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.

For several future imaging and communications spacecraft, a challenging area of technology development is the fine acquisition, tracking, and pointing (ATP) control of the spacecraft and its payload. For example, some spacecraft with large aperture(s) in the range of 10~30 m diameter requires a few arc-seconds accuracy, 10~15 nano-radians jitter, and a fast slewing rate to acquire the target. Furthermore these stringent requirements are at risk of great structure and control interactions. This paper we will focus on the control of optical beam jitter. A Laser Jitter Control (LJC) testbed has been constructed to test jitter algorithms. The testbed consists of two fast steering mirrors (FSM), three position sensing modules (PSM), one diode laser, and several beam splitters and mirrors, all on an isolated Newport optical bench. Jitter is injected with one FSM and the other FSM is used to control it. The jitter spectrum, representing the on-orbit spacecraft and beam jitter environment, contains not only narrow band noise due to rotating devices such as gyroscopes and reaction wheels but also broadband noise. The performance of a Wiener Filter -adaptive algorithm with ideal reference signal is established as the baseline for comparison of adaptive control methods in suppressing both broadband and narrowband disturbances. Specifically, the Least Mean Squares (LMS) approach and the Gradient Adaptive Lattice (GAL) approach are investigated during these experiments.

This paper presents the recent development in algorithms for active vibration isolation on spacecraft using a Stewart platform. The multiple error least mean square (LMS) algorithm and the clear box algorithm have been implemented on these platforms and several enhancements have been made to the clear box algorithm. Based on experimental results, it is concluded that the multiple error LMS algorithm is preferred for vibration isolation when a disturbance correlated signal is available. In the absence of such a signal, the clear box algorithm is the method of choice. Among the implementations of the clear box algorithm, the sine/cosine method is preferred for handling time-invariant disturbance frequencies, the adaptive method for rapidly varying disturbance frequencies, and the frequency-domain method for a large number of time invariant disturbance frequencies.

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The low-frequency flexible modes of solar arrays on spacecraft are excited by the minimum-time bang-bang control during a slew maneuver. These flexible modes limit the control bandwidth and degrade the pointing error at the end of slew maneuver, resulting in a longer settling time. The objective of this investigation is to develop and verify methods to shape the torque profile of reaction wheels for the slew maneuver such that, at the end of slew, vibration are minimized and the settling time is reduced. Effectiveness of the smooth versine input profiles, torque feedforward, and input shaping are accessed. Both numerical simulation results and ground simulated experimental results are presented in this paper.

Free-Space Optical Communications requires stable and precise pointing on spacecraft to maintain optimal operating condition. One technology to achieve high precision pointing and steering for imaging or laser communication payload in the presence of vibrations is to make use of a Stewart platform with active struts. At the Spacecraft Research and Design Center (SRDC) of Naval Postgraduate School (NPS), a hexapod with voice coil actuators is equipped with in line accelerometers to perform the tasks of vibration isolation. For the tasks of pointing for optical payload, we adopted the approach of external measurement system, which provides directly the position and orientation information of the hexapod top platform. A laser metrology system, composed of three pairs of laser diodes and detectors, is design and fabricated at NPS. In this paper we document its development process, which includes the design, analysis, inspection and preliminary calibration effort.

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Fundamental concepts of Iterative Learning Control (ILC) are applied to path generating problems in mechanisms. As an illustration to such class of problems, an adjustable four-bar linkage is used. The coupler point of a four-bar traces a coupler curve that will in general deviate from the desired coupler path. Except at the precision points, the coupler curve will exhibit some structural error, which is the deviation from the specified curve. The structural error will repeat itself every cycle at exactly the same points over the range of interest. Since ILC is a methodology that was developed to handle similar repetitive errors in control systems, it is believed that it will be well served to apply it to this class of problems. Results show that ILC can be simple to implement, and it is found to be very well suited for such path generation problems.