The ability to extract target features from a ladar Range Resolved Doppler Image (RRDI) can enable target detection, identification, discrimination, and status assessment. Extraction of these features depends on the image processing algorithms, target characteristics, and image quality. The latter in turn depends on ladar transmitter and receiver characteristics, propagation effects, target/beam interactions, and receiver signal processing. In order to develop hardware systems and processing algorithms for a specific application, it is necessary to understand how these factors interact. A modular, high fidelity ladar simulation has been developed which provides modeling of each step from transmitter, through feature extraction.
In last years proceedings, the above authors reported a basic limitation on the maximum effective bandwidth when tracking through atmospheric turbulence. This limitation, called the optical frequency, was shown to be an upper limit on tilt detection. This paper will further expand on this fundamental limitation. Further testing at the MIT/Lincoln Laboratory has provided more insight into the optical frequency as well as other tracking limitations. It will be shown in this paper that scintillation appears to be dominant above the optical frequency and that by wisely selecting the bandwidth of the tracking system, one can exclude some of the noise of scintillation, while still performing the best possible tracking.
Tracking through a turbulent atmosphere poses several challenging problems. The authors have recently conducted a series of tracking tests at a MIT/Lincoln Laboratories facility where a complete tracking and adaptive optics system is available in a laboratory. The atmosphere is simulated using seven precision rotating phase screens. A great deal has been learned about tracking algorithms and their response under a scintillated atmosphere. Data will be shown to describe a key limitation to high bandwidth tracking. This effect, called the `Optical Frequency', appears to be an upper bound on track bandwidth when using an image based tracking system.
Tracking through a turbulent atmosphere has several challenges. If the target is extended, such as a large target being illuminated with laser energy from the tracking aperture, the problems of scintillation and anisoplanatism cause significant concerns. The 'blotches' caused by scintillation can be interpreted by the tracking algorithms as tilt and incorrectly applied to the steering mirror. Similarly, anisoplanatism imparts tilt components from separated points, that may be independent, and not act as coherent tilt that is possible to correct with one mirror. The authors have been involved in a study that compares the performance of 14 different tracking algorithms under scintillated conditions. Image data, available from Lincoln Laboratory, has been used to calculate tilt from the different algorithms and do comparisons using Power Spectral Density analysis. The results show that different algorithms have significantly different performance characteristics.
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