KEYWORDS: Beam steering, Control systems, Telecommunications, Bragg cells, Laser systems engineering, Laser communications, Control systems design, Acquisition tracking and pointing, Electronic filtering, Filtering (signal processing)
For many practical needs, laser communication systems must support operation between mobile platforms.
Engineering robust links; however, will depend on several innovations. In particular, successful pointing,
acquisition, and tracking (PAT) require the use of a beacon signal and the capability of accurate and agile
alignment of the line-of-sight (LOS) between the communicating terminals performed over a large field of
regard. While mechanical devices, such as gimbals, offer relatively slow tracking over a very wide range,
they lack in pointing bandwidth necessary for rejecting high frequency vibrations and beam deflection
caused by the optical turbulence. In contrast, fast steering and especially non-mechanical devices, such as
Bragg cells, enjoy very high bandwidth (on the order of several kHz), but their effective range is very
small. Inherent limitations of both gimbals and fast steerers result in shortcomings of the entire PAT
system when either of these devices is used as a sole beam steerer. Therefore, focus needs to be shifted to
hybrid architectures, exploiting the advantages of the constituting elements. This paper demonstrates a
system combining a robotic manipulator with two acousto-optic cells and presents the algorithm
development and the simulation results.
Omniwrist III is a new sensor mount developed under Air Force funding that emulates the kinematics of a human wrist. Driven by two linear motors and computer controlled, it is capable of a full 180° hemisphere of pitch/yaw motion. A comprehensive laboratory testing of one of few existing devices of this type, installed in the Laser Research Laboratory at Binghamton University, has resulted in the establishment of a complete transfer matrix-type model relating pitch/yaw coordinates of the sensor mount to the voltage signals applied to the motors. Although dynamic characteristics of the device are position-dependent, it has the potential for exceeding bandwidth and positioning accuracy of a traditional gimbals system at least by the factor of ten. The device is suitable for the application of the most advanced control strategies that will result in the further enhancement of its dynamic performance thus extending the scope of its application to various problems of satellite communications, LADAR, laser weapon systems, etc. This study is aimed at the investigation of the best performance characteristics (bandwidth, tracking error, cross-coupling effects, etc.) attainable under advanced control laws. The authors intend to consider implementation of such control laws as optimal control utilizing dynamic programming, gain scheduling, and fuzzy logic control. The results of this research will be incorporated in the future papers. It is shown that Omniwrist III with the appropriate controls could be considered as a new generation of gimbals system.
KEYWORDS: Bragg cells, Control systems, Control systems design, Acousto-optics, Laser communications, Kinematics, Beam steering, Actuators, Telecommunications, Laser systems engineering
Omniwrist is a new sensor mount developed under the Air Force funding that emulates the kinematics of a human wrist. Driven by two linear motors and equipped with a dedicated computer implementing advanced control laws, it is capable of a full 180° hemisphere of pitch/yaw motion and demonstrates performance characteristics comparable with an electro-mechanical beam steering system. While exceeding the bandwidth requirements for the coarse beam steering task, Omniwrist’s dynamic response is much slower than the one of the acousto-optic device (Bragg cell) that is virtually inertia-free. At the same time, the steering range of a Bragg cell, ± .5°, is too small for many applications. The authors have been successful in the enhancement of the design and development of control laws improving its dynamic characteristics of a Bragg cell. This paper presents the research aimed at the development of a hybrid laser beam steering system comprising Bragg cells installed on the Omniwrist platform. An optimal control strategy facilitating such applications as scanning, search, rapid repositioning, tracking, feedback and feedforward compensation of environmental vibration of the optical platform (satellite-based and airborne) has been developed, implemented and tested. This includes the solution of such underlying problems as mathematical description of the hybrid system, optimal task distribution between the “coarse” and the “fine” positioning tasks, coordination of the operation of the “coarse” and “fine” system controllers. The efficiency of the developed system in various applications will be investigated further and compared against known designs.
Space based optical communications offer several advantages over traditional RF systems. They include: smaller beam divergence, smaller antennas, higher data rates, low probability of intercept, reduced EMI, and low probability of jamming. Additionally, the potentials for light weight, small volume and low power terminals make laser communications a consideration for several potential DOD programs. There have been may proposed configurations for both the laser communication terminal and the satellite network but architectures have remained fairly fluid. Despite these changes, there are several enabling technologies that must be fostered to meet program requirements. Efforts at Rome Lab are currently directed to the development of higher powered laser transmitters; rapid and accurate pointing, acquisition, and tracking systems; multiple channel operation; and sensitive low noise receivers. This paper will provide an overview of these efforts.
A technique, resulting in the significant reduction of negative effects of satellite jitter on the accuracy of laser beam positioning in satellite communication systems, is presented. It implies application of vibration monitoring instrumentation for estimation of the motion components constituting jitter. A self-tuning feedforward jitter compensation scheme is introduced to the existing mirror-positioning control systems. The technique is verified by computer simulation and is in the process of experimental investigation.
A mathematical model of a pointing, acquisition, and tracking (PAT) system intended for laser-based intersatellite communication is developed and implemented in software in the form of a simulator. It represents major dynamic channels of the system and complex interaction of operational conditions, disturbances, and design parameters thus facilitating the solution of various analysis, design, and control problems. Application of computer graphics and animated schematics, driven by the model, provides accurate display of the most intricate phenomena behind the PAT operation. Coupled with a working laboratory prototype of the PAT system through a data communication board, the simulator is capable of displaying both simulated and real variables serving as a diagnostic tool. The simulator has been applied for development and verification of a novel jitter compensation scheme.
Application of very fast CCD detectors to both fine pointing and tracking and to low data rate reception for free space optical communications has been demonstrated in several prior programs. CCD detector based wide field multi-channel receivers have been considered for several new applications. We are conducting a program whose goal is evaluation of radiation induced degradation in the properties of CCD devices for use in such systems. In this paper we present initial results of this ongoing effort. We present a brief overview of systems design and detector design for multi-channel low rate data receivers. Following this we present results from a comprehensive experimental determination of CCD performance degradation due to ionizing radiation. Ionizing radiation testing was accomplished using a Co60 source at the Salk Institute. Dose levels up to 1 Mrad were utilized in this test activity. We will also briefly discuss aspects of displacement damage testing activity at the University of Rochester Tandem VandeGraaff accelerator. Primary emphasis of this effort will be on evaluation of charge transfer efficiency changes due to displacement damage. Special attention is given to charge trapping phenomena and their impact on CTE. A review of test results for the ionizing radiation test portion of our program is given.
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