Online Semisingularity Avoidance for Three-Axis Tracking Pedestals

Three-axis tracking pedestals eliminate the keyhole problem and are widely used in whole-space-tracking missions. Owing to their rotary limits, however, such pedestals have semisingular postures. In these postures, they cannot follow targets moving in certain directions. More critically, when using common unilateral constraint avoidance approaches, there always exists a target movement for which the pedestal will be guided into semisingular configurations. This is a fatal defect when tracking noncooperative targets. To solve this serious problem, we propose a novel online solution for semisingularity avoidance. A reduced configuration space is developed so that the pedestal does not have semisingularity. We propose a general method to derive such a configuration space for the three-axis tracking pedestal under arbitrary elevation and heeling-motion limits. Experimental results clearly show that, with the proposed approach, the pedestal can effectively avoid semisingularity with a very smooth posture trajectory.