Grating lobe prediction and deconvolution for synthetic aperture sonar

Abstract

Synthetic aperture sonar (SAS) arrays are typically undersampled in the sense that the array element spacing is much larger than the acoustic wavelength. Grating lobes are suppressed by making a judicious choice of beampattern nulls for the transmit and receive elements, such as a 3:2 ratio for the length of the transmit and receive elements. However, grating lobe artifacts can appear in SAS imagery when the target strength difference between a highly reflective object and the surrounding seabed exceeds the sidelobe level of the synthetic array. We present a theoretical model of the SAS point scattering function (PSF) that takes into account shaded element beampatterns for a multichannel SAS array. The PSF model is validated using experimental data from AquaPix, a wideband 300 kHz interferometric SAS. In practice, observed SAS images are described by the convolution of the seabed reflectivity with the PSF. Therefore, knowledge of the PSF facilitates the removal of grating lobe artifacts using deconvolution techniques such as the Richardson-Lucy algorithm. Conventional and deconvolved SAS images of a highly reflective target are presented to demonstrate the effectiveness of deconvolution based on the modeled PSF.