Underwater Acoustic Localisation in the context of Autonomous Submersibles

With the advancement of the field of underwater robotics, the amount of autonomy embodied
in the vehicles themselves have considerably increased while making it possible to build and
deploy swarms of small autonomous underwater vehicles (AUVs). Apart from the many environmental
and mechanical challenges encountered in the underwater domain, the swarming paradigm
demands the need for each vehicle to be aware of the positions of at least its near neighbors.
The Serafina AUV project which was initiated with the goal of developing swarming technology
for the small and highly agile Serafina class AUVs requires a localisation system which could cope
with the dynamic and fast changing vehicle configurations while being small, reliable, robust,
and energy efficient and not dependent on pre-deployed acoustic beacons.

The short-range acoustical relative localisation system presented, uses hyperbolic and spherical 
localization concepts and provides each vehicles with the azimuth, range and heading of its near 
neighbours. The implementation utilises an acoustically transmitted MLS signal which provides 
extremely high robustness against interference by stochastic and systematic disturbances which 
are typical for underwater environments. The azimuth is obtained via hyperbolic positioning with 
improved resolution and accuracy with respect to conventional methods. Range and heading estimation 
is achieved by two independent methods for increased robustness; one uses the implicit synchronisation 
provided by the underlying inter-vehicle communication scheduling system to measure the difference 
of TOAs of an acoustic and a long-wave radio signals; the second relies on TDOAs and a reverse 
hyperbolic localisation scheme.