Transformation from Cartesian to geographic coordinates

Orekit usage
1

I am trying to transform a point in geographic (lat, lon, alt) coordinates to Cartesian/EME2000 as ORaaS does. Doing the inverse, from Cartesian to LLA works as described here, and the result is in very close agreement with ORaaS. But “Transform a surface-relative point to a Cartesian point” transform​(GeodeticPoint point), which would seem to be the inverse of the successful use of transform​(Vector3D point, Frame frame, AbsoluteDate date) “Transform a Cartesian point to a surface-relative point”, gives a very different answer than what I expect, and moreover takes neither the frame nor the AbsoluteDate as an argument, so seemingly it is not doing the transformation desired.

How do I get the LLA to Cartesian transformation as ORaaS computes it?

2

Hi Liam,

Using transform(GeodeticPoint point), you get a point in the ITRF frame associated to the OneAxisEllipsoid.

To get the EME2000 coordinates at some date, you need to do the following (which is what ORaaS does):

  1. Create the Earth model
OneAxisEllipsoid earth = new OneAxisEllipsoid(Constants.WGS84_EARTH_EQUATORIAL_RADIUS,                                   Constants.WGS84_EARTH_FLATTENING,                                  FramesFactory.getITRF(IERSConventions.IERS_2010, false));
  1. Create the GeodeticPoint from latitude, longitude, altitude (beware lat and lon should be in radians, alt in meters)
GeodeticPoint point = new GeodeticPoint(lat, lon, alt);
  1. Create the TopocentricFrame for this point:
TopocentricFrame topoFrame = new TopocentricFrame(earth, point, "My Point");
  1. Apply the transform to get the position in EME2000:
Vector3D cartesianEme2000 = topoFrame.getPVCoordinates(date, FramesFactory.getEME2000()).getPosition();
double x = cartesianEme2000.getX();
double y = cartesianEme2000.getY();
double z = cartesianEme2000.getZ();

Best regards,
Pascal

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