Orbit Determination for GPS Satellites

Abstract

Conventional GPS processing techniques use the broadcast ephemeris to obtain relative station coordinates. Errors in the broadcast ephemeris can cause baseline errors of the order of 1 ppm. For high order geodetic positioning accuracies, it is therefore necessary to determine precise satellite orbits.

Predicted orbits can be generated by numerically integrating a model of the forces acting on the satellite. Carrier phase data from a number of tracking stations can then be used to improve this predicted orbit. At Nottingham, the phase data is double differenced, and used in a simultaneous least squares adjustment to produce corrections to selected station coordinates and to the predicted orbit.