chapter  21
Air traffic control system
Pages 1

We have seen examples of primary and secondary

radar systems in previous chapters. To reiterate;

with primary radar, high energy is directed via an

antenna to illuminate a ‘target’; this target could

be an aircraft, the ground or water droplets in a

cloud. In the case of ATC primary radar, the

energy is reflected from the aircraft’s body to

provide range and azimuth measurements. ATC’s

primary radar system places the target(s) on

a plan position indicator (PPI). Primary

surveillance radar (PSR), see Figure 21.1, has its

disadvantages; one of which is that the amount of

energy being transmitted is very large compared

with the amount of energy reflected from the

target. Small targets, or those with poor relecting

surfaces, could further reduce the reflected

energy. Natural and man-made obstacles such as

mountains and wind farms also shield the radar

signals. Secondary surveillance radar (SSR)

transmits a specific low energy signal (the

interrogation) to a known target. This signal is

analysed by a transponder and a new (or

secondary) signal, i.e. not a reflected signal, is

sent back (the reply) to the origin, see Figure

21.2. Secondary radar was developed during the

Second World War to differentiate between

friendly aircraft and ships: this system was called

Identification Friend or Foe (IFF). In the air

traffic control system, the primary and secondary

radar antennas are mounted on the same rotating

assembly, thereby providing a coordinated

system. The complete system is illustrated in

Figure 21.3.