise,
>
> OneAxisEllipsoid.transform(Vector3D, frame, date)
>
> returns the GeodeticPoint for an arbitrary point in an external frame.
> However, there is no method, apparently, to return the velocity of the point
> in geodetic coordinates.

Not yet, but in this direction the problem is more difficult. If you
look for example at the method projectToGround (wich takes a
TimeStampedPVCoordinates and computes another TimeStampedPVCoordinates),
the computatio  is quite involved (finding principal and secondary
curvature by looking at plane sections,
combining them appropriately). Even if we were computing only first
derivatives there are some tricky parts. For example projecting a
velocity involves a twist due to the different radii of curvature on
an ellipsoid surface. It can surely be done, but it requires some work.

I wonder if it would not be easier to compute the Jacobian matrix the
other way round and invert it.

>
> It is straightforward to implement the transformation of position and velocity
> between the geodetic coordinates and the standard ECEF coordinates computed by
>
> OneAxisEllipsoid.transform(GeodeticPoint).
>
> Is there a Frame defined for this coordinate system?

No, frames only support orthogonal Cartesian coordinates systems, not
angular ones.

best regards,
Luc

>