Jitter Control of Space and Airborne Laser Beams

Abstract

Acquisition, Tracking and Pointing (ATP) of space and airborne laser beams is fast becoming an important research topic as requirements for the pointing and control of the optical beam is increasing. Arc-second accuracy, nano-radian jitter and large flexible structures combine to require stringent pointing requirements testing the limits of control systems. Solar arrays, reaction wheels, control-moment-gyros for spacecraft and airframe, engine, and payload configuration for aircraft result in narrowband as well as random structural interactions that further complicate the control method. Additionally, the effect of the atmosphere on the laser adds a broadband disturbance, resulting in a laser beam that has been corrupted by colored noise. This paper will focus on the control techniques that may be used to remove the disturbances. A Laser Beam Jitter Control test bed (LJC) has been developed at the Naval Post Graduate School and is used to investigate different control algorithms for Fast Steering Mirrors (FSM). The test bed consists of two FSMs, three position sensing detectors (PSD), one diode laser, and several beam splitters and mirrors, all sitting on a vibrationally isolated Newport optical bench. The control mirror, along with beam splitters and folding mirrors, is mounted on a platform isolated from the optical bench. The platform is shaken by a CSA SAS-5 (5 lb.) inertial actuator. Colored noise is injected with one FSM and the other FSM is used to control it. The disturbance spectrum contains not only narrow band noise from the shaken platform simulating rotating devices onboard such as reaction wheels but also broadband noise from a disturbance FSM, separate from the platform. Several adaptive feedforward and feedback control algorithms are tested with this disturbance on the LJC and are compared.