Abstract

Acoustic synthetic apertures are usually generated by coherent summing of signals, correcting for phase shifts due to the lapse in time as a receiver array moves to create the aperture. This approach suffers from phase estimation errors arising from both positioning errors of the moving receivers and from phase instabilities in the signal. An incoherent synthetic aperture can be created if compact broadband signals are used, avoiding phase error problems. Furthermore, since no overlapping sensors are required for phase estimation, a synthetic aperture can in principle be created from a single moving receiver. This paper investigates the feasibility and expected performance of an incoherent synthetic aperture constructed by moving a single acoustic receiver in the vertical plane. The application discussed in this paper is to evaluate the focal region of an Acoustic Time Reversal Mirror (TRM) where the receiver platform is an Autonomous-Profiling Vehicle (APV). The APV is a modified commercial product from Ocean Sensors in San Diego, USA, and carries an acoustic data acquisition payload designed by the Acoustic Research Laboratory. The APV has the ability to profile in the vertical plane, guided by a taut vertical mooring line, or to zigzag in a series of progressively displaced vertical profiles as it drifts freely with currents. The APV also monitors conductivity, temperature and depth, so that the sound speed profile and the depth of the receiver at each pulse reception can be reconstructed. Numerical propagation models are used to simulate the performance of such an incoherent synthetic aperture array during shallow water TRM experiment to validate the overall performance prior to conducting sea-trials. Both Fast Field (SPARC) and Normal Mode (Kraken C) models are applied. The temporal-spatial ambiguity problem is considered with respect to the time it takes to form the aperture compared to the coherence time of the TRM focussing, limited by the coherence time of the shallow-water waveguide. We also present some CTD and acoustic data taken by the modified APV.