The previous chapter considered scattering at high grazing angles where the Physical Optics approximation is accurate. At lower grazing angles this approximation breaks down and it is necessary to adopt a different approach. In Section 16.2, electromagnetic (EM) scattering theory is extended to the composite model for scattering from an imperfectly conducting dielectric rough surface. In contrast to Physical Optics, the EM polarisation sensitivity of the composite model is able to explain the difference between vertical (V) and horizontal (H) polarisations at medium grazing angles. However, at low grazing angles (LGAs) there are many characteristics, as described in Chapter 2, which are not consistent with the composite model. Section 16.3 therefore considers extending LGA scattering theory beyond the composite model. The principal LGA clutter phenomena of multipath interference and scattering from breaking waves (and their manifestation in radar `sea spikes') may be understood from scattering off corrugated (one-dimensional, 1D) surfaces. Thus, it is here that we show how for corrugated surfaces vector EM scattering simplifies to scalar scattering. We introduce numerical methods for calculating the backscatter from rough surfaces at LGA and compare the accuracy of various techniques for idealised surfaces. In Section 16.4, the radar cross-sections (RCS) of spikes and the effect of multipath are calculated for realistic surfaces. These are put together in Section 16.5 with a model for the occurrence of breaking waves to derive trends for the average RCS of sea clutter versus grazing angle and sea state. It is shown that the results are well matched with a wide range of experimental LGA data. Finally, in Section 16.6, we return to the remote sensing problem of Section 15.4 and illustrate how the imaging of tidal flow over bottom topography is affected by LGA scattering.

Chapter Contents:

• 16.1 Introduction
• 16.2 The composite model for scattering at medium grazing angles
• 16.3 Scattering at low grazing angles: beyond the composite model
• 16.4 Scattering from breaking waves
• 16.5 Average backscatter from the ocean at low grazing angles
• 16.6 Imaging ocean currents at low grazing angles
• 16.7 Sea clutter in littoral environments

Inspec keywords: electromagnetic wave scattering; radar clutter; interference (signal); electromagnetic wave polarisation; backscatter; numerical analysis; marine radar; radar cross-sections

Other keywords: medium grazing angle; LGA clutter phenomena; high grazing angle; LGA scattering theory; radar sea spikes; physical optics approximation; sea state; corrugated surface; scalar scattering; multipath interference; composite model; low grazing angle scattering; sea clutter; numerical method; breaking wave scattering; backscatter; EM polarisation sensitivity; vertical polarisation; electromagnetic scattering theory; radar cross-section; conducting dielectric rough surface; RCS; tidal flow imaging; remote sensing problem; ocean surface; EM scattering theory; horizontal polarisation; topography

Subjects: Radar equipment, systems and applications; Electromagnetic compatibility and interference; Other numerical methods; Electromagnetic wave propagation