Angle estimation of single and overlapping echoes with a broadband split-beam echosounder

Split-beam echosounders are widely used in acoustical oceanography to measure the abundance and distribution of marine organisms and to study their movement and behavior in situ. Abundance estimates rely on target strength measurements, which depend on accurate angle of arrival (AoA) estimates to compensate for transducer directivity, and analyses of movement and behavior also depend on AoA to determine spatial positions. Recent years have seen the transition from narrowband to broadband systems, which offer improved acoustic detection and characterization of targets, yet the method for AoA estimation with split-beam echosounders has remained unchanged. The split-aperture correlator (SAC), originally developed for narrowband systems, remains the only available option despite inadequate assessment for broadband systems and fundamental restrictions to single-echo scenarios. In this work, we assess SAC's applicability to broadband systems, demonstrating its inaccuracy in estimating the AoAs of broadband single echoes from targets with highly frequency-dependent scattering responses, such as gas-bearing fish. An extended SAC is introduced to address this limitation. Furthermore, we introduce a broadband maximum likelihood estimator for estimating the AoAs and spectra of overlapping echoes. The estimator's efficacy and applicability to a broadband split-beam echosounder are demonstrated through controlled laboratory experiments.