Short baseline acoustic positioning system

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Figure 1: Method of operation of a short baseline (SBL) acoustic positioning system for ROV SBL Acoustic Positioning System PILOT.jpg
Figure 1: Method of operation of a short baseline (SBL) acoustic positioning system for ROV

A short baseline (SBL) acoustic positioning system [1] 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. [2] 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.

Contents

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 [3] 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 [4] 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.

History

SBL systems are found employed in a variety of often specialized applications. Perhaps the first implementation of any underwater acoustic positioning system was a SBL system installed on the U.S. Navy oceanographic vessel USNS Mizar. In 1963, this system guided the bathyscaphe Trieste 1 to the wreck site of the American nuclear submarine USS Thresher. However, performance was still so poor that out of ten search dives by Trieste 1, visual contact was only made once with the wreckage.

The Woods Hole Oceanographic Institution is using a SHARPS SBL system to guide their JASON tethered deep ocean robotic vehicle relative to the MEDEA depressor weight and docking station associated with the vehicle. Rather than tracking both vehicles with a positioning system from the surface which would result in degraded accuracy as the pair's deployment distance, the SBL baseline transducers are mounted on MEDEA. yielding the position of JASON relative to MEDEA with good accuracy independent of the system's deployment depth. The reported accuracy is 0.09m [5]

SBL systems are also available commercially for positioning of small ROVs and other subsea vehicles and equipment. [6]

Example

Figure 2: The SCINI ROV next to its dive hole at Heald Island, Antarctica SCINI ROV.JPG
Figure 2: The SCINI ROV next to its dive hole at Heald Island, Antarctica

An example of SBL technology is currently (since 2007) underway in Antarctica, where the Moss Landing Marine Laboratory is using a PILOT SBL system to guide the SCINI remotely operated vehicle. SCINI (figure 2) is a small, torpedo-shaped tethered vehicle (ROV) designed for rapid and uncomplicated deployment and exploration of remote sites around Antarctica, including Heald Island, Cape Evans and Bay of Sails. SCINI system is designed to be compact and light-weight so as to facilitate rapid deployment by helicopter, tracked vehicle and even man-hauled sleds. Once on site, its torpedo shaped body allows it to access the ocean through small (20 cm dia.) holes drilled into the sea ice. The mission's science goals [7] however demand high accuracy in navigation, to support tasks including running 10-m video transects (straight lines), providing precise positions for still images to document the distribution and population density of benthic organisms and marking and re-visiting sites for further investigation.

The SBL navigation system (figure 3) consists of three small, 5 cm diameter sonar baseline transducers (A, B, C) that are linked by cable to a control box (D). A small (13.5 cm L x 4 cm D), cylinder shaped transponder is mounted on the SCINI vehicle. Accuracy is optimized by making use of the flat sea ice to place the baseline transducers well apart; approx. 35m for most SCINI deployments.

Figure 4 reviews SCINI operations guided by the SBL system. Figure 4A is an improvised ROV control room, in this case in a cabin hauled on top of an ice hole at Cape Armitage. From left, the displays are the ROV controls screen (A), the main camera view (B), the navigation screen (C) and the science display (D). The ROV pilot will generally watch the main camera view. He will glance at the navigation screen (C), which shows the current ROV position and track overlaid on a chart, for orientation and to guide the ROV to the location instructed by the scientist. The scientist, shown here seated on the right is provided with the science display (D), which combines the ROV imagery with position, depth and time data in real time. The scientist types written or speaks audible observations into the computer to provide a context for the data, note objects or evens of interest or designate the start or conclusion of a video transect (figure 4B).

A typical investigation of a site will span several dives, as tasks such as initial investigation, still image acquisition and video transects are gradually completed. A critical element in these dive series is to show prior-dive search coverage, so that a successive dive can be targeted at a previously unvisited area. This is done by producing a cumulative coverage plot of the dive site (figure 4C). The plot, which is updated after every dive, is displayed as a background map on the navigation screen thus providing guidance for the ongoing dive. It shows the prior ROV tracks with color used to indicate depth. Analysis of the track data displayed here yields the quality of positioning to provide a margin of error for measurements. In this case, the typical precision has been established as 0.54m.

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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.

Long baseline acoustic positioning system Class of underwater acoustic positioning systems used to track underwater vehicles and divers

A long baseline (LBL) acoustic positioning system 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 short baseline systems (SBL). LBL systems are unique in that they use networks of sea-floor mounted baseline transponders as reference points for navigation. These are generally deployed around the perimeter of a work site. The LBL technique results in very high positioning accuracy and position stability that is independent of water depth. It is generally better than 1-meter and can reach a few centimeters accuracy. LBL systems are generally employed for precision underwater survey work where the accuracy or position stability of ship-based positioning systems does not suffice.

GPS sonobuoy or GPS intelligent buoy (GIB) are a type of inverted long-baseline (LBL) acoustic positioning devices where the transducers are installed on GPS-equipped sonobuoys that are either drifting or moored. GIBs may be used in conjunction with an active underwater device, or with a passive acoustic sound source. Typically the sound source or impact event is tracked or localized using a time of arrival (TOA) technique. Typically several GIBs are deployed over a given area of operation; with the total number determined by the size of the test area and the accuracy of the results desired. Different methods of GPS positioning may be used for positioning the array of GIBs, with accuracies of cm to meter level in realtime possible.

Underwater searches are procedures to find a known or suspected target object or objects in a specified search area under water. They may be carried out underwater by divers, manned submersibles, remotely operated underwater vehicles, or autonomous underwater vehicles, or from the surface by other agents, including surface vessels, aircraft and cadaver dogs.

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Diving support equipment is the equipment used to facilitate a diving operation. It is either not taken into the water during the dive, such as the gas panel and compressor, or is not integral to the actual diving, being there to make the dive easier or safer, such as a surface decompression chamber. Some equipment, like a diving stage, is not easily categorised as diving or support equipment, and may be considered as either.

References

  1. ↑ Underwater Acoustic Positioning Systems, Chapter 3, P.H. Milne, 1983, ISBN   0-87201-012-0
  2. ↑ 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
  3. ↑ Handbook of Acoustics, Malcolm J. Crocker 1998, ISBN   0-471-25293-X, 9780471252931, page 462
  4. ↑ An evaluation of USBL and SBL Acoustic Systems and the Optimization of Methods of Calibration, Philip, The Hydrographic Journal, No. 108 April 2003
  5. ↑ Integrating Precision Relative Positioning Into JASON/MEDEA ROV Operations, Bingham et al., MTS Journal Spring 2006 (Volume 40, Number 1)
  6. ↑ "Water Linked Underwater GPS Explorer Kit", Blue Robotics, 3 April 2017. Retrieved on 18 August 2019.
  7. ↑ SCINI project web site, science goals
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