As one of the analysis centers of the International GNSS Service (IGS), we process thousands of GNSS observations each day to derive precise station coordinates, satellite orbits, satellite clock corrections, and Earth rotation parameters. In line with the current IGS product policy we provide geodetic products in ultra-rapid (latency 3-9h), rapid (latency (1 day) and final (latency two weeks) mode. Currently, GPS and GLONASS are processed.
All three processing lines follow the same strategy. Based on a well-defined subset of IGS tracking stations (see Figure 1) a least-squares adjustment is used to derive the unknowns. Within this network approach, satellite orbits and clock corrections (i.e., deviation of the satellite clock with respect to the GPS time) are determined. For GPS observations phase ambiguities are fixed. The generation of IGS products follows the least-squares adjustment and depends on the processing line. The ultra-rapid line provides orbits (15-minute sampling) and clock corrections (5-minute sampling) only but includes predictions for both for the next 24h.
These products are of interest to users with real-time or near real-time applications. The daily processing provides also orbit and clock corrections but also Earth rotation parameters. The most precise results are derived in the final processing carried out for the latest week. Here, orbits are determined by stacking daily orbits to three-day arcs which minimize orbit jumps at the day boundaries. Satellite clocks are provided also with 30-second sampling which is very helpful for PPP users. Besides, Earth rotation parameters and station coordinates are provided in the ERP and the SINEX format, repsectively. All products are provided in the current reference frame (i.e., IGb14 since May 17th, before IGS14). A detailed description of our GNSS processing strategy can be found in the GFZ Analysis Center Notes.
The processing and product generation is performed using the GFZ developed EPOS.P8 software which is in line with the current geodetic conventions. The high accuracy of our products is ensured by the comparisons and combinations done by the IGS Analysis Center Coordinator.