Contribution of SLR tracking data to GNSS orbit determination

GNSS orbits derived from microwave tracking data may be validated using SLR range measurements. Recent validation results show mean range residuals of several centimeters, as well as significant seasonal variations for the two GPS satellites that are equipped with retroreflector arrays. These differences may be assigned to orbit or observation modeling problems, or both. In order to study this issue, we analyzed several time series of SLR range residuals. GNSS microwave orbits from three IGS analysis centers differing in arc-length, in orbit modeling, and in attitude modeling were validated. The range residuals derived from the various GNSS orbits show similar periodic variations, which are correlated with eclipsing seasons and the sun's elevation above the orbital plane, indicating orbit or attitude modeling deficiencies. Moreover, we address the question, whether it would make sense to perform a combined analysis of microwave and SLR data for GNSS orbit determination, assuming that the existing modeling problems can be solved. With the available low number of SLR observations no significant improvement of the orbit accuracy is found. An a priori variance-covariance analysis shows an improvement of the situation, if continuous SLR tracking data of already a very small number of globally distributed SLR sites would be available.