The availability of orbit information with high precision and low latency is a key requirement for many Earth-observation missions, predominantly in the field of radio occultation. In this paper, onboard precise orbit determination using GNSS broadcast ephemerides is studied. For this purpose, both real data and simulated measurements from a representative LEO satellite are processed in a flight proven Kalman-filter algorithm. Preliminary results show that the use of Galileo leads to a significant reduction in 3D rms orbit errors compared to GPS. The full paper will study the use of GPS, Galileo and BeiDou-3 using different signal combinations as well as the new GPS CNAV data. Expande Abstract file uploaded.