Modelling and prediction of obstacle shadowing on secondary surveillance radar target azimuth

National Air Traffic Services Ltd. (NATS) currently operates some 20 secondary surveillance radar (SSR) stations throughout the country for monitoring air traffic. Recorded data from some of these sites has shown deviations in aircraft positional accuracy when the SSR antenna is partially shadowed by neighbouring man-made structures. Modern SSR monopulse receivers estimate the bearing of aircraft through the orientation of the incident planar wavefront. Obstacles in the propagation path diffract part of the electromagnetic wave energy and as a result, the electric field across the antenna array is disturbed. The resultant azimuth error in the shadowed sector depends on the shape, distance and relative height between the antenna and the shadowing obstacle. A method to predict the azimuth error curve due to obstacle shadowing based on cylindrical diffraction theory is described. A comparison between experimental and calculated data reveals that azimuth errors can be very accurately estimated when the obstacle geometric shape is relatively simple