Radar as used by NZ Air Traffic Control
There are two types of radar in use by Air Traffic Control throughout the world today:
PRIMARY Surveillance Radar (PRS)
SECONDARY Surveillance Radar (SSR)
This radar, which was invented toward the end of the Second World War, uses a radio transmitter and receiver at the same ground-based location. The transmitter emits radio waves which are then reflected by solid objects eg. aircraft, boats, terrain, and weather such as rain and ice. The reflected radio waves are picked up by the associated co-located receiver. As the return signal will be weaker, it is amplified, so allowing the radar operator to see both moving and fixed objects.
Due to the massive amount of reflected radio waves from terrain and weather, the radar operator can suppress these return ‘echoes’ with equipment known as a “Moving Target Indicator” (MTI), to allow easier identification of aircraft on the radar.
The MTI is usually left permanently on, so that only moving targets with speeds of greater than 27kph are seen by the radar operator.
In NZ, Air Traffic Control Primary Surveillance Radars are sited at Auckland, Wellington, and Christchurch and these serve as back-up systems for the main SSR system.
SECONDARY SURVEILLANCE RADAR
This radar system is now the main system used world wide for the controlling of aircraft, as most aircraft nowadays are fitted with equipment called a TRANSPONDER. The Transponder electronically transmits data, such as aircraft call-sign, speed, altitude, and a specific SSR identification code, when interrogated by the ground-based SSR.
Secondary surveillance radar is a computer-based system that relies on the interface between the aircraft computer (transponder) and the ATC ground-based computer. There is no reflecting of radio waves involved.
It is planned to eventually withdraw PRIMARY RADAR worldwide.