Near-Real Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and GRACE

Precise and rapidly available orbits of GPS and Low-Earth-Orbiting (LEO) satellites are the prerequisite for processing of the radio occultation data from CHAMP, GRACE and other LEOs performing occultation measurements. For efficient occultation data assimilation by the weather prediction systems a 3 h timeline is required. In 2002 GFZ has started to generate orbits at fixed 3 h intervals with a mean latency of 2.2 h, so-called Ultra-rapid Science Orbits (USOs). The resulting delay of the occultation products was in the range 3–5 h, sufficient for building an operational radio occultation system and for performing data assimilation studies in weather centers. To better meet the 3 h timeline, a new Near-Real Time (NRT) orbit processing system was developed to generate GPS and LEO orbits every 1.5 h with low latency, typically less than 0.5 h. This reduces the average delay for occultation products to 1 h 45 min. The NRT system consists of three independent chains generating LEO orbits with latencies ranging from 13 to 30 min and accuracies in the range 6–10 cm validated by Satellite Laser Ranging (SLR).