Developments in Adaptive Filtering and System Identification

Office of Naval Research

Friday, August 01 2008

Potential applications include optical communications, target tracking, and image processing.

Progress has been made on several fronts in a continuing program of research oriented toward the development of real-time algorithms, and computer and control systems that utilize the algorithms, for adaptive filtering, prediction, and system identification with improved efficiency and numerical stability in applications that involve large numbers of channels and high filter orders. Potential applications include adaptive optics, optical communications, target tracking, image processing, blind identification and deconvolution in wireless communications, and active control of noise and vibration.

The program has included three main subprograms supporting different aspects of research and development at the Air Force Research Laboratory on directed-energy weapons and laser communications:

• Control (more precisely, suppression) of laser-beam jitter.

The most important sources of laser beam jitter are platform (aircraft) vibration and atmospheric turbulence. Jitter typically consists of multiple narrow frequency-band components, often combined with broad-frequency-band disturbances. The need for adaptive (in contradistinction to non-adaptive) control or suppression of jitter arises because the frequency content of jitter varies as different platform modes are excited and different atmospheric conditions are encountered. No linear time-invariant (LTI) controller can control all disturbance bandwidths optimally. By adapting to the particular frequency content of any disturbance, adaptive control effectively extends the bandwidth of even robust, high-performance LTI feedback controllers.

In this subprogram, lattice-filter- based subspace system- identification and adaptive control algorithms were applied in laser- beam-steering experiments. It was shown that enhanced rejection of disturbances is achievable in laser-beam steering by use of modern optimal feedback controllers augmented with adaptive control loops that determine control gains that are optimal for the current disturbance acting on the laser beam. In each adaptive control loop, an adaptive lattice filter implicitly identifies disturbance statistics from real-time data on the relative orientation of the laser beam as measured by use of a quad cell.