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Underwater acoustic positioning is a method of providing relative and absolute positioning for the purposes of underwater navigation in the absence of GNSS satellite signals.
Acoustic signals are sent between a surface vessel and a subsea system such as an autonomous underwater vehicle (AUV), remote operated vehicle (ROV), or a diver, and the relative position can be ascertained. When combined with a surface positioning system such as GNSS on the surface vessel, an absolute position for the underwater system can be derived.
There are several scenarios where reliable underwater positioning is necessary. From hydrography to harbour maintenance, AUVs and ROVs need accurate positioning data in order to complete their tasks efficiently. However, operating underwater poses significant challenges in terms of positioning.
GNSS signals cannot penetrate beyond an inch below the water’s surface. While GNSS can be used before and after a vehicle submerges, this isn’t a reliable method for underwater positioning, especially at great depths when regular surfacing is not viable for underwater operations. Similarly, radio waves and light also operate at frequencies that are too high to travel through water.
Acoustic signals have a much lower frequency than GNSS, operating at 30 kHz. This lower frequency allows for travel through much greater distances of water. Similar to how whales communicate, these acoustic signals allow us to send data between underwater transponders to determine positions.
Underwater acoustic positioning systems utilise sound waves or acoustic signals to perform positioning utilising a technique known as Ultra-Short Baseline, or USBL. A device known as a hydrophone functions like an underwater microphone and speaker, and transmits and receives the acoustic signals used in the USBL. A device known as a USBL transducer is what is created when an array of hydrophones is manufactured with very precise distances and geometries between the individual hydrophone elements.
When an acoustic signal is received by the USBL transducer, the hydrophone elements act as a cluster of GNSS satellites with the received acoustic data being used to calculate the bearing to the source of the signal. Using knowledge of the speed of sound through water, the range between the USBL transducer and the acoustic signal source can also be derived.
To coarsely calculate the bearing between the USBL transducer and the acoustic signal source, the difference in arrival time of the acoustic signal is recorded at each hydrophone, and the known geometry of each hydrophone is used to back-calculate the source of the acoustic signal. This calculation can be improved and refined in several ways. Increasing the number of hydrophone elements in the USBL transducer, and having varied geometry between the hydrophones, particularly in three dimensions, will typically improve the USBLs ability to resolve the source of the acoustic signal.
The next method to improve the calculation is to utilise an encoded acoustic signal, rather than a simple wave-form. As the acoustic wavefront moves over the hydrophones, the ability of the USBL to resolve a complex signal in the acoustic wave allows a technique called phase-differencing to be used to better resolve the difference in arrival times of the waveform, and thus improve the precision of the calculations.
Finally, the USBL calculation may be improved by resolving the bearing in both directions, instead of just one. In simple USBL systems, the USBL transducer is the resolving the position of a simple acoustic source or transceiver. In more complex USBL systems two transducers may be used, with both being used to resolve the relative bearing. Data is shared between the transducers, either via acoustic modem or via a wired network, to allow the two bearings to be included in the calculations.
Based on the USBL calculations, the bearing and range from the transceiver are known, giving a relative position from the transceiver to the acoustic source. In some scenarios this is sufficient information, however, most scenarios require an absolute position of the tracked object, be it an ROV, AUV, or diver. This is achieved using an absolute positioning source such as GNNS, combine with attitude and heading knowledge of the surface vessel, to translate the absolute GNSS position from the antenna to the USBL transceiver. This absolute position is then used in conjunction with the USBL range and bearing to determine the absolute position of the tracked object.
The accuracy and precision of USBL systems can be improved in several ways. For example, increasing the number of hydrophone elements in the transducer will provide more data that makes directional calculations more precise. Also, varied geometry between the hydrophones, particularly in a three-dimensional sense, will typically improve the USBL ability to resolve the source of the acoustic signal.
Another method to improve precision is to utilise a wideband, or encoded acoustic signal, rather than a simple waveform. Using a complex acoustic signal allows the USBL to better apply phase-differencing as the acoustic wavefront moves over the hydrophones. Phase-differencing improves the USBL ability to accurately resolve the difference in arrival times of the waveform, which improves the precision of the calculations.
It could be said that the larger the baseline distances and the greater the number of hydrophones, the easier it is to resolve the source of an acoustic signal. However, this typically increases the physical size of the USBL, which in most cases is very undesirable as it can increase the complexity of deployment and cost. To miniaturise a USBL device represents several engineering challenges, which is to not only decrease baseline distance without affecting hydrophone performance but to improve signal processing to maintain accuracy from a smaller baseline. The Subsonus USBL is an example of meeting these challenges, where the unit is so compact that it can be used in a unique dual USBL configuration (USBL-Squared). This configuration uses a USBL on the surface vessel and tracked asset to improve accuracy by resolving acoustic signals at both ends.