Extended orbit modeling techniques at the CODE processing center of the international GPS service for geodynamics (IGS): theory and initial results.

Abstract

The CODE Processing Center of IGS has produced orbits for the U.S. Global Positioning System (GPS), Earth rotation parameters (ERPs), stational coordinates, and other data on a regular basis since 21 June 1992. The experience gained at CODE during the first year of IGS operations indicated that at least in some cases the authors' standard orbit model was not sufficient for 3-days-arcs. The authors' new orbit model contains the following adjustable parameters: (1) six parameters defining the initial state vector (position and velocity), (2) velocity changes at pre-determined times and in pre-determined directions, (3) (at maximum) nine parameters of a new direct radiation pressure model (drp-model), (4) the parameters of the Earth's gravity field, (5) two parameters of a simple Earth's albedo radiation pressure model (arp-model), and (6) the (empirical) resonance terms suggested by (Colombo, 1989). These new orbit models were used in the newly developed computer program ORBIMP, interpreting the IGS precise ephemerides as (pseudo-)observations. Results are presented using IGS data from 17 to 30 January 1993. The new orbit models allowed working with 14-days-arcs instead of 1-3 days-arcs (as it is standard within IGS), the orbital accuracy dropped from about 1-2 m rms for the standard model to about 10 cm to 20 cm when using the new radiation pressure model