Assessment of auxiliary data processing

An orbit and attitude determination system is presented. Two auxiliary processing algorithms (Doppler tracker for estimating beam center Doppler frequency and autofocus for estimating Doppler FM rate) are described and their performances with SEASAT-1 SAR data are evaluated. Accuracy is achieved with little change to the satellite platform (apart from real time transmission of attitude measurements). The orbit determination system uses S band range and range rate measurements made at ground tracking stations, giving up to 50 m accuracy for the tracking period itself, and 500 m accuracy for 12-hr predictions. The PFM Earth reference attitude sensor data transmitted to the ground during the pass are available for fast delivery processing. Measurement accuracy is 0.05 deg about each axis. The tracking algorithm estimates are converted to absolute values, using the attitude and orbit data. The autofocus algorithm is only used if orbit data are unavailable at processing time. Real time auxiliary data used for fast delivery processing give 1 km location accuracy for ice monitoring or ship routing. Accuracy within 100 m is possible, using full attitude and orbit data.