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 QuickBasic). Bruno Tilgner 100533.2016@compuserve.com