Coherent Sea Clutter Signatures Observed with A High-Resolution X-Band Radar

Abstract

Microwave radars operating from ship and land-based platforms typically view the ocean surface at grazing angles below a few degrees. Consequently, it is important to understand the behavior of electromagnetic waves scattered from the ocean surface in this angular regime. Ocean scattering at larger grazing angles (20° to 70°) is well understood in terms of a two-scale or composite surface scattering model (CSM) [1], where backscatter is predominantly due to the presence of wind-generated capillary-gravity wavelets superimposed on larger gravity waves. The larger waves tilt and strain the bragg waves and therefore only contribute to the backscattered signal in an indirect manner. Low grazing angle (LGA) backscatter is less well understood and exhibits characteristics that cannot be explained using these “conventional” composite surface models [2]. Sea spikes are high-contrast (>20 dB) scattering events frequently observed in high resolution (∼10 m) horizontally polarized radar returns [3] having amplitudes 10–20 dB higher than bragg model predictions and doppler shifts significantly larger than those predicted by two-scale models [4]. The physical mechanisms responsible for these scattering features are currently unknown, although evidence suggests they are caused by scatterers near the crests of steep or breaking waves [5]. LGA backscatter from the sea appears to be highly sensitive to such effects as sharpening of long wave crests and wave breaking, and may therefore be sensitive to the long wave variability caused by submesoscale features such as oceanic fronts and eddies making this scattering regime important for remote sensing.