Geodetic applications of the ROCSAT-3/COSMIC mission

The jointly planned Taiwan-US space mission ROCSAT3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) is scheduled for launch in 2001. A 6-8 low Earth orbiting satellite constellation will conduct atmospheric limb-sounding using GPS radio occultation. Although not the primary scientific objective, it is envisioned that the initial low altitude orbits (similar to 400 km) of the satellites (some in tandem) will provide unique opportunities for gravity mapping and precision orbit determination experiments. Simulations show that, depending on the orbit evolution scenario, the use of these data can yield at least an order of magnitude improvement over the state-of-the-art global gravity model EGM96 out to degree and order 20-40 (spatial resolution of 1000-500 km). This improvement would be very useful in anticipation of missions that focus on the temporal variations of the field, such as GRACE, and for current and future altimeter missions. Additionally, useful signals of low-degree temporal variations can be obtained if non-conservative forces remain "modelable" at the final altitude of above 700 km. Some of the geodetic benefits that can be expected from ROCSAT-3/COSMIC include: (a) simultaneous improvement of the GPS satellites' orbits within a few hours from data collection, essential to all GPS applications (a factor of two improvement can be expected over the current accuracy at 5-10 cm), (b) improved estimation of the global surface pressure fields, useful for geodynamic and altimetric studies such as Earth rotation, geocenter motion, time-varying gravity, and ocean circulation, and (c) inclusion of satellite laser ranging reflectors, proposed as a secondary tracking system will help establish local datum ties to the conventional terrestrial reference frame.