Acoustic Diver Tracking
Throughout the project, diver orientation was enhanced by an acoustic USBL diver tracking system, manufactured by Sonardyne.
In this system, range and time difference measurements of sound signals emitted by a beacon attached to the diver on the seabed allow to calculate the divers position.
Further details on acoustic theory can be found at: http://www.sonardyne.co.uk/theory.htm#USBL
A transceiver assembly mounted over the side of the vessel received the sound signals. A navigation processor calculated and displayed the relative diver position. Further software packages were used to transform the relative coordinates into real world geographical coordinates and to display the diver position overlayed on a georeferenced multibeam or sidescan image of the wreck.
As the transceiver and thus the coordinate reference frame was fixed to the vessel and influenced by yaw, pitch and roll, additional equipment such as a motion reference unit and a gyrocompass had to be used to compensate for the boat’s movement and enhance tracking accuracy.
The average positioning accuracy achieved with the USBL system ranged from 2m to 6m. This made the system suitable for basic orientation on large wreck sites, but not for surveying or detailed orientation in low visibility conditions. The main factors responsible for inaccuracies were related to boat positioning and boat movement compensation rather than the acoustic tracking system itself.