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Short baseline acoustic positioning system
A short baseline (SBL) acoustic positioning system〔Underwater Acoustic Positioning Systems, Chapter 3, P.H. Milne, 1983, ISBN 0-87201-012-0〕 is one of three broad classes of underwater acoustic positioning systems that are used to track underwater vehicles and divers. The other two classes are ultra short baseline systems (USBL) and long baseline systems (LBL). Like USBL systems, SBL systems do not require any seafloor mounted transponders or equipment and are thus suitable for tracking underwater targets from boats or ships that are either anchored or under way. However, unlike USBL systems, which offer a fixed accuracy, SBL positioning accuracy improves with transducer spacing.〔The ROV Manual, Section 4.2.7 Advantages and Disadvantages of Positioning Systems, Robert D. Christ and Robert L. Wernli Sr., 2007, ISBN 978-0-7506-8148-3〕 Thus, where space permits, such as when operating from larger vessels or a dock, the SBL system can achieve a precision and position robustness that is similar to that of sea floor mounted LBL systems, making the system suitable for high-accuracy survey work. When operating from a smaller vessel where transducer spacing is limited (i.e. when the baseline is short), the SBL system will exhibit reduced precision.
== Operation and performance ==
Short baseline systems determine the position of a tracked target such as a ROV by measuring the target's distance from three or more transducers that are, for example, lowered over the side of the surface vessel from which tracking operations take place. These range measurements, which are often supplemented by depth data from a pressure sensor, are then used to triangulate the position of the target. In figure 1, baseline transducer (A) sends a signal, which is received by a transponder (B) on the tracked target. The transponder replies, and the reply is received by the three baseline transducers (A, C, D). Signal run time measurements now yield the distances B-A, B-C and B-D. The resulting target positions are always relative to the location of the baseline transducers. In cases where tracking is conducted from a moving boat but the target position must be known in earth coordinates such as latitude/longitude or UTM, the SBL positioning system is combined with a GPS receiver and an electronic compass, both mounted on the boat. These instruments determine the location and orientation of the boat, which are combined with the relative position data from the SBL system to establish the position of the tracked target in earth coordinates.
Short baseline systems get their name from the fact that the spacing of the baseline transducers (on a boat for example) is usually much less than the distance to the target, such as a robotic vehicle or diver venturing far from the boat〔Handbook of Acoustics, Malcolm J. Crocker 1998, ISBN 0-471-25293-X, 9780471252931, page 462〕 As with any acoustic positioning system, a larger baseline yields better positioning accuracy. SBL systems use this concept to an advantage by adjusting transducer spacing for best results〔An evaluation of USBL and SBL Acoustic Systems and the Optimization of Methods of Calibration, Philip, The Hydrographic Journal, No. 108 April 2003〕 When operating from larger ships, from docks or from the sea ice where greater transducer spacing can be used, SBL systems can yield a positioning accuracy and robustness approaching that of sea-floor mounted LBL systems.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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