Principles of Modern Radar - Volume 2 1891121537

590 CHAPTER 13 Introduction to Radar Polarimetryorientation about the radar line of sight. Polarimetric measurements subject to Faradayrotation can be calibrated using a calibration target, such as a sphere or trihedral cornerreflector, which has no cross-polarized return. The effect of Faraday rotation can then beestimated and removed from the measured scattering matrix of an arbitrary scatterer. Thereader is referred to Section 10.4 in [11] and the references therein for a detailed discussionon the estimation and correction of Faraday rotation.When the wave strikes an object such as a radar target, characteristic information aboutits material composition, shape, and orientation can be obtained from measurement of thepolarimetric scattering matrix in a suitably defined orthogonal coordinate basis [12–22].The scattering matrix accounts for all information contained in the polarized target returnunder the conditions of the given radar encounter, such as frequency, waveform, targetobservation angle, and presence of clutter. Measurement of the complete matrix requiresusing dual polarized antennas for transmission and reception to characterize co-polarizedand cross-polarized received states for each of the two orthogonal states transmitted. Dueto cost and complexity, most practical radar systems measure only the partial matrix,thereby trading some information on the target for a less expensive system that operatesmore quickly while providing the information that is most often used. Polarimetry dealswith the exploitation of coherent polarization properties of EM waves and their interactionwith the scattering object, observed through complex received voltage or power orradar cross section (RCS) measurements, from which the scattering matrix can be derived.Various radar applications of polarimetry involving target identification, feature recognitionthrough radar imaging, and remote sensing of terrain and weather patterns have beenextensively developed over the years [1–11, 23–26].The foundations of radar polarimetry were laid down in the early 1950s by Kennaugh[12,13]. Kennaugh’s reports [13] document the pioneering research on radar polarimetryconducted by his research group between September 16, 1949, and October 1, 1954.Kennaugh first introduced the concept of scattering matrix in polarimetry and demonstratedthat there exist characteristic polarization states for which the radar receives minimum/maximum power. This work laid the foundation for radar polarimetry and formed a rigoroustheoretical basis behind the design of early (pre-1960) civilian and defense polarimetricradar systems worldwide. Kennaugh introduced the theory to estimate the polarizationstates for optimal reception of a coherent EM wave. The optimization procedure for thechannel power leads to an eigenvalue equation for the radar scattering matrix, whose diagonalizationusing unitary matrix transformation yields the characteristic polarizationstates for which the radar receives minimum or maximum power in the reciprocal, monostaticcase. Of particular importance are the eigen-polarization states that produce nulls inthe cross-polarization and co-polarization channels. Huynen [14] generalized Kennaugh’swork to time-varying targets and showed that a time-varying power-averaged target return(e.g., chaff cloud, ocean surface) is characterized by nine significant polarizationparameters, whereas a fixed target can be described by five parameters. This resultedin the pioneering polarization fork concept, which enabled easy visualization of the nullpolarization states on the Poincaré sphere. He introduced the first target decomposition theoremand showed that the time-averaged target can be decomposed into an effective singlefixed (time-invariant) target that represents the ensemble, and a residue target or N-target,which represents target noise or depolarizing scattering behavior at the higher frequencies.Huynen applied the phenomenological theory of radar targets to scattering from roughsurfaces, such as terrain and sea, and rigorously accounted for the depolarizing behaviorof such objects. This work provided the basis for accurate characterization of land, sea,