In robot swarming each individual needs to know the positions and orientations of at least its near neighbours. The Serafina project aims to have many small autonomous submersibles (55 cm in length) in swarms. Given the high degree of manoeuvrability and agility of the individual vehicles as well as the provision to allow a large number of members mean the spatial configuration of the swarm changes frequently.

The requirements of the relative localization system are to provide estimation of the relative angular bearing, relative pose and distance to near neighbours. This information needs to be updated at a reasonable rate with respect to the motion of the swarm. Given the nature of the application, constraints are imposed on available energy and space by the small size of the vehicle in addition to the need to produce a solution with a low cost implementation.

The system utilises wide-band acoustic pulses comprising of Maximum Length Sequence (MLS) signals. The intrinsic characteristics of MLS signals provide high interference robustness and the ability to be deployed even in cluttered environments which usually impose multiple specular reflections and possible resonance effects.

The proposed system involves sending out MLS pulses from two extremities of a submersible with a projector and the observing submersible receiving these pulses on multiple hydrophones. The received signals are cross-correlated to obtain the inter-aural time differences between hydrophone channels. The difference in time-of-flight between the two MLS pulses is also measured. This information is then combined to estimate the angular bearing, range and the pose of the sending submersible with respect to the observer.

Sending of the acoustic pulses are synchronised with the underlying long-wave radio communication scheduling system of the swarm which guarantees a locally collision free schedule. By setting the amplitude of the acoustic signals such that their range is equivalent to the long-wave radio communication range, only one sending event takes place within a local neighbourhood, eliminating cross-talk. Furthermore, this acoustical - long-wave-radio synchronization provides a secondary range and pose estimation method using difference in time-of-flight, making the range and pose estimation more robust.

This paper presents the intermediate results of experiments carried out using the described system as well as an analysis of the results with respect to the resolution and accuracy of the obtained estimates. The performance of the online experimental setup is evaluated with respect of the required performance criteria for the actual implementation of the system in the functional swarm of submersibles.

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