Correction of the Off-Axis Reflector Beam Squint in Passive Images of the Fourth Stokes Parameter at 91 GHz

In this paper we analyze effects of the antenna beam squint on the fourth Stokes parameter V images taken by the fully polarimetric imager SPIRA and suggest methods to correct them. The first images of complex scenery, containing man- made and natural objects, showed unexpected features in the y-parameter with amplitudes of up to 30 K, depending on the contrast in the total intensity images. They have pronounced variation in elevation direction, whereas in azimuth (horizontal) only small changes are observed. A simple relation can be established between the measured fourth Stokes parameter and the scene brightness distributions in V and the total intensity I. It allows correction of the beam-squint effects in the V Stokes parameter image using image processing methods. Another, less general method, could be more easily applied to SPIRA images, achieving a comparable enhancement. In this way, the beam-squint effects were reduced down to the uncertainty of the instrumental polarimetric calibration.