1. A perimeter antenna array for a synthetic aperture radar comprising: at least one linear array of radar transmitter elements; at least one linear array of radar receiver elements orthogonal to the linear array of transmitter elements; one or more transmitter switches for switching signals to a selection of said radar transmitter elements; one or more receiver switches for switching received signals from a selection of said radar receiver elements; and one or more processors for processing the received signals to synthesise virtual elements equi-distant from pairs of the transmitter elements and the receiver elements.