This property means that the dihedral can be used as a simple way of creating a cross-pol response in an HH radar system.įigure 5-2: Polarization signatures of a dihedral or double-bounce reflector However, if the reflector is rotated by 45° around the radar line of sight, the linear horizontal co-pol response is zero and the linear horizontal cross-pol response is a maximum. Because the two reflecting surfaces of the dihedral sides negate the sign of the ellipticity a second time, this results in a typical 'double-bounce' or 'even-bounce' signature. In the case of the dihedral reflector, when its corner (the intersection of its sides) is aligned horizontally, parallel to the horizontal axis of the EM wave, the co-pol response is a maximum for linear or elliptical horizontal, linear or elliptical vertical and circular polarizations (Figure 5-2). Interesting signatures are obtained from a dihedral corner reflector and from Bragg scattering off the sea surface. The sign changes once for every reflection - the sphere represents a single reflection, and the trihedral gives three reflections, so each behaves as an 'odd-bounce' reflector.įigure 5-1: Polarization signatures of a large conducting sphere or trihedral corner reflectorįor more complicated targets, the polarization signature takes on different characteristic shapes. The wave is backscattered with the same polarization, except for a change of sign of the ellipticity (or in the case of linear polarization, a change of the phase angle between E h and E v of 180°). One example is the PWS software for PCs.įigure 5-1 shows the polarization signatures of the simplest class of targets - a large conducting sphere, a flat plate or a trihedral corner reflector. Polarization signatures and the Poincaré sphere can be conveniently drawn on polarimetric analysis workstations. The polarization plots have peaks at polarizations that give rise to maximum received power, and valleys where the received power is smallest, in agreement with the concept of Huynen's polarization fork in the Poincaré sphere. Often the plots are normalized to have a peak value of one. For an incident wave of unit amplitude, the power of the co-polarized (or cross-polarized) component of the scattered wave is presented as the z value on the plots. For each of these possible incident polarizations, the strength of the backscatter can be computed for the polarization that is the same as the incident polarization (giving the co-pol signature plot) and for the polarization that is orthogonal to the incident polarization (giving the cross-pol signature plot). These variables are mapped along the x- and y-axes of a 3-D plot portraying the polarization signature. The BSA convention is usually used for the signatures.Īn incident electromagnetic wave can be selected to have an Electric Field vector with an ellipticity between -45° and +45°, and an orientation between 0° and 180°.

These two signatures do not represent every possible transmit-receive polarization combination, but do form a useful visualization of the target's backscattering properties. This choice of polarization combinations leads to the calculation of the co-polarized and cross-polarized responses for each incident polarization, which are portrayed in two surface plots called the co-pol and cross-pol signatures.

To simplify the visualization, the backscattered polarizations are restricted to be either the same polarization or the orthogonal polarization as the incident wave. The scattering power can be determined as a function of the four wave polarization variables, the incident and and backscattered and angles, but these constitute too many independent variables to observe conveniently. One such visualization is provided by the polarization signature of the target. Because the incident wave can take on so many polarizations, and the scattering matrix consists of 4 complex numbers, it is helpful to have a graphical method of visualizing the response of a target as a function of the incident and backscattered polarizations.

Dihedral reflector