Design considerations for an harmonic radar to investigate the flight of insects at low altitude

Abstract

Most of our current knowledge about the behaviour of insects flying at high altitude has been derived directly from observations made with conventional entomological radars. Until recently it was not possible to apply this very powerful technique to study the behaviour of insects flying close to the ground because the strong radar echoes, reflected from ground features and from vegetation (‘clutter’), almost always mask the very weak signals returned from insects. This constraint has been overcome, at least for insects weighing more than ≈50 mg, by the development of an harmonic radar, and of transponder tags small enough to be carried by these insects without impeding their flight performance. Because the transponders have to be carried by insects in flight, extreme miniaturisation was obviously essential and it was achieved by using a passive, frequency-doubling technique, so that no on-board battery was needed. Transponders with all-up weights of between 1 and 12 mg were produced, the weights varying according to the degree of mechanical robustness required. The frequency-doubled signals from the transponder are selectively detected by the radar so that the tagged insects can be located even in the presence of very strong clutter. The system has three major advantages over conventional visual and video methods of observing low-flying insects. Firstly, and most importantly, its range of operation (up to 900 m) allows surveillance and flight tracking over hundreds of thousands of m 2, i.e. four orders of magnitude greater than is possible with conventional methods. Secondly, it provides dynamic and geometrically correct records of the insects’ horizontal flight paths and thirdly, it works equally well by day and night. The technique has introduced a new era in the study of insect flight at low altitude.