Passive millimetre-wave imaging technologies for reconnaissance and surveillance on UAVs

Passive millimetre-wave imaging is a potential solution to surveillance problems like clouds and other obscurants. Research has shown that passive millimetre-wave imaging can form high contrast, natural imagery even through foul weather. This arises principally because the relatively long wavelength of millimetre-waves enables penetration of foul weather and obscurants such as cloud, fog, dust and smoke. Millimetre-wave imaging is essentially thermal imaging at a long wavelength and so both day and night operation is possible. The application of passive acquisition methods, which are hard to detect by hostile forces, is important for strategical and tactical reasons and the sensor’s own security. In mountainous regions or in highly urbanized areas an almost Nadir looking imaging scheme for the data acquisition is beneficial in order to avoid information gaps in certain locations due to shadowing. Because of the incoherent statistical character of the natural thermal radiation, synthetic imaging principles like synthetic aperture radar (SAR) are not applicable and thus millimetre-wave radiometers are forced to use real antenna apertures leading to a poorer spatial resolution. For example consider the case of a W-band airborne reconnaissance system operating from a 5km Nadir distance with a desired ground resolution of about 2m. The antenna aperture would be about 7m, which would be completely impractical for a conventional antenna. For this situation the imaging principle of aperture synthesis from radio astronomy has to be applied. Here a highly thinned array of small-aperture antennas can be used and by proper processing of the signal correlations of each antenna pair the wanted brightness temperature distribution on ground can be determined. The advanced method of aperture synthesis as a new approach for high resolution passive imaging is comprehensively discussed and simulated system design examples are illustrated. The imaging performance of aperture synthesis is investigated using measured high resolution W-band data of typical military targets as an ideal input brightness temperature distribution. From that starting point the ideal system response is computed and impacts of various system errors are shown. The feasibility of such a system on a UAV platform is discussed.