Acoustical methods for azimuth, range and heading estimation in underwater swarms

Robotic swarms and specifically schools of semi-autonomous underwater vehicles are becoming more and more of a reality. An essential feature required in most such systems is an efficient and precise relative localisation system. Specific, common requirements include posture estimations of multiple neighbours, controllable sensing ranges, as well as high robustness and accuracy. The presented system is based on acoustically transmitted MLS (Maximum Length Sequence) -signals. Multiple receivers allow for precise azimuth and range measurements, while multiple transponders additionally allow for complete posture estimations of neighbouring vehicles. The short baselines (< 200 mm) given by the dimensions of the vehicles in the swarm make sub-sample interpolations in the post-cross-correlation phase necessary. Furthermore the deployment of low cost transducers and hydrophones in a broadband signal setup requires specific filtering methods. For the experimental setup as depicted in detail in the article, a mean error μ_Δθ in the bearing (azimuth) estimation i is less than θ.3° throughout all experiments, while the mean error μ_Δα in the heading estimation α of a neighbouring vehicle is always less than 5.0°. Experiments have been performed in many configurations in a inter-vehicle distance of up to 5 m, while the mean range error μ_Δr remains well below 10 mm. All experiments have been performed in sweet water.