7.1 INTRODUCTION

Secondary surveillance radar (SSR) is not a true radar system at all but two-way communication system between an interrogator on the ground and transponders fitted to aircraft, which reply automatically. We include SSR here because the system is very similar to radar in the way it operates, suffers from many of the classical radar problems and is widely used throughot the world, often in conjunction with primary surveillance radar.

The origins of SSR lie in the identify, friend or foe (IFF) systems of World War II; a signal was transmitted from the ground towards a suspect aircraft, which was required to reply with the appropriate code or be treated as a foe. Modern SSR works in a similar way; interrogation messages are transmitted on a one-way uplink frequency of 1030 MHz and cooperating aircraft reply on a one-way downlink frequency of 1090 MHz. The replies are fed to a plot extractor, which decodes the aircraft identity and height and passes them on to the air traffic controllers together with the measured range and bearing. A classic work on secondary surveillance radar is Stevens ¹.

Secondary surveillance radar cleverly avoids problems with clutter through the use of two frequencies, because the receivers at either end of the link are not in tune with the adjacent transmitter and they do not pick up unwanted echoes. Another advantage of SSR is that the transmitter and antenna...