Underwater acoustic positioning system

The acoustic distance measurements may be augmented by depth sensor data to obtain better positioning accuracy in the three-dimensional underwater space.

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.

LBL systems yield very high accuracy of generally better than 1 m and sometimes as good as 0.01m along with very robust positions[7][8] 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.

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.

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.

The tracked position is calculated in realtime at the surface from the Time-Of-Arrival (TOAs) of the acoustic signals sent by the underwater device, and acquired by the buoys.

[17] 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 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.

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.

In 2023, University of Washington researchers demonstrated a fourth class of 3D underwater positioning for these smart devices that does not require infrastructure support like buoys.

Figure 1: Method of the operation of a Long Baseline (LBL) acoustic positioning system for ROV
Figure 2a: An acoustic short baseline (SBL) positioning system was installed on the USNS Mizar during the search dives to the wreckage of the submarine USS Thresher
Figure 2b: The bathyscaphe Trieste was guided by its acoustic positioning system to the Thresher
Figure 3: The Russian deep sea submersibles MIR-1 and MIR-2 searched the wreck site of the Japanese submarine I-52 in 1998. A LBL positioning system was used to guide and document the progressing search over multiple dives
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.