UNDERWATER ARRAY SENSING Sample Clauses

UNDERWATER ARRAY SENSING. Existing practice is that each AUV maps landmarks and obstacles using its own sonar, independently of other AUVs. Maps are sometimes “swapped” and logically merged later, but there is no coherent combining during the initial mapping stage (at the signal level). In traditional radar, multiple transmit antennas are combined in a phase-coherent fashion to beamform – direct the illumination or focus reception along a particular direction of interest, but this requires tight synchronization. A relatively recent technological advance in radar technology is multiple-input multiple-output (MIMO) radar, wherein the transmitter employs multiple transmit antennas to emit a set of orthogonal waveforms – a different one from each transmit antenna. It has been established that MIMO radar offers significant diversity / resolution benefits relative to traditional phased-array radar [L8, L9, L10]. MIMO radar ideas are currently breaking into the sonar arena – there are few mature pieces of work in the area, but a few groups have started looking at MIMO sonar; cf. [L11,L12]. What we are interested in, however, is distributed (virtual) MIMO sonar – meaning that sonars from different AUVs are combined to improve resolution in a way akin to the case of multiple sonars mounted on a single AUV. Since MIMO techniques require relatively accurate synchronization, what we advocate here is to `mate’ nearby AUVs in pairs. This has a number of benefits: it is easier to synchronize nearby AUVs, the communication channel between them is a lot better – supporting relatively high data transmission rates – and finding a mate should be easy in swarm mode. The following research issues must be addressed to bring the proposed virtual MIMO sonar concept to fruition: Motion locking and Doppler shift: Relative motion between two AUV mates is an issue, as it implies different Doppler shifts and possibly different multipath channels. This implies that the AUVs in a pair should be motion-locked to within acceptable tolerance. Appropriate mechanisms for achieving this should be devised. Such locking is automatic between neighbors in swarm mode, which suggests that pairing and motion locking can be achieved by mimicking swarm-inducing mechanisms. Waveform design and synchronization: Due to the severe nature of multipath in the underwater acoustic environment, special care should be exercised in designing appropriate orthogonal codes for the different sonars. A simple option is to use pseudo-random cod...
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