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Ultrawideband radar images of the surface disturbance produced by a submerged, mine-like object

This paper describes an ultrawideband, imaging radar designed to image the fine structure of the surface disturbances caused by submerged objects and presents images of the surface wake generated by a sphere immersed in a current. The system was deployed on a research pier on the Outer Banks of North Carolina as part of the Mine Surface Effects (MISE) program, a program designed to investigate the feasibility of using microwave radar to detect submerged mines. The radar achieves extremely high range resolution, approximately 4 cm, through the use of a short pulse with a 3 dB bandwidth of 3 GHz and a center frequency of 9 GHz. Azimuthal imaging is achieved through the use of a linear array of receive antennas and a time-domain beam-forming technique. Radar images from the MISE program are presented which show the surface disturbance generated by a 91 cm-diameter sphere immersed in a 40-60 cm/s current. A surface wake is clearly visible in the imagery, both during the low phase of the tide, when the sphere broached the surface, and during high tide, when the sphere was completely submerged. Closer examination of the imagery reveals that when the target was fully submerged, the backscatter from the wake was primarily composed of long-lived (>1.5 s), isolated echoes that translated downstream at a velocity close to that of the measured surface current. Photographs of the wake suggest that these echoes are generated by sharp-crested surface features that form where the slow moving water of the turbulent, subsurface wake impinges the surface.