Radar Basics

Reflection at smooth surfaces

Reflection is a change of the direction of the wavefront at a boundary between two media of different densities, where the wavefront returns to the medium from which it came. If the interface is smooth, this process is called specular reflection.

In the mathematical description of electromagnetic wave diffraction, the term “specularly smooth surface” is defined as specific to a particular wavelength λ or range of wavelengths. A smooth surface is one whose roughness is much less than the wavelength. In practice, a surface whose roughness does not exceed one-thirtieth of the incident wavelength is considered smooth. In this case the diffuse scattering that occurs with such a low roughness can be neglected.

Specular reflection means that the angle at which the reflected wave propagates is equal to the angle at which the incident wave arrived. The law of specular reflection is formulated as follows: The angle of incidence is equal to the angle of reflection. During reflection, there is also a phase jump of 180° in the wave.

A practical example of the effect of reflection of waves from relatively smooth surfaces is the formation of an antenna radiation pattern involving reflections from the earth’s surface. Such a mechanism for forming antenna patterns is found, for example, in meter-wave radars such as the P-18. In the case of the P-18, it must select a radar site in which no unevenness is higher than 1 m within a 1000 m radius to reduce diffuse scattering losses. This approximation describes the relationship between the wavelength of the radar and the size of these asperities. The distance “up to 1000 m” results from the size of the Fresnel zone around the radar.