A Self-Serving Optical Correlator For Tracking

Vander Lugt correlators are limited by the sensitivity of their Matched Spatial Filters (MSFs) to image scale, object orientation, and aspect angle. For real-world applications, the number of filters required to identify and track an object, in any relative position, is large. A solution to this problem is the use of an addressable filter (i.e. a Spatial Light Modulator) which preserves the system's optical alignment even though the filter has changed, and feedback from the correlation plane which servos the input scene display. Perkin Elmer has built a correlation system capable of tracking a real target, in any rotational orientation, over a 16:1 change in scale, in real-time. The system uses two Liquid Crystal Light Valves (LCLVs), optically addressed by CRTs at video frame rates. One serves as the correlator's input and operates in an amplitude mode; the second functions in a phase-only mode and serves as the Matched Spatial Filter. Three spatially multiplexed Binary Phase-Only Filters (BPOFs) operate on the fourier transform of the scene in parallel. Each filter acts independently, and produces a correlation spot. The center BPOF is computed from a slightly magnified image of the target object, and the adjoining filters are based upon correctly scaled, but slighty counter-rotated images of the target. By cross-referencing the three correlation outputs, the type of scene distortion (scale and/or rotation) can be determined, and the input scene CRT raster can be adjusted to maintain the optical match required for continuous correlation. This allows a MSF to operate over a much wider range of conditions than previously possible. By monitoring the scale of the scene CRT's raster, the system can determine when the next addressable filter must be called up from memory and displayed to continue correlation. Two computers run the software for the system, which contains image processing programs to capture input video images, generate filters, rotate and scale the scene CRT's raster scan to preserve correlation performance, change filters when the limits of raster scan size are exceeded, and track the target. The system works in conjunction with a remote gimballed telescope platform that provides the input video imagery. Tracking error information from the computers is telemetered to the platform to maintain the line of sight to the target.