INTERFEROMETRIC EVIDENCE FOR THE OBSERVATION OF GROUND BACKSCATTER ORIGINATING BEHIND THE CUTLASS COHERENT HF RADARS

Interferometric techniques allow the Super-DARN coherent HF radars to determine the elevation angles of returned backscatter, giving informa tion on the altitude of the scatter volume, in the case of ionospheric backscatter, or the reflection altitude, in the case of ground backsc atter. Assumptions have to be made in the determination of elevation a ngles, including the direction of arrival, or azimuth, of the returned signals, usually taken to be the forward look-direction (north) of th e radars, specified by the phasing of the antenna arrays. It is shown that this assumption is not always valid in the case of ground backsca tter, and that significant returns can be detected from the backward l ook-direction of the radars. The response of the interferometer to bac kscatter from behind the radar is modelled and compared with observati ons. It is found that ground backscatter from a field-of-view that is the mirror image of the forward-looking field-of-view is a common feat ure of the observations, and this interpretation successfully explains several anomalies in the received backscatter.