Digital attitude reference system (DARS)

A family of digital attitude reference systems (DARS) is developed for three-axis-stabilized, earth-oriented satellites. It includes the complete DARS configuration, which utilizes measurements from a sun sensor in addition to the horizon sensors and gyros in use on conventional analog gyrocompassing-type reference systems (CARS). The complete configuration also utilizes statistically optimal (Kalman) filtering to process these measurements to estimate and compensate for important measurement errors as well as to estimate the spacecraft attitude. Various suboptimal configurations of DARS are also studied. Based on simulation results, the attitude accuracies of the two most promising DARS configurations are approximately 0.03 and 0.04 deg, respectively, and the convergence time of both systems is less than 20 sec. Thus, DARS is shown to fill the important gap between low-accuracy gyrocompassing systems (using horizon sensors and gyros) and high-accuracy systems (requiring star sensors or trackers) by achieving intermediate attitude accuracy at the expense of low-to-medium complexity. (Author)