Re: Newbie with questions

Bruno Tilgner (
Sat, 21 Dec 1996 11:39:52 -0500

Philip Chien wrote:

>>You will very rarely get a satellite's instantaneous position expressed as
an orthoganal reference frame.<<

Well, ... this is exactly what the models described in Spacetrack Report No. 3
will give you: position and velocity vectors expressed in an inertial
reference frame (x-axis towards first point of Aries, z-axis towards celestial
north pole).

If you want to calculate the sub-satellite point accurately you cannot avoid
computing the projection of the instantaneous satellite position onto the
Earth ellipsoid. As a first step a transformation to an Earth-fixed system
must be done. This involves simply a rotation about the z-axis by an angle
equivalent to the Greenwich Mean Sidereal Time. The second step, the calculation
of geodetic latitude, longitude and height above the ellipsoid is done through
an iterative procedure. If someone is interested, I'll publish the code (in

Bruno Tilgner