A perimeter antenna array for a radar, in particular a slope monitoring radar, formed from a pair of parallel linear arrays of receiver elements and a pair of parallel linear arrays of receiver elements, together forming a rectangle of receiver and transmitter elements. Signals are switched to the transmitter elements and received by the receiving elements. The signals are processed to obtain signals for virtual elements located equidistant between transmitting and receiving elements. The signals from the virtual elements are analyzed to produce a radar image.