Advances in Multistatic Sonar

Although the concept of multistatic active sonar (MAS) has been around for over 50 years, new trends have brought this technology to the forefront of anti-submarine warfare research. These trends include advancements in sonar sensors and signal processing, advancements in submarine stealth, and a desire to track targets in noisy and reverberant environments, such as near-shore or shallow waters. The latest trend is to exploit the really game-changing capabilities of unmanned and autonomously operating underwater vehicles. The focus of this chapter is on advances in signal processing enabling especially the tracking of low signature targets, namely targets with low signal-to-noise ratio (SNR), in a multisensor environment. In particular, the track-before-detect (TBD) approach and its adaption to pre-selected contact-based tracking are addressed. The TBD approach is designed to track low SNR targets. TBD-based procedures jointly process several consecutive pings and, relying on target kinematics or, simply, exploiting the physically admissible target transitions, declare the presence of a target and, eventually, its track (Orlando et al., IEEE-TSP 2010). A TBD algorithm is typically fed by unthresholded data or thresholded data with significantly lower thresholds than the ones used by conventional trackers. Moreover an important feature of a TBD scheme is the so-called constant false track acceptance rate (CFTAR) property: if a TBD scheme ensures the constant false alarm rate property with respect to the unknown statistics of the disturbance, then it allows controlling the overall false track acceptance rate. The TBD algorithm herein presented considers a bistatic sonar architecture and is capable of handling raw hydrophone data (Orlando et al., CIP 2010). Remarkably, it guarantees the CFTAR property. Performance analysis highlights its potential to implement automatic track continuation and to prepare automatic classification for temporarily weak targets as these tasks are usually the challenges that MAS systems have to overcome. In the context of multistatic sonar, a batch algorithm is also introduced, that jointly processes measurements provided by multiple sensors over a certain number of consecutive pings (Orlando et al., FUSION 2010). These measurements are time differences of arrival and bearing information of a target maneuvering in the surveillance region. This approach is tested on a benchmark data set provided by METRON in the context of collaborative international multi-laboratory research that is ongoing in the ISIF Multi-Static Tracking Working Group (Orlov, Metron Data set 2009). The remainder of this chapter is organized as follows: the next section is devoted to the description of bistatic and multistatic sonar systems. Section III focuses on the derivation of the TBD (or TBD-based) processors. In Section IV a node selection strategy for multistatic 2