Atmospheric Propagation

The Group’s investigations of atmospheric propagation at millimetre wavelengths grew out of its work for DERA Malvern (now QinetiQ) on passive ranging.  That technique, which uses spatial interferometry, allows one to measure the curvature of an arriving phasefront and hence determine the bearing and range of an emitter.  Whilst the original objective was to locate passively an emitting beacon, it became apparent that the technique could also be used as a means to probe the fluctuations in the refractive index of the air in the path between the emitter and the receiver.  Initial experiments revealed that the technique is sensitive to tiny variations in the refractivity of the air, and can distinguish between true absorption and deflection of the beam.  In fact, it is possible to identify if the air in the path has deflected the beam to one side or has focussed/defocused it along the line of sight, or has genuinely absorbed the signal energy.  These subtleties can only be revealed with a multi-port spatial interferometric receiver.  A simple point-to-point link with a single receiver would simply see a change in apparent attenuation in each case.  The picture below depicts a 3-port interferometer sensing the curved phasefronts arriving from a remote source.