Modelling radar sea clutter in littoral environments

Abstract

A model is presented for low grazing angle sea clutter in littoral environments. It is based on a simple characterisation of the clutter in terms of scattering from ripples riding on long waves (the composite model) and from breaking waves; both being subjected to obscuration and multipath illumination. In littoral environments changes to the ocean waves are caused by limited fetch, wave refraction, current variations and enhanced breaking in shallow water. Simple models for these phenomena are described along with their effect on clutter radar cross section (RCS). I. INTRODUCTIONThe operation of maritime radar systems close to the coast is affected by the characteristics of the radar reflections from the sea surface, known as sea clutter. Experimentally it has been found that the sea clutter in these littoral environments is different from that in the open ocean, and often causes degradation in radar performance. The successful method used to model sea clutter empirically in the open ocean, by taking many measurements and looking for trends in behaviour as a function of viewing direction and sea conditions, is difficult to repeat in the littoral. This is because there are too many factors affecting the behaviour, as each environment is different. To make progress it is necessary to model the behaviour of the sea surface in littoral conditions and then to calculate radar scattering from that surface. The resulting modelled behaviour of sea clutter may then be analysed for measurable trends, which can be verified empirically.In this paper we investigate such an approach applied to low radar grazing angles, typical of radar systems operating from aircraft or ships. We consider back-scattering from the sea surface but, except for local forward scattering causing interference at the surface, do not include propagation effects that are important for some types of radar. Two sections on the description of the sea surface in the open ocean are followed by an outline of the radar cross section (RCS) modelling technique and results. The four principal processes that change the character of the sea surface in the littoral environment are then described along with how to extend RCS modelling to these conditions. Initial results of the effect of limited fetch on RCS are presented.

Investigation on Microwave Polarimetric Scattering From Two-Dimensional Wind Fetch- And Water Depth-Limited Nearshore Sea Surfaces

“…Meanwhile, the small fetch suppresses the growths of waves in both deep sea and nearshore shallow sea environment, which leads to almost negligible difference in these two cases. The joint effect of above two aspects leads to the aforementioned phenomenon, which is also supported by the analysis in [24]. When incident angle exceeds 30 • , the variation trend of curves is similar to the counterpart of deep sea.…”

Combination of empirical/asymptotic models to characterize sea clutter at intermediate angles

“…Consequently, we decide to continue the study with the Lagrange interpolation by varying the wind speed. Recently, Ward and Tough [16] propose a mean backscatter model for low grazing angles. Some simulations have been done for several sea states (1 to 5), which correspond to seas with wind speeds of 3.2, 5.5, 7.6, 9.6 and 11.5 m/s, for the polarization HH and VV and by fixing the frequency to 9 GHz.…”

“…Recently, Ward and Tough [16] propose a mean backscatter model based on physics which characterizes normalized radar cross section for low grazing angles. It is also possible to compute the radar signal backscatter by the sea by using exact methods.…”

“…We compare in the section III the spline and Lagrange interpolations for the GOSSA+GIT 2 , SSA+GIT and SPM+GIT by varying the polarizations (VV and HH). We finally compare the interpolations at intermediate angles by calculating the absolute differences with a set of data given by Nathanson [1] and Masuko et al [18], and two mean backscatter models, the IRSG-LIN model [10] and the IGENCE model [16].…”

Wave height dependence of L-Band small-grazing-angle sea clutter

“…In many cases, wind direction is inconsistency with wave direction, and the relationship between wind speed and wave height is not agree with equilibrium sea surface sate [I].Due to the effects of littoral environments, sea clutter characteristics become more complex and completely different from fully developed sea. The performance of radar system is severely degraded in littoral conditions [2]. So it is very necessary to exact model sea clutter in littoral environment.…”