Environmental effects on airborne radar performance

A radar is designed to meet customer specifications of range performance, angle accuracy, etc. However, an airborne radar must operate in an environment which may be different from the assumptions used in writing the performance specification. In an aircraft installation, the radar antenna is protected from the atmospheric environment by a radome which may introduce distortions and reflections of the radar energy. In some installations, such as AWACS, the aircraft may intrude into the near-field of the antenna, with subsequent distortions and reflections. The atmosphere between radar and target introduces diffraction effects which frequently cause fluctuation of the received target signal. The surface of the Earth acts as a large, complex target. Modern pulse Doppler radars use range-Doppler processing to separate airborne targets from the backscatter from the Earth (dubbed clutter). The antenna is designed with low side-lobes to minimize “side-lobe clutter” and with adequate system frequency stability to handle “main-beam clutter.” The forward scatter from the Earth frequently causes the radar to see a mirror reflection of the desired target, apparently below the Earth´s surface. This may cause undesired effects in tracking the target. This paper discusses these environmental effects revealed in flight testing of two Westinghouse radars.