The use of primary pulsed radar as a surveillance instrument in Air Traffic Control is familiar to most, and is a direct development of the various ground radars current in the last war. The wavelengths generally employed are 10 cms. for more local control, and 50 cms. for long range and high coverage. The primary radar depends for its information on reflection from the target aircraft of sequences of pulses of electromagnetic energy radiated by the aerial, and displays this information as a “paint” on a P.P.I. (Plan-Position Indicator) cathode ray tube giving range and bearing relative to the radar site. The radar beam is fan-shaped, narrow in the horizontal plane to a few degrees, and wide in the vertical plane to provide adequate vertical cover. The beam is made to scan through the target by a rotation of the aerial about a vertical axis up to a rate of 14 r.p.m. The beam width in the horizontal plane is necessarily kept narrow to provide adequate discrimination between targets at the same range but different bearings. The primary radar is frequently associated with a height-finding radar in which the beam shape is reversed, having a narrow angular cross section in the vertical plane. In this case the aerial is made to nod up and down along an appropriate azimuth selected by the controller, and the height of an aircraft is displayed on a height-range cathode ray tube.