Orbit Determination and Prediction, and Computer Programs

The following paper describes the methods and programs used in the Telstar project for the purposes of orbit determination and ephemeris calculation. The orbit determination process involves the computation of orbital elements from tracking data obtained during each pass, and subsequent refinement by combining such single-pass estimates. The tracking data arc in terms of angular observations only. The ephemeris calculations involve standard procedures for computation of Keplerian orbits and perturbations due to the earth's oblateness. It is well known that in the problem of orbit determination from angular data only, three observations (each observation consisting of two angles and a time) are not sufficient to determine an orbit if the three sightlines are coplanar. If the three sightlines are nearly coplanar, the computed orbital elements may reflect large uncertainties which are not necessarily due to observational errors. Hence, the method used is based on the determination of a set of orbital elements from four observations. 1357