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The system was selected because its high frequency did not degrade with depth. The search phase used the 100-kilohertz frequency on a 500-meter range (1,000-meter swath) to acquire targets, then the 400-kilohertz frequency classified the target using a 75-meter range (150-meter swath). The second payload was a high-resolution digital camera with LED strobe built by the Woods Hole Oceanographic Institution (WHOI), which was used to photograph mosaic targets that had been classified by the high- frequency side scan to be of interest. AUV Positioning Accuracy Vehicle positioning accuracy was a major mission concern since the high-frequen- cy sonar runs required short ranges to classify targets of interest. The positioning con- trol to run a photomosaic mission required a line spacing of 3 meters. The AUV's positioning capability was tested using a target designed to test both the sonar target position accuracy and the photomosaic line control. The target was placed on the seafloor at 1,500 meters depth. An acoustic beacon was attached to the target so that a precise position could be calculated as a bench- mark to evaluate the target position error generated from the sonar and photomosaic. The test results exceeded expectations by confirming that the AUV positioning of the target from the sonar had an error of less than 6 meters, with the average about 5 meters, and that the AUV could maintain a 3-meter line spacing to photograph the target to produce a quality photomosaic. Side Scan Sonar Performance The 2200-M's low frequency was tested for range capabil- (Top) A cable detected on 1,000-meter swath. (Middle) A 200-plus- year-old British shipwreck detected on 1,000-meter swath. (Bot- tom) The 500-kilohertz classification of a 200-plus-year-old Brit- ish shipwreck at 75-meter-range scale. ity to verify that the 100 kilohertz would cover a 1,000-meter swath with no range fall off. The range performance met all mission requirements. Detection of small targets in the long range, such as rope and cable, was accomplished. The long search range showed shipwrecks very well and allowed large areas to be searched with high coverage rates per dive. The 400-kilohertz sonar produced detailed high-resolution images that aided in classifying whether a feature located dur- ing the long-range reconnaissance mode was the remains of a shipwreck. The classification mission used a clover-leaf pattern that pro- vided multiple aspects of the target for analysis, which is not possible to achieve with a deep-tow system. When a feature was classified as a wreck site, the sonar payload was removed from the AUV and replaced with the camera payload. The pay- load switch took a couple of hours, resulting in minimal down time. Sonar Data Processing A typical dive averaged 24 hours and generated about 15 gigabytes of sonar data. After each dive, the vehicle was recovered, data downloaded and the batteries swapped for the next (Top left) The AUV's clover-leaf survey pattern for clas- sification. (Top right) The 1,800-square-mile mosaic of AUV-collect- ed sonar data. (Left) Photomosaic of a 200-plus-year-old British ship- wreck. www.sea-technology.com SEPTEMBER 2012 / st 11