AN APPLICATION OF THE MONOPULSE PRINCIPLE TO DETERMINING ELEVATION ANGLES IN SAR IMAGES

In mapping nonflat regions of the Earth using air-borne synthetic aper ture radar, (SAR), terrain height variations cause two problems in rad iometric calibration: the first being that the local incidence angle f or any pixel may vary from that given by the flat (or curved) Earth as sumption, the second, being that the wrong elevation angle may be used in correcting for the radiometric variation of the antenna pattern. I n tracking radars, simultaneous amplitude or phase measurements made b y the same radar antenna, but modulated differently, are compared to d etermine the angular position of targets. This is known as the monopul se principle. In this paper, we show how polarimetric SAR da a can be used in a novel application of the monopulse principle to determine th e elevation angle and thus, the height at the different parts of the i mage. Our approach begins with the observation that, provided like- an d cross-polarized backscatter are uncorrelated, then the algorithm des cribed in [1] for calculating antenna crosstalk yields a measurable qu antity whose amplitude (and phase) depends only on elevation angle (or off-boresight angle). Thus, if we determine the crosstalk for a given point in the image, we can relate that measurement the elevation angl e appropriate to that point. Knowledge of the slant range to the point then allows determination of the height of the platform above it. Thi s operation, repeated at many locations throughout the image, allows u s to build up a topographic map of the height of the aircraft above ea ch location. The approach described in this paper gives sufficient res olution in elevation to allow the antenna pattern radiometric correcti on to be done properly, since it allows the determination of elevation angles at rid of points in the image. Knowledge of the slant range to each point and the height of the aircraft then allows us to build up a grid of height estimates for the imaged area. Unfortunately, the spa tial resolution of the grid was not sufficient to turn the height maps into usable maps of local incidence angle. In the paper we also descr ibe how our approach may be improve upon, using an active, phased arra y antenna.