Processing of GNSS constellations and ground station networks using the raw observation approach

This article describes the raw observation approach as implemented at Graz University of Technology to determine GNSS products like satellite orbits, clocks, and station positions. To assess the performance of the approach, 15 years (2003–2017) of observations from a network of 245 globally distributed IGS stations to the GPS constellation were processed on a daily basis using the IGS14 reference frame and antenna calibrations. The resulting products are evaluated against those determined by IGS analysis centers. Orbit fit quality relative to the IGS combination is comparable to the best-fitting solutions used for evaluation. Starting from early 2017, when the IGS switched to IGS14, the determined orbits fit better to the IGS combination than any other considered solution. Midnight discontinuities show good internal orbit consistency and no noticeable satellite block-dependency. Satellite clocks are comparable to the considered IGS analysis center solutions. Station positions differ from the IGS combination on a similar level to the solutions they were evaluated against. The temporal repeatability of station positions is slightly better than that of the IGS combination. The quality of resulting GNSS products confirms that the raw observation approach is well suited for the task of determining satellite orbits, clocks, and station positions. It provides an alternative to well-established approaches used by IGS analysis centers and simplifies the introduction of additional observables from new and modernized GNSS.