Calibrating GNSS orbits with SLR tracking data

SLR tracking data allow for a completely independent validation of GNSS orbits that are derived from microwave data. SLR validation results show mean range residuals of several centimeters for both, GPS and GLONASS satellites, as well as significant seasonal variations for the two GPS satellites that are equipped with retroreflector arrays. It was, however, not clear whether these systematic effects could be assigned to orbit modeling deficiencies or to SLR tracking biases. We present new SLR validation results, which point to serious GPS orbit modeling problems. Moreover, we address the question, whether it would make sense to perform a combined analysis of microwave and SLR data for GNSS orbit determination. 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 were available.