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Underwater Acoustic Imaging is used to produce high-quality images in water where visibility is too poor to use optical methods. In underwater acoustic imaging, electrical energy (electricity) is sent to a crystal, which causes the crystal to vibrate. The sound of the vibration (mechanical energy) travels out into the water column until it hits an object and is reflected back to the crystal. After the energy returns to the crystal, a computer converts it to an image.

   Underwater Positioning & Tracking Systems
  

   are used to identify the location of an object or vehicle underwater. Since radios do not transmit through water,
  

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    GPS
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   cannot be used unless an antenna is cabled to the surface. Several acoustic methods have been developed for the purpose of underwater navigation. These include:
  

   Long Base Line
  

   (LBL) uses several transponders on the seafloor. The distance between each transponder is measured. A transducer on the vessel,
  

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    ROV
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   ,
  

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    AUV
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   or tow fish, interrogates each transponder and the ranges to each are measured. From this= information, the position of the subsea platform can be determined by triangulation.
  

   Short Base Line
  

   (SBL) is similar to LBL except the array of transponders is spread along the underside of the vessel.
  

   Ultra Short Base Line
  

   (USBL) uses only one transponder and one multi-element hydrophone on the surface. The measurement of the angle to the transducer is made across the face of the hydrophone (the ultra short base line). In addition to the angle, the range is determined by measuring the amount of time it takes for apulse to travel from the hydrophone and to be returned by the transponder. So range and bearing to the target are determined.
  

   LBL is the most accurate of the methods described and is more common in deeper water, because the accuracy of SBL and USBL degrade with range. The disadvantage of LBL is the time and expense required to deploy and survey in the seafloor transponders. USBL is the least expensive method and the easiest to use. SBL falls somewhere in between the two.