Towards 1-cm Orbits

TOPEX/POSEIDON (T/P) has demonstrated that using radar altimetry, the time variation of ocean topography can be determined with an accuracy of a few centimeters, and has also established the new capability for monitoring global sea level change with a precision of about 1 mm/year. This has become possible due to the high radial accuracy (2-3 cm) achieved for the T/P orbits, whereas in previous missions orbit error dominated the altimeter error budget. Jason, the T/P follow-on, will continue measurement of the ocean surface using radar altimetry with the same, if not better accuracy. Reaching the Jason centimeter accuracy goal would greatly benefit the knowledge of ocean circulation. For example this would reduce the present errors in estimates of time rates of change in oceanic heat flux divergence to values close to those anticipated for greenhouse gas increase, as well as obtain clearly sub-millimeter accuracy in determining global sea level rise, currently estimated to be 1-2 mm/year. What is required for achieving 1-cm radial orbit accuracy? Orbit accuracy depends on the fidelity of the force and measurement models, and quality of tracking data. Simulated SLR, GPS, and "perfect" tracking of the T/P and Jason satellites were studied using GEODYN, applying dynamic and reduced-dynamic strategies , current and anticipated error budgets, to evaluate error sensitivity and Precision Orbit Determination (POD) capability. Simulation studies indicate that achieving the 1-cm goal poses a major challenge, but is possible with sufficiently precise and dense tracking. Combination solutions such as GPS/SLR offer more promise than using GPS alone. Altimeter crossover data, although not evaluated in the simulations, also offer a very strong data type for POD. Possibly significant improvements to current T/P POD with the inclusion of altimeter crossover data are evaluated using actual data, and also presented.