Shadow Enhancement in Synthetic Aperture Sonar Using Fixed Focusing

A shadow cast by an object on the seafloor is important information for target recognition in synthetic aperture sonar (SAS) images. Synthetic aperture imaging causes a fundamental limitation to shadow clarity because the illuminator is moved during the data collection. This leads to a blend of echo and shadow, or geometrical fill-in in the shadow region. The fill-in is most dominant for widebeam synthetic aperture imaging systems. By treating the shadow as a moving target and compensating for the motion during the synthetic aperture imagery, we avoid the geometrical shadow fill-in. We show this to be equivalent to fixing the focus at the range of the shadow caster. This novel technique, referred to as fixed focus shadow enhancement (FFSE) can be used directly as an imaging method on hydrophone data or as a postprocessing technique on the complex SAS image. We demonstrate the FFSE technique on simulated data and on real data from a rail-based SAS, and on two different SAS systems operated on a HUGIN autonomous underwater vehicle.

[1]  Johannes Groen,et al.  Adaptive motion compensation in sonar array processing , 2006 .

[2]  R.E. Hansen,et al.  Synthetic aperture sonar processing for the HUGIN AUV , 2005, Europe Oceans 2005.

[3]  J. Groen,et al.  Shadow enhancement in synthetic aperture sonar imagery for improved target classification , 2005 .

[4]  Roy Edgar Hansen,et al.  Simulation of high resolution mine hunting sonar measurements , 2005 .

[5]  S. Upadhyay.  Seismic Reflection Processing , 2004 .

[6]  S. Upadhyay.  Concepts and Methods in Seismic Migration , 2004 .

[7]  Hugh Griffiths,et al.  Experimental validation of autofocus algorithms for high-resolution imaging of the seabed using synthetic aperture sonar , 2003 .

[8]  Scott Reed,et al.  An automatic approach to the detection and extraction of mine features in sidescan sonar , 2003 .

[9]  R.E. Hansen,et al.  Signal processing for AUV based interferometric synthetic aperture sonar , 2003, Oceans 2003. Celebrating the Past ... Teaming Toward the Future (IEEE Cat. No.03CH37492).

[10]  X. Lurton.  An Introduction to Underwater Acoustics , 2002 .

[11]  A. Bellettini,et al.  Theoretical accuracy of synthetic aperture sonar micronavigation using a displaced phase-center antenna , 2002 .

[12]  Hao Ling,et al.  Time-Frequency Transforms for Radar Imaging and Signal Analysis , 2002 .

[13]  Yvan Petillot,et al.  Unsupervised mine detection and analysis in side-scan sonar: A Comparison of Markov Random Fields and Statistical Snakes , 2001 .

[14]  P. T. Gough,et al.  A short history of synthetic aperture sonar , 1998, IGARSS '98. Sensing and Managing the Environment. 1998 IEEE International Geoscience and Remote Sensing. Symposium Proceedings. (Cat. No.98CH36174).

[15]  Peter T. Gough,et al.  Imaging algorithms for a strip-map synthetic aperture sonar: minimizing the effects of aperture errors and aperture undersampling , 1997 .

[16]  W. Carrara,et al.  Spotlight synthetic aperture radar : signal processing algorithms , 1995 .

[17]  Franck Florin,et al.  Effets d'échos et d'ombres differentiels sur les antennes de sonar actif en champ de fresnel , 1993 .

[18]  John C. Curlander,et al.  Synthetic Aperture Radar: Systems and Signal Processing , 1991 .

[19]  A. Currie,et al.  Synthetic aperture radar (SAR) images of moving targets , 1987 .

[20]  A. J. Berkhout,et al.  Seismic Migration: Imaging of Acoustic Energy by Wave Field Extrapolation , 1980 .

[21]  Takuso Sato,et al.  Synthetic aperture sonar , 1973 .

[22]  R. Keith Raney,et al.  Synthetic Aperture Imaging Radar and Moving Targets , 1971, IEEE Transactions on Aerospace and Electronic Systems.