Broadband amplitude and phase sonar calibration using LFM pulses for high-resolution study of hard and soft acoustic targets

Target phase has been advanced as an additional parameter for sonar target identification. Previous studies aiming to classify fish and other marine organisms by means of the phase angle component in their echoes have reported promising results. Similarly, the use of broadband techniques to simultaneously obtain spectral information and achieve good spatial resolution has been deemed advantageous. Therefore, broadband active sonar that utilizes both amplitude and phase would appear to be a desirable system. Adopting this approach requires suitable calibration methods that account for complex sensitivity over large bandwidths. Current calibration procedures that incorporate pulse compression with linear-frequency modulated (LFM) pulses are available but do not address phase. The present work expands on these methods in order to achieve broadband sonar amplitude and phase calibration, using spectral processing of LFM pulses and standard targets. Target phase is obtained as the difference of phase angles at two frequencies, in order to remove range effects. This paper presents time-domain scattering, together with calibrated target strength (TS) and phase spectra of solid tungsten carbide spheres and air-filled ceramic shells, obtained in a laboratory water tank. Experimental results are compared to theoretical models to validate measurements and identify underlying echo formation mechanisms.