In practice, environmental information about an ocean bottom area to be searched using SONAR is often known a priori to some coarse level of resolution. The SONAR search sensor then typically has a different performance characterization function for each environmental classification. Large ocean bottom surveys using search SONAR can pose some difficulties when the environmental conditions vary significantly over the search area because search planning tools cannot adequately segment the area into sub-regions of homogeneous search sensor performance. Such segmentation is critically important to unmanned search vehicles; homogenous bottom segmentation will result in more accurate predictions of search performance and area coverage rate. The Naval Surface Warfare Center, Panama City Division (NSWC PCD) has developed an automated area segmentation algorithm that subdivides the mission area under the constraint that the variation of the search sensor’s performance within each sub-mission area cannot exceed a specified threshold, thereby creating sub-regions of homogeneous sensor performance. The algorithm also calculates a new, composite sensor performance function for each sub-mission area. The technique accounts for practical constraints such as enforcing a minimum sub-mission area size and requiring sub-mission areas to be rectangular. Segmentation occurs both across the rows and down the columns of the mission area. Ideally, mission planning should consider both segmentation directions and choose the one with the more favorable result. The Automated Area Segmentation Algorithm was tested using two a priori bottom segmentations: rectangular and triangular; and two search sensor configurations: a set of three bi-modal curves and a set of three uni-modal curves. For each of these four scenarios, the Automated Area Segmentation Algorithm automatically partitioned the mission area across rows and down columns to create regions with homogeneous sensor performance. The testing results indicated that the algorithm correctly segmented the rectangular a priori regions. For the triangular a priori segmentation, the algorithm created reasonable rectangular sub-areas.
In this problem an identification vehicle must re-acquire a fixed set of suspected targets and determine whether each suspected target is a mine or a false alarm. If a target is determined to be a mine, the identification vehicle must neutralize it by either delivering one of a limited number of on-board bombs or by assigning the neutralization task to one of a limited number of single-shot suicide vehicles. The identification vehicle has the option to reload. The singleshot
suicide vehicles, however, cannot be replenished. We have developed an optimal path planning and reload strategy for this identify and destroy mission that takes into account the probabilities that suspected targets are mines, the costs to move between targets, the costs to return to and from the reload point, and the cost to reload.
The mission is modeled as a discrete multi-dimensional Markov process. At each target position the vehicle decides based on the known costs, probabilities, the number of bombs on board (r), and the number of remaining one-shot vehicles (s) whether to move directly on to the next target or to reload before continuing and whether to destroy any mine with an on-board bomb or a one-shot suicide vehicle. The approach recursively calculates the minimum expected overall cost conditioned on all possible values r and s. The recursion is similar to dynamic programming in that it starts at the last suspected target location and works its way backwards to the starting point. The approach also uses a suboptimal traveling salesman strategy to search over candidate deployment locations to calculate the best initial
deployment point where the reloads will take place.
An advanced capability for automated detection and classification of sea mines in sonar imagery has been developed. The advanced mine detection and classification (AMDAC) algorithm consists of an improved detection density algorithm, a classification feature extractor that uses a stepwise feature selection strategy, a k-nearest neighbor attractor-based neural network (KNN) classifier, and an optimal discriminatory filter classifier. The detection stage uses a nonlinear matched filter to identify mine-size regions in the sonar image that closely match a mine's signature. For each detected mine-like region, the feature extractor calculates a large set of candidate classification features. A stepwise feature selection process then determines the subset features that optimizes probability of detection and probability of classification for each of the classifiers while minimizing false alarms.
Coastal Systems Station has developed an approach for automatic mine detection and classification. The Detection Density ACF Approach was created by integrating the adaptive clutter filter (ACF) developed by Martin Marietta, the specification of target signature suggested by Loral Federal Systems, and the Attracted-Based Neural Network developed at NSWC Coastal Systems Station with a detection density target recognition criterion. The Detection Density ACF Approach consists of eight steps: image normalization, ACF, selecting the largest ACF output pixels, convolving the selected pixels with a minesize rectangular window, applying a Bayesian decision rule to detect minelike pixels, grouping the minelike pixels into objects, extracting object features, and classifying objects as either a mine or a nonmine with a neural network. When trained on features extracted from 30 sonar images and tested against another 30 images, this approach demonstrates very good performance: probability of detection and classification (pdpc) of 0.84 with a false alarm rate of 1.4 false calls per image. A performance analysis study shows that the detection density ACF approach performs very well and significantly reduces the false alarm rate.
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