SECONDARY SURVEILLANCE RADAR

Conventional (or primary) radar depends for its efficient operation on the reflection from a ‘target’ for example, an aircraft - which is received by the radar antenna and displayed on the controller's radar screen. The range of primary radar is limited by the strength of the signal from the ground based transmitter which is reflected back to the ground receiver by the aircraft, and also by the aircraft's height above the horizon.

Secondary surveillance radar, however, differs from primary radar in that the signal transmitted by the ground station is received by the aircraft via a piece of equipment on the flightdeck - the transponder - which then retransmits the signal back to the ground station. The power required to produce a return signal is therefore far less with secondary radar than with primary radar which depends only on a reflected signal. The range of SSR is up to twice that available with primary radar. In addition, whilst primary radar displays the position of the aircraft target, secondary radar is capable of questioning the various systems on the flightdeck and having those details transmitted back to a computer at the Air Traffic Control Centre.

Before the flight departs on its journey a flight plan will have been filed giving detailed information on the route, aircraft type, call sign, etc, and these facts will be fed into the computer, together with a four figure identification number which will be exclusive to that flight in UK airspace on that particular day. The figure code number is known as the ‘squawk’, and this word is part of the every day language of air traffic control massages.

A flight in controlled airspace, having been allocated a particular squawk number as part of the ATC clearance, will be required to select the appropriate code on the aircraft's transponder. A selective signal from the ground radar station will be transmitted to the aircraft, whereupon the transponder will process the signal and return it to the ground, including the four figure squawk code.

When the computer has sorted the data the radar signal is then passed to the radar controller's screen, which displays the aircraft position, its passing level, an arrow indicating whether the aircraft is in the climb or the descent, and the cleared flight level. The destination may also be shown. The process in fact takes less time than it does to read about it.

Squawk codes can be used for several other situations to provide ATC with invaluable information on the process of the flight. Code 7700, for example, is the code used to denote an emergency on-board condition.

Code 7600 indicates radio failure, while code 7500 warns the controller of some unlawful interference with the flight. The squawk of 7000 is used by aircraft – usually General Aviation flights – which are operating outside controlled airspace. Aircraft with radio problems may be asked by ATC to squawk IDENT if the transmissions are being received, thus indicating to the ground that radio reception is functioning.