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The Naval Special Warfare (NSW) Technology Program is presently investigating the development of a dual-frequency, acoustic lens, sonar system. The lower frequency will provide detection of targets buried near the watersediment interface; the higher frequency will provide imaging of non-buried targets. The sonar system is being developed in modular form such that each subsystem can be independently tested and assessed. The NSW Technology Program plans to assess the lower frequency subsystem during the DRI experiment. A brief description of the planned NSW measurement is provided below.
The measurement will employ the candidate lower frequency acoustic lens subsystem, a line array receiver, a sonar tower, and calibrated retro-reflectors. The lower frequency subsystem will have a 25-cm diameter aperture and will be partially populated using acoustic elements. These elements will operate in the 20 to 90 kHz frequency range. A transducer measuring 67 cm by 3.5 cm will be employed as the receiver. This receivers operational band is 30 to 65 kHz and will be oriented such that its narrow beam will be in the horizontal plane. The acoustic lens subsystem and the receiver will be attached (approximately 3 m above the bottom sediment) to a stationary sonar tower. The sonar tower is equipped with scanning (pan and tilt) motors such that the acoustic lens subsystem and receiver have an almost 360° rotational capability as well as a full 0° to 90° tilt.
Five calibrated retro-reflectors will be used as the buried targets. These targets have relatively constant target strength over a wide aspect angle. The target strength of these retro-reflectors ranges from 5.5 dB at 50 kHz to +7.9 dB at 100 kHz; the 3 dB down beam width varies from about 32° at 50 kHz to almost 17° at 100 kHz. These targets will be mounted onto PVC pipes which in turn will be water-jetted into the sediment resulting in complete burial of the retro-reflectors. Three of these targets will be positioned in the same azimuth direction (i.e., at the same bearing) from the sonar tower. These three targets will be placed at ranges of 5, 9, and 17 m from the sonar tower such that their main response axes (MRA) will be in the horizontal plane and directed toward the sonar tower. The two remaining targets will be positioned at a different azimuth angle from that of the first three targets and will be located at ranges of 9 and 17 m from the sonar tower. These two targets will have their MRAs directed toward the watersediment interface.
If logistics permit and if security approval is obtained, several additional mine-like targets will be deployed in the target field. These targets will be buried via water-jetting methods and will be located at a different azimuth direction than the calibrated targets.
Data will be acquired for both steep (target at 5 m range) and shallow grazing angles (targets at 9 and 17 m range) as functions of target burial depth, range, horizontal beam width, and pulse type. Various pulses will be employed, including short sinusoidal pulses and broad band frequency modulated pulses.
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