Waveguide invariant broadband target detection and reverberation estimation

Abstract

Reverberation often limits the performance of active sonar systems. In particular, backscatter off of a rough ocean floor can obscure target returns and/or large bottom scatterers can be easily confused with water column targets of interest. Conventional active sonar detection involves constant false alarm rate (CFAR) normalization of the reverberation return which does not account for the frequency-selective fading caused by multipath propagation. This paper presents an alternative to conventional reverberation estimation motivated by striations observed in time-frequency analysis of active sonar data. A mathematical model for these reverberation striations is derived using waveguide invariant theory. This model is then used to motivate waveguide invariant reverberation estimation which involves averaging the time-frequency spectrum along these striations. An evaluation of this reverberation estimate using real Mediterranean data is given and its use in a generalized likelihood ratio test based CFAR detector is demonstrated. CFAR detection using waveguide invariant reverberation estimates is shown to outperform conventional cell-averaged and frequency-invariant CFAR detection methods in shallow water environments producing strong reverberation returns which exhibit the described striations.

Interference structure of shallow water reverberation in time-frequency distribution

“…It has been observed that reverberation intensity has regular striations in the time-frequency distribution in ref. [8]. However, in the paper, the theoretical derivation is different from that in ref.…”

Extraction of time-frequency target features

“…This work is similar to work in other papers [7], [8], [9] where the Generalized Hough Transformation was also applied to evaluate spectral information.…”

“…Goldhahn [7], [8] demonstrated its usability to improve noise estimation for ocean bottom reverberation and thus target and clutter return discrimination. Turgut [9] investigated the invariance principle for mid-frequency to estimate the invariance parameter value and possible target kinetics on simulated and experimental data.…”

Understanding and utilizing waveguide invariant range-frequency striations in ocean acoustic waveguides

“…In [25], Goldhahn et al uses the waveguide invariant to describe reverberation in active sonar returns, allowing for an improved ability for target detection.…”

“…The waveguide invariant has been used for a wide range of applications such as: passive range estimation [58,56,62], matched field processing [59,23], active sonar [49,27,25], array processing [37,57,66], time-reversal mirrors [32,54,38,39], and more (See Appendix A for a complete review of the waveguide invari-ant literature). Despite its many uses, the waveguide invariant is not well studied compared to other ocean-acoustic phenomena.…”