Underwater Acoustic Positioning System
Encyclopedia
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.
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.
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 the phase shift of the reply signal as seen by the individual elements of the transducer array. The combination of distance and direction fixes the position of the tracked target relative to the surface vessel. Additional sensors including GPS, a gyro or electronic compass and a vertical reference unit are then used to compensate for the changing position and orientation (pitch, roll, bearing) of the surface vessel and its transducer pole. USBL systems offer the advantage of not requiring a sea floor transponder array. The disadvantage is that positioning accuracy and robustness is not as good as for LBL systems. The reason is that the fixed angle resolved by a USBL system translates to a larger position error at greater distance. Also, the multiple sensors needed for the USBL transducer pole position and orientation compensation each introduce additional errors. Finally, the non-uniformity of the underwater acoustic environment cause signal refractions and reflections that have a greater impact on USBL positioning than is the case for the LBL geometry.
Short-baseline (SBL) systems
use a baseline consisting of three or more individual sonar transducers that are connected by wire to a central control box. Accuracy depends on transducer spacing and mounting method. When a wider spacing is employed as when working from a large working barge or when operating from a dock or other fixed platform, the performance can be similar to LBL systems. When operating from a small boat where transducer spacing is tight, accuracy is reduced. Like USBL systems, SBL systems are frequently mounted on boats and ships, but specialized modes of deployment are common too. For example, the Woods Hole Oceanographic Institution
uses a SBL system to position the Jason deep-ocean ROV relative to its associated MEDEA depressor weight with a reported accuracy of 9 cm
GPS intelligent buoys
(GIB) systems are inverted LBL devices where the transducers are replaced by floating buoys, self-positionned by GPS. The tracked position is calculated in realtime at the surface from the Time-Of-Arrival (TOAs) of the acoustic signals sent by the underwater devic, and acquired by the buoys. Such configuration allow fast, calibration-free deployment with an accuracy similar to LBL systems. At the opposite of LBL, SBL ou USBL systems, GIB systems use one-way acoustic signals from the emitter to the buoys, making it less sensible to surface or wall reflections. GIB systems are used to track AUVs, torpedoes, or divers, may be used to localize airplanes black-boxes, and may be used to determine the impact coordinates of inert or live weapons for weapon testing and training purposes references: Sharm-El-Sheih, 2004; Sotchi, 2006; Kayers, 2005; Kayser, 2006; Cardoza, 2006 and others...).
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 bathyscape 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 extend 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
.
Oceanography
Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...
, salvage operations, marine archaeology
Maritime archaeology
Maritime archaeology is a discipline within archaeology as a whole that specifically studies human interaction with the sea, lakes and rivers through the study of associated physical remains, be they vessels, shore side facilities, port-related structures, cargoes, human remains and submerged...
, law enforcement and military activities.
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 ROVRov
Rov is a Talmudic concept which means the majority.It is based on the passage in Exodus 23;2: "after the majority to wrest" , which in Rabbinic interpretation means, that you shall accept things as the majority....
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
Local coordinates
Local coordinates are measurement indices into a local coordinate system or a local coordinate space. A simple example is using house numbers to locate a house on a street; the street is a local coordinate system within a larger system composed of city townships, states, countries, etc.Local...
or in global coordinates
Geographic coordinate system
A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position...
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
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
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 classesLong-baseline (LBL) systems
Long Baseline Acoustic Positioning System
A Long Baseline 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 and Short Baseline 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
Ultra-short baseline
USBL is a method of underwater acoustic positioning. A complete USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder/responder on the seafloor, a towfish, or on a ROV...
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 the phase shift of the reply signal as seen by the individual elements of the transducer array. The combination of distance and direction fixes the position of the tracked target relative to the surface vessel. Additional sensors including GPS, a gyro or electronic compass and a vertical reference unit are then used to compensate for the changing position and orientation (pitch, roll, bearing) of the surface vessel and its transducer pole. USBL systems offer the advantage of not requiring a sea floor transponder array. The disadvantage is that positioning accuracy and robustness is not as good as for LBL systems. The reason is that the fixed angle resolved by a USBL system translates to a larger position error at greater distance. Also, the multiple sensors needed for the USBL transducer pole position and orientation compensation each introduce additional errors. Finally, the non-uniformity of the underwater acoustic environment cause signal refractions and reflections that have a greater impact on USBL positioning than is the case for the LBL geometry.
Short-baseline (SBL) systems
Short Baseline Acoustic Positioning System
A Short Baseline 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 and Long Baseline Systems...
use a baseline consisting of three or more individual sonar transducers that are connected by wire to a central control box. Accuracy depends on transducer spacing and mounting method. When a wider spacing is employed as when working from a large working barge or when operating from a dock or other fixed platform, the performance can be similar to LBL systems. When operating from a small boat where transducer spacing is tight, accuracy is reduced. Like USBL systems, SBL systems are frequently mounted on boats and ships, but specialized modes of deployment are common too. For example, the Woods Hole Oceanographic Institution
Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution is a private, nonprofit research and higher education facility dedicated to the study of all aspects of marine science and engineering and to the education of marine researchers. Established in 1930, it is the largest independent oceanographic research...
uses a SBL system to position the Jason deep-ocean ROV relative to its associated MEDEA depressor weight with a reported accuracy of 9 cm
GPS intelligent buoys
GPS Intelligent Buoys
GPS intelligent buoy systems may be classified as inverted LBL 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...
(GIB) systems are inverted LBL devices where the transducers are replaced by floating buoys, self-positionned by GPS. The tracked position is calculated in realtime at the surface from the Time-Of-Arrival (TOAs) of the acoustic signals sent by the underwater devic, and acquired by the buoys. Such configuration allow fast, calibration-free deployment with an accuracy similar to LBL systems. At the opposite of LBL, SBL ou USBL systems, GIB systems use one-way acoustic signals from the emitter to the buoys, making it less sensible to surface or wall reflections. GIB systems are used to track AUVs, torpedoes, or divers, may be used to localize airplanes black-boxes, and may be used to determine the impact coordinates of inert or live weapons for weapon testing and training purposes references: Sharm-El-Sheih, 2004; Sotchi, 2006; Kayers, 2005; Kayser, 2006; Cardoza, 2006 and others...).
History and examples of use
USS Thresher (SSN-593)
The second USS Thresher was the lead ship of her class of nuclear-powered attack submarines in the United States Navy. Her loss at sea during deep-diving tests in 1963 is often considered a watershed event in the implementation of the rigorous submarine safety program SUBSAFE.The contract to build...
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 bathyscape Trieste 1
Bathyscaphe Trieste
The Trieste is a Swiss-designed, Italian-built deep-diving research bathyscaphe with a crew of two, which reached a record maximum depth of about , in the deepest known part of the Earth's oceans, the Challenger Deep, in the Mariana Trench near Guam, on January 23, 1960, crewed by Jacques Piccard ...
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.
Japanese submarine I-52
I-52, code-named Momi was a Type C-3 cargo submarine of the Imperial Japanese Navy used during World War II for a secret mission to Lorient, France, then occupied by Germany, during which she was sunk....
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 extend 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
California Department of Fish and Game
The California Department of Fish and Game is a department within the government of California, falling under its parent California Natural Resources Agency. The Department of Fish and Game manages and protects the state's diverse fish, wildlife, plant resources, and native habitats...
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
Sacramento River Delta
The Sacramento-San Joaquin River Delta, or California Delta, is an expansive inland river delta and estuary in northern California in the United States. The Delta is formed at the western edge of the Central Valley by the confluence of the Sacramento and San Joaquin rivers and lies just east of...
.