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The growing torpedo threat to ships and submarines demands effective countermeasures for defence. Detection, classification and localisation of an attacking torpedo is the first step towards launching effective countermeasures. Studies are on the rise to use the existing sonar systems to detect torpedoes and subsequently use countermeasures. The use of towed array sonar systems for torpedo detection and acoustic counter measures are the most recent and not reported much in open literature. This paper presents a modular acoustic counter measure approach using towed decoy against acoustic homing torpedoes. Describes the technologies and algorithms required for different modes of towed decoy, which is effective against both active and passive acoustic homing torpedoes. Towed decoy signal generation schemes and their realisation using digital signal processing hardware are outlined in this paper.

In this work, the problem of receive antenna selection (AS) is considered, in a multiple antenna communication system having a single radio frequency (RF) chain at the receiver. The AS is performed on a per-packet basis, and AS decisions are based on noisy estimates of the channel gains obtained using pilot symbols embedded in the data packet for coherent demodulation, along with the receiver's knowledge of the time correlation of the channel. The problem is posed as a partially observable Markov decision process (POMDP) with the goal of minimizing the average packet error rate (PER). The performance of a myopic policy is compared with that of the POMDP solution, and it is shown that the former is optimal under certain conditions. As the POMDP approach requires the channel gains to be quantized to a finite set of states, we also propose two heuristic AS schemes that use the continuous-valued received pilot symbols to make AS decisions, and thereby offer comparable or better performance than the POMDP approach. Unlike previous work, the schemes proposed here for AS do not require a lengthy AS training phase to precede each data packet. The performance improvement offered by the POMDP solution and the proposed heuristic solutions relative to existing AS training-based approaches is illustrated using Monte Carlo simulations.

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