On the use of decision feedback equalization for continuous active sonar

Continuous Active SONAR (CAS) systems allow duty cycles up to 100% and so lower target association errors are possible as compared to Pulse Active Sonar (PAS). Large time-bandwidth product Linear Frequency Modulated (LFM) signals are the de facto standard for this type of SONAR systems, yet, these signals suffer from low processing gains when the ocean exhibits low-spatio temporal coherence. In this work, we depart from the typical sub-band matched filter processing and propose to adopt signal processing techniques used in underwater acoustic communications. In particular, we analyze Binary Phase Shift Keying (BPSK) signals via an adaptive Decision-Feedback Equaliser (DFE). The DFE is able to adapt to environmental changes based on the known transmitted bits. The operational bandwidth is 1500–4000 Hz and the BPSK signals are transmitted in two non-overlapping bands to avoid transmit interference during reception. The performance of the proposed system is demonstrated based on field data recorded during a sea experiment off the Coast of La Spezia in October 2017. The key result here is that the Doppler measurement update rate of the echoes is as fast as the bit rate of the transmitted BPSK signal.