Direct versus indirect line of sight (LOS) stabilization

Two methods are analyzed for inertially stabilizing the pointing vector defining the line of sight (LOS) of a two-axis gimbaled laser tracker. Mounting the angular rate and acceleration sensors directly on the LOS axes is often used for precision pointing applications. This configuration impacts gimbal size, and the sensors must be capable of withstanding high angular slew rates. With the other stabilization method, sensors are mounted on the gimbal base, which alleviates some issues with the direct approach but may be less efficient, since disturbances are not measured in the LOS coordinate frame. This paper investigates the impact of LOS disturbances and sensor noise on the performance of each stabilization control loop configuration. It provides a detailed analysis of the mechanisms by which disturbances are coupled to the LOS track vector for each approach, and describes the advantages and disadvantages of each. It concludes with a performance comparison based upon simulated sensor noise and three sets of platform disturbance inputs ranging from mild to harsh disturbance environments.