The availability of orbit information with high precision and low latency is a key requirement for many Earthobservation missions, predominantly in the field of radio occultation. Traditionally, precise orbit determination solutions of low-Earth orbit (LEO) satellites are done offline on the ground after dwonloading GNSS measurements and required auxiliary data, like satellite attitude, to the processing center. The latency of this processing depends on the frequency of LEO down-link contacts and the latency of the precise GNSS orbit and clock products required for the orbit determination process. These dependencies can be removed by computing the precise orbit determination solution onboard the satellite using GNSS broadcast ephemerides. In this study, both real data and simulated measurements from a representative LEO satellite are processed in a flight-proven Kalman-filter algorithm. The paper studies the use of GPS, Galileo and BeiDou-3 for real-time orbit determination in different combinations with simulated measurements. Results show that the use of dual-frequency Galileo and BeiDou-3 measurements leads to a significant reduction of 3D rms orbit errors compared to GPS-only and achieves a positioning accuracy of about one decimeter.