Principles of Modern Radar - Volume 2 1891121537

13.4 Radar Applications of Polarimetry 617the waveform processing, the advantages of using circular polarization far outweigh itslimitation in clutter attenuation for heavy rain. Therefore, the use of circular polarization toreduce rain clutter is still widespread. The reader is referred to [53, 54] for a comprehensivetreatment of radar meteorology and accurate polarimetric measurements of the rain clutter.13.4.3 Ground ClutterGround clutter due to backscattering from land or sea frequently limits radar target visibility.It usually introduces depolarization as a consequence of ground surface roughness.Ground clutter is, in general, distributed with a high degree of nonstationarity both in timeand space. This indicates considerable depolarization of the backscattered wave. A higherwind velocity and frequency can also significantly increase depolarization and reduce thedegree of polarization of distributed ground clutter. For strong point clutter in an urban environment,due to spatial regularity of the ground surface or the prevailing contribution ofpoint scatterers, the degree of polarization can increase to a significant extent [2]. Diffusescattering is the main cause of depolarization and nonstationarity, especially in vegetatedareas.Results from the analysis and measurements of the long-term polarization behavior ofground clutter show similarities with target behavior [2]. When using linear polarizationin the X band, these measurements show that the co-polarization return is greater thanthe cross-polarization one by about 6 dB on average, for both vertical and horizontalpolarizations. This is typical for volume scattering from vegetation. When using circularpolarization, the power of the cross-polarization return usually slightly prevails over thatof the co-polarization return. Measurements carried out under this condition resulted inan average ratio, between cross-polarization and co-polarization power, ranging from 2 to4 dB. The reader is referred to [55] and the bibliography therein for a detailed treatmentof the subject.13.4.4 Sea ClutterWhen a radar from above illuminates an object on sea surface such as a ship, significantbackscattering from the sea surface limits the target detection capability of the radar. Forsmooth sea, the backscattering appears to emanate from coherent sources located on thesurface. In practice, one encounters rough seas, and multiple interactions of the electromagneticwave with the ocean surface contribute incoherent random point scatterers tothe return, similar to the diffuse scattering observed for land clutter. Measurements andtheory indicate that sea clutter depends on radar frequency, polarization, grazing angleof the incident wave, wind speed, and sea state. For low grazing angles and at lowermicrowave frequencies (L-band or lower), it has been found that horizontal polarizationoffers considerably smaller radar cross section than vertical polarization. For rough seas,at lower grazing angles and higher frequencies, this difference is quite small, and evennonexistent. Measurements on sea clutter with other types of polarization, such as circular,have not been considered mostly because such measurements cannot be carriedout under controlled conditions, and produce large variability in measured RCS. In spiteof these difficulties, linear horizontal polarization is used to reduce sea clutter at lowergrazing angles. This choice of polarization is advantageous for ship-mounted radars totarget coastline and other surface vessels, as the target detectability is considerably enhanced,especially at the lower microwave frequencies in the L-band [56]. Ship detection