Providing real-time acoustic positioning with the Subsonus USBL
Advanced Navigation and Carnegie Clean Energy have collaborated to develop a positioning system for the 6th generation of CETO, a revolutionary wave energy conversion technology. CETO captures the energy from waves through the motion of its Buoyant Actuator. The motions are transmitted to an onboard power generation system referenced to the seabed. For design verification, mooring load monitoring, and model validation purposes, it is necessary to measure the absolute position of the Buoyant Actuator in real-time.
Standalone inertial navigation system (INS) solutions are not suitable for this submerged positioning application due to their inherent drift which leads to inaccurate data. However, there are ways to aid an INS with acoustic instruments to improve performance:
- Velocity Aiding: The velocity of the system over the seabed is measured using a Doppler Velocity Log (DVL) and is integrated into the positioning algorithm to correct the INS data. This significantly reduces INS drift, but cannot eliminate it. Due to the level of drift rates observed in other applications using this method, it was regarded as unsuitable for this application.
- Acoustic Aiding: The relative position of the system is measured by acoustically pinging transponders of which the positions are accurately known. Acoustic positioning is by its nature drift-free. Depending on how the position is resolved from the beacons’ pings, but also on the distance between the beacons themselves, the acoustic aiding system can be qualified as Long Base-Line (LBL), Short Base-Line (SBL), or Ultra-Short Base-Line (USBL).
USBL acoustic aiding was chosen as the most appropriate solution, due to its ability to mitigate against measurement drift. Ultimately, Advanced Navigation’s Subsonus, a next generation USBL, was selected for its high accuracy, compact size and low cost.
A unique feature of Subsonus is its ability to transfer heading data acoustically. The bi-directional measurement and communication between two Subsonus units equipped with hydrophone arrays allow the known heading of the seabed unit to be transferred to the surface unit. The integrity of this acoustic heading data is significantly better than magnetic compass sensors which are prone to disturbance from magnetic fields.
Test system layout – both Subsonus and the reference system measure the motion of the test buoy
The Solution: The Subsonus USBL
Testing was required to validate Subsonus as an appropriate measurement solution for the motions of the wave energy system. This allowed for an assessment of Subsonus’ ability to provide the required measurement accuracy while responding to the challenges that are inherent to the industry. These include strict reliability and accuracy requirements.
A test set-up was deployed off Fremantle, Western Australia with a dynamic buoy and a seafloor mounted reference unit. One Subsonus unit was mounted on a buoy, the other mounted on a fixed frame positioned on the seabed. A reference system was Integrated into buoy on the surface to act as a Measurement reference.
Both the Subsonus technology and the reference system were integrated within a PTP network communicating via Ethernet. The Interface and Logging Unit was used as the time server to ensure synchronisation of both systems within 0.1 millisecond.
The greater difference in the east position with respect to the north is a result of the distribution of the equipment at the test location. The seafloor master unit was placed close to the southern gravity anchor point which meant tracking of buoy movement in a northerly direction relied heavily on acoustic range measurement and correct master unit positioning.
In contrast, tracking of movement in the easterly direction relied predominantly on the angular acoustic measurements and correct master unit heading, both of which are expected to show greater variance. In a final installation, this could be optimised by placing the seafloor master unit closer to the expected operating area of the slave unit above. The pressure depth sensor built into Subsonus provides additional vertical position and velocity data to the INS.
An illustration of multiple Subsonus communicating with each other.
The Outcome: An accurate underwater positioning system
Through the range of testing, the suitability of an acoustic-inertial positioning system for the motion tracking of the wave energy converter was demonstrated. Subsonus’ acoustic heading technology offers positioning and heading accuracy without the reliance on GNSS (global navigation satellite system) signals.
Path taken by the survey vessel above the seabed as measured by the Subsonus pair
Subsonus is a next-generation USBL underwater acoustic positioning system that provides high accuracy position, velocity, and heading at depths of up to 1000 metres. The system features an industry-leading calibrated hydrophone array combined with an internal tightly coupled INS, all packed into a miniature titanium enclosure small enough to fit in the palm of your hand.