Method of operation
Figure 1 describes the general method of operation of an acoustic positioning system, this is an example of a long baseline (LBL) positioning system for ROV ;Baseline station deployment and survey Acoustic positioning systems measure positions relative to a framework of baseline stations, which must be deployed prior to operations. In the case of a long-baseline (LBL) system, a set of three or more baseline transponders are deployed on the sea floor. The location of the baseline transponders either relative to each other or in global coordinates must then be measured precisely. Some systems assist this task with an automated acoustic self-survey, and in other cases GPS is used to establish the position of each baseline transponder as it is deployed or after deployment. ;Tracking or navigation operations Following the baseline deployment and survey, the acoustic positioning system is ready for operations. In the long baseline example (see figure 1), an interrogator (A) is mounted on the ROV that is to be tracked. The interrogator transmits an acoustic signal that is received by the baseline transponders (B, C, D, E). The reply of the baseline transponders is received again at the ROV. The signal time-of-flight or the corresponding distances A-B, A-C, A-D and A-E are transmitted via the ROV umbilical (F) to the surface, where the ROV position is computed and displayed on a tracking screen. The acoustic distance measurements may be augmented by depth sensor data to obtain better positioning accuracy in the three-dimensional underwater space. Acoustic positioning systems can yield an accuracy of a few centimeters to tens of meters and can be used over operating distance from tens of meters to tens of kilometers. Performance depends strongly on the type and model of the positioning system, its configuration for a particular job, and the characteristics of the underwater acoustic environment at the work site.Classes
Underwater acoustic positioning systems are generally categorized into three broad types or classes Long-baseline (LBL) systems, as in figure 1 above, use a sea-floor baseline transponder network. The transponders are typically mounted in the corners of the operations site. LBL systems yield very high accuracy of generally better than 1 m and sometimes as good as 0.01m along with very robust positions This is due to the fact that the transponders are installed in the reference frame of the work site itself (i.e. on the sea floor), the wide transponder spacing results in an ideal geometry for position computations, and the LBL system operates without an acoustic path to the (potentially distant) sea surface. Ultra-short-baseline (USBL) systems and the related super-short-baseline (SSBL) systems rely on a small (ex. 230 mm across), tightly integrated transducer array that is typically mounted on the bottom end of a strong, rigid transducer pole which is installed either on the side or in some cases on the bottom of a surface vessel. Unlike LBL and SBL systems, which determine position by measuring multiple distances, the USBL transducer array is used to measure the target ''distance'' from the transducer pole by using signal run time, and the target ''direction'' by measuring theHistory and examples of use
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 LUSBLReferences
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