Principles of Modern Radar - Volume 2 1891121537

616 CHAPTER 13 Introduction to Radar PolarimetryFIGURE 13-12Radar images(range/cross-range)of co-polarizedreturns for coatedcylinder. (FromNaishadham andPiou [51]. Withpermission.)1.510.50−0.5Coated Cylinder VVCoated Cylinder HH1.510.50−0.5−1−1−1.5−1 1 1Frequency = 4 GHz−1.5−1 1 1Theta = 90 deg−50 −45 −40 −35 −30 −25 −20 −15 −10HH polarization than VV. Therefore, variation in the target return with aspect angle,sensed as a function of the transmit and receive polarization states, provides interestingphysics-based information that can be applied to discriminate between coated and uncoatedtargets.13.4.2 Rain ClutterMuch of the clutter surrounding a target is due to objects that move relative to each other,such as ocean waves, rainfall, and foliage movement caused by wind. Clutter can appearwithin the resolution cell of the target; if it is not separated from the target return, clutter cancause missed detections and false alarms. The scattering matrix for clutter is time varyingin general, but for typical radar observation times (few ms) the matrix may be assumedconstant. Rain does not depolarize the incident wave within time scales comparable to theobservation time. Precipitation clutter is known to be very sensitive to antenna polarization,and the choice of circular polarization as a means of reducing rain clutter by as much as30 dB is widely recognized [2]. The rationale behind the use of circular polarizationis that the total return from a spatial resolution cell, within the precipitation volume, isproduced by a large number of spherically shaped hydrometeors. In theory, if a circularlypolarized wave is incident on either a conducting or dielectric sphere, the backscatteredwave has the opposite rotation sense, and polarization cancellation may be used at thereceiver to reduce the clutter from raindrops. In practice, however, the effectiveness ofsuch cancelation of precipitation clutter is limited, ranging approximately from 10 to35 dB, since the assumption of spherical raindrops is only approximately met. Largerraindrops, approximating oblate spheroids, occur for increasing rainfall rates, and in thepresence of wind the shape may become quite asymmetrical and complex [3]. Multipathpropagation due to reflection off land or water (e.g., sea, river, lake) surface can causesignificant depolarization of the incident wave for heavy rainfall, especially in windyconditions. Except for light rain, a significant improvement of clutter attenuation can beachieved by adaptation of antenna polarization. An improvement in clutter cancelationby about 12 dB has been reported for heavy rain using elliptical polarization (comparedwith circular) [52]. However, considering the additional complexity of radar hardware and