Variation of satellite transponder delay

For precise orbit determination of geosynchronous earth orbit (GEO) satellites using transfer ranging observations, it is generally assumed that the variation of the satellite transponder delay is very small and that it can be solved as a constant parameter together with satellite orbit parameters. However, this assumption is too general and it reduces the accuracy of orbit determination for GEO satellites. To study and analyse the impact of the satellite transponder delay on GEO satellites orbit determination, two schemes were proposed. First, the satellite transponder delay was eliminated by forming single-difference observations between two ground stations; second, the satellite transponder delay was described as a constant parameter. The preliminary results demonstrate a difference of about 1–2 m between the two schemes when used for precise orbit determination of GEO satellites. By fixing the GEO satellite orbit and other relevant parameters estimated by single-difference model, we inversed the instantaneous transponder delay from non-difference observation. It was found that the satellite transponder delay has a distinct diurnal variation, with an amplitude of 3–4 m. The findings of this paper are helpful in establishing an accurate model of satellite transponder delay and in improving the accuracy of GEO satellites orbit determinations and predictions.