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 cur rent. The system was deployed on a research pier on the Outer Banks of Nort h Carolina as part of the Mine Surface Effects (MISE) program, a program de signed to investigate the feasibility of using microwave radar to detect su bmerged mines. The radar achieves extremely high range resolution, approxim ately 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 t echnique. Radar images from the MISE program are presented which show the s urface 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 durin g the low phase of the tide, when the sphere broached the surface, and duri ng high tide, when the sphere was completely submerged. Closer examination of the imagery reveals that when the target was fully submerged, the backsc atter from the wake was primarily composed of long-lived (> 1.5 s), isolate d echoes that translated downstream at a velocity close to that of the meas ured surface current. Photographs of the wake suggest that these echoes are generated by sharp-crested surface features that form where the slow movin g water of the turbulent, subsurface wake impinges the surface.