
Applied Acoustics specializes in Ultra-Short Baseline (USBL) technology for advanced underwater tracking, providing an alternative to GPS, which does not efficiently function underwater.
The Global Positioning System (GPS), or the broader Global Navigation Satellite System (GNSS), is highly effective in the air for pinpointing locations and tracking movements. However, the moment one descends beneath the water’s surface, GNSS becomes ineffective.
This is due to the radio frequencies used by GNSS (1.2 GHz to 1.6 GHz) being unable to penetrate water, and instead reflect off the surface. To achieve reliable tracking underwater, acoustic methods are most commonly employed.
Applied Acoustics USBL technology operates in the medium frequency band (16 kHz to 32 kHz). This method is typically used for locating subsea equipment, personnel, or points of interest.
Easytrak USBL Systems
Applied Acoustics’ Easytrak USBL systems come in various configurations to accommodate different sizes, depths, ranges, and numbers of targets. They offer benefits such as:
- Tracking multiple targets simultaneously.
- Compatibility with aae acoustic release and positioning beacons.
- Capability to track targets in both shallow and deep waters.
Positioning Beacons
The company’s positioning beacons serve diverse applications, from short-range dives to deep-water vehicle tracking. Their robust construction ensures stability and compatibility with most industry-standard systems.
How USBL Systems Work
A USBL system comprises:
- A topside console or hardware interface.
- A transceiver mounted on a pole attached to a vessel or platform, extending into the water.
- A positioning beacon placed on the seabed, an ROV, or a towed array to locate underwater vehicles, towfish, structures, and more.
The system transmits an acoustic pulse from the transceiver to the positioning beacon, which responds acoustically. The transceiver receives this reply and displays the data on the surface console or interface.
The transceiver calculates the time interval between sending the signal to the beacon and receiving the response. Multiple elements within the transceiver process the reply, determining the range, bearing, and depression angle of the target using a least mean squares (LMS) algorithm. The surface console displays this information, enabling operators to locate targets at a glance.
One of the main advantages of USBL systems over other subsea positioning technologies, such as LBL arrays, is their ease of use. They can be deployed rapidly from the surface without requiring additional subsea equipment. The only exception is that each target must be fitted with a compatible acoustic positioning beacon.
Enhancing Accuracy & Functionality
To improve positioning repeatability and precision, we recommend our proprietary Sigma digital protocols. This approach spreads digital signals across the frequency band, enhancing signal reception and ensuring better timing and range stability for consistent results.
External sensors, such as an inertial measurement unit (IMU), gyrocompass, or depth data, can integrate with the navigation system. For vehicle tracking, heading data can also be interfaced. These parameters can be transmitted serially, via a network, or acoustically, provided the appropriate hardware is used. This integration further enhances positional accuracy.
Linking GNSS with USBL
It is standard practice to input the vessel’s GNSS data into a USBL system. This allows the system to convert positional data into absolute locations, incorporating the external sensors mentioned earlier. Proper calibration of these sensors and the GNSS input is crucial to ensure accuracy.
Due to its complexity, USBL systems are most effective for extended installations or longer survey operations, including dynamic positioning (DP) reference positioning, ROV operations, diver support, and towed surveys.
Alternative Systems
A combined USBL and inertial navigation system (INS), such as the Pyxis INS + USBL, closely couples inertial sensors to the acoustics and a GNSS antenna connected to the USBL system. This configuration ensures precise alignment of GNSS with acoustics, offering real-time corrections for vessel movement.
This type of system is calibration-free, apart from aligning the GNSS antenna with the transceiver. It is well-suited for rapid deployment on vessels of opportunity and for accurate positioning on platforms without high-grade survey gyros or IMUs. This all-in-one solution allows operations to commence immediately, saving time and enhancing efficiency.
GNSS for Divers
Anyone who has attempted to use a mobile phone underwater knows that GNSS signals weaken or disappear entirely, as water obstructs radio waves. As a result, divers cannot rely on standard GNSS devices for underwater navigation.
In some cases, a GNSS receiver can be deployed on a buoy positioned above the diver and connected via a cable. This setup allows the GNSS position to be transmitted to an underwater monitor, enabling the diver to navigate.
Applied Acoustics’ range of MiniPod submersible GNSS receivers can also be used for this purpose. Combined with a small battery pack, the diver’s position can be accurately transmitted to the diver and wirelessly sent back to the vessel or dive team for monitoring.
Where this approach is not feasible, such as in commercial dive operations, diver positions can be determined using a small acoustic positioning beacon attached to the diver and a USBL system. This setup enables the ship to monitor diver locations, enhancing subsea safety and allowing dive operators to guide divers with instructions for underwater navigation.