Exchanging satellite attitude quaternions for improved GNSS data processing consistency

The orientation of GNSS satellites in space is a key quantity in GNSS data processing. It is required to correctly apply several force models and observation corrections. Incorrect or insufficient modeling of satellite attitude can lead to inconsistencies both among providers of GNSS products and between providers and users, which negatively affects the quality of combined products and positioning solutions. Exchanging satellite attitude information in the form of quaternions helps to eliminate or reduce those inconsistencies. This study presents the first systematic exchange of attitude data in a common format (called ORBEX) within the International GNSS Service (IGS). A comparison of attitude auxiliary data from seven IGS analysis centers over a one-year period shows significant differences for GPS and GLONASS satellites, but not for Galileo. These differences are mostly confined to eclipse periods but can reach up to 180°. Considering the attitude differences between analysis center solutions in an experimental clock combination significantly improved the consistency of analysis center solutions during eclipse periods. A kinematic PPP analysis covering ten globally distributed stations over one week resulted in an almost unanimous improvement when using the provided satellite attitude instead of a nominal model, with an average RMS reduction of 50% in the east and up components. The results presented in this study show that satellite attitude is important GNSS auxiliary data and making it available benefits both providers and users.