Radar Echoing Area Polar Diagrams of Birds

Abstract

As part of an investigation into the sources of unidentified radar point echoes, or ‘angels’1–3, we have been measuring the echoing area polar diagrams4 of birds, using a high-resolution X-band radar. This radar, which is horizontally polarized, is capable of measuring equivalent echoing areas as low as 2 × 10−6 sq. metre with an accuracy better than ± 1 db. and of detecting even smaller radar targets. Each bird in flying position, but with wings closed and legs retracted, was fixed to a nylon cord. The nylon cord was held vertically between ground and an aerial line suspended between two towers. Typical azimuth polar diagrams for three birds are shown in Fig. 1. Each pattern was made with the aerial of the radar fixed in elevation and bearing, while the bird was rotated in bearing about a point at the centre of its body. The rate of rotation of the specimen and the time constant of the equipment provide a smoothing factor over approximately 10° in azimuth. This smoothing factor provides satisfactory ‘averaging’ and removes the fine lobe structure. The spacing between radar and bird was chosen to give an even illumination of the rotating bird. The birds were placed at heights which coincided with the radar aerial elevation angle of approximately 18°, an arrangement which ensured a low side-lobe background and consequently optimum radar sensitivity. The bird echoing areas were evaluated by comparison with standard metal spheres.