Low Frequency SAS

Fabien Novella (1, 2) , Yan Pailhas (3) , Gilles Le Chenadec (2, 4) , Isabelle Quidu (2, 1) , Michel Legris (1, 2)

1 Lab-STICC_M3 - Equipe Marine Mapping & Metrology
2 ENSTA Bretagne - École Nationale Supérieure de Techniques Avancées Bretagne
3 NATO Undersea Research Center
4 Lab-STICC_MATRIX - Equipe Models and AlgoriThms for pRocessIng and eXtracting information

Multipath is an issue for the performance of synthetic aperture sonars. In HF-SAS (high frequency SAS) images, multipaths effects manifest in the hiding of targets, the loss of image contrast and the degradation of interferometry bathymetric estimates and degradation of spatial coherence due to a decrease of the SNR. In this study, the behaviour of a LF-SAS (low frequency SAS) equipped with a full 2D receiving array is analysed. This 2D Rx array enables the observation of the 2D spatial coherence. On the vertical axis, the existence of a multipath reflects on the emergence of lobes in the figure of coherence. Intuitively, the lobes can be understood considering that the direct path and a multipath can be seen from the reception array as two sources that act as a dipole antenna. This study aims at evaluating effects of multipaths on these lobes establishing the link between multipaths and lobes). The study builds upon data acquired by the HRLFSAS designed by NATO CMRE. The effect of multipath is highlighted thanks analytical modelisation and a comparison between observed coherence and the one predicted from application of the Van Cittert Zernike theorem to the distribution of energy estimated by a vertical beamforming.

Domaines: Acoustique [physics.class-ph]; Traitement du signal et de l'image [eess.SP]

Fabien Novella, Yan Pailhas, Gilles Le Chenadec, Isabelle Quidu, Michel Legris. Low Frequency SAS: Influence of multipaths on Spatial Coherence. 6th Underwater Acoustics Conference & Exhibition, Jun 2021, Virtual meeting, United States. pp.070032, ⟨10.1121/2.0001503⟩. ⟨hal-03519906⟩