page_title: Underwater acoustic positioning system
section_title: History and examples of use
hierarchical_section_title: Underwater acoustic positioning system / History and examples of use
caption_attribution_description: English: Operation of a special-purpose underwater acoustic positioning system for the real-time measurement of the geometry and opening area of a fish sampling trawl net. Left: Four small responders (A, B, C, D) are mounted in the corners of the trawl net opening and wired via junction bottle (E) and umbilical (F) to a surface station computer. Center: The net is deployed. Right: The surface station computer sends instructions to one responder (ex. A) to transmit, while instructing the other responders (ex. B, C, D) to receive. By this method all six distances (A-B, A-C, A-D, B-C, B-D, C-D) are measured. The four sides of the opening and one diagonal are used to triangulate the trawl net opening geometry and area. The second diagonal is available to compute a measurement error metric for data quality verification.
context_page_description: An underwater acoustic positioning system is a system for the tracking and navigation of underwater vehicles or divers by means of acoustic distance and/or direction measurements, and subsequent position triangulation. Underwater acoustic positioning systems are commonly used in a wide variety of underwater work, including oil and gas exploration, ocean sciences, salvage operations, marine archaeology, law enforcement and military activities.
context_section_description: An early use of underwater acoustic positioning systems, credited with initiating the modern day development of these systems, involved the loss of the American nuclear submarine USS Thresher on 10 April 1963 in a water depth of 2560m.  An acoustic short baseline (SBL) positioning system was installed on the oceanographic vessel USNS Mizar.  This system was used to guide the bathyscaphe Trieste 1 to the wreck site.  Yet, the state of the technology was still so poor that out of ten search dives by Trieste 1, visual contact was only made once with the wreckage.  Acoustic positioning was again used in 1966, to aid in the search and subsequent recovery of a nuclear bomb lost during the crash of a B-52 bomber at sea off the coast of Spain.
  In the 1970s, oil and gas exploration in deeper waters required improved underwater positioning accuracy to place drill strings into the exact position referenced earlier thorough seismic instrumentation and to perform other underwater construction tasks.
  But, the technology also started to be used in other applications.  In 1998, salvager Paul Tidwell and his company Cape Verde Explorations led an expedition to the wreck site of the World War 2 Japanese cargo submarine I-52 in the mid-Atlantic. Resting at a depth of 5240 meters, it had been located and then identified using side scan sonar and an underwater tow sled in 1995. War-time records indicated the I-52 was bound for Germany, with a cargo including 146 gold bars in 49 metal boxes.  This time, Mr. Tidwell's company had hired the Russian oceanographic vessel, the Akademik Mstislav Keldysh with its two manned deep-ocean  submersibles MIR-1 and MIR-2 (figure 3). In order to facilitate precise navigation across the debris field and assure a thorough search, MIR-1 deployed a long baseline transponder network on the first dive. Over a series of seven dives by each submersible, the debris field was progressively searched. The LBL positioning record indicated the broadening search coverage after each dive, allowing the team to concentrate on yet unsearched areas during the following dive.  No gold was found, but the positioning system had documented the extent of the search.
  In recent years, several trends in underwater acoustic positioning have emerged.  One is the introduction of compound systems such the combination of LBL and USBL in a so-called LUSBL configuration to enhance performance. These systems are generally used in the offshore oil & gas sector and other high-end applications.  Another trend is the introduction of compact, task optimized systems for a variety of specialized purposes. For example, the California Department of Fish and Game commissioned a system (figure 4), which continually measures the opening area and geometry of a fish sampling net during a trawl. That information helps the department improve the accuracy of their fish stock assessments in the Sacramento River Delta.
caption: Figure 4: NetTrack is an example of a special-purpose underwater acoustic positioning system of the SBL type, designed to measure the opening geometry and area of a trawl net for accurate fish stock assessment purposes. Left: Four small responders (A, B, C, D) are mounted in the corners of the trawl net opening and wired via junction bottle (E) and umbilical (F) to a surface station computer. Center: The net is deployed. Right: The surface station computer sends instructions to one responder (ex. A) to transmit, while instructing the other responders (ex. B, C, D) to receive. By this method all six distances (A-B, A-C, A-D, B-C, B-D, C-D) are measured. The four sides of the opening and one diagonal are used to triangulate the trawl net opening geometry and area. The second diagonal is available to compute a measurement error metric for data quality verification.
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