Methods of Attitude Determination and Control

Having seen how the attitude determination and control process works, we summarize in this section the commonly used reference vectors and control torques and the disturbance torques which cause the spacecraft to drift. Table 1-1 lists the various reference sources commonly used for attitude determination. Basically, two alternatives exist: we may either measure the attitude with respect to some external reference vector (the first four items in the table) or we may measure the centrifugal acceleration (the last item listed) to determine the change in the orientation. The latter is referred to as inertiaI guidance and is done by gyroscopes or accelerometers. For space flight, the main problem with inertial guidance is that it depends on integrating small changes in the attitude to propagate the orientation in inertial space from some known initial value. Therefore, small errors accumulate, and periodic updates based on some external reference source are required. One such combination is the use of gyroscopes and star sensors. The efficient use of star sensors typically requires a fairly accurate initial attitude estimate which is then refined by the star sensor data. When a spacecraft undergoes a maneuver from one orientation to another, the gyroscopes provide an accurate measure of the change in orientation and a good initial estimate of the new attitude. This estimate is then refined using the star sensor data. The refined attitude is used both as a measure of the true spacecraft orientation and to update the gyroscopes to eliminate accumulated errors since the previous update.

In general, the strain and torque on spacecraft are several orders of magnitude less in the space environment than they would be at the Earth's surface. Nevertheless, torques that perturb the attitude do exist. The major environmental torques

Table 1-1. Attitude Determination Reference Sources
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