Application of nonlinear generalised minimum variance to the nadir problem in 2-axis gimbal pointing and stabilization

Precision tracking applications using two-axis gimbal or antenna actuation systems suffer from a singularity when the inner axis reaches +-90 degrees. This is known by various terms - the keyhole singularity, gimbal lock or the nadir problem. Practically, sightline control is degraded and often lost in a neighborhood of this singularity. In this paper, two nonlinear control algorithms are applied to sightline pointing and stabilization control in the neighborhood of the nadir; the traditional cosecant correction and the nonlinear generalized minimum variance technique. Both controllers were tested against a validated model of an Aeromech TigerEye turret.