TRANSPONDER
Primary Radar does not require any additional equipment on board the aircraft. The information it supplies to the ground radar operator is incomplete and unreliable. The controller might be working with, say, twelve aircraft. These aircraft will all appear on the radar screen but the controller will not be able to tell which dot represents which aircraft. Furthermore, primary radar provides no height information; therefore two aircraft could look on the screen as though they are about to collide, when in fact one is thousands of feet above the other.
The use of the transponder is called Secondary Surveillance Radar, (SSR). The equipment in the aircraft consists of a small box with the means to select a series of four numbers between 0 and 7. When the pilot first contacts a ground radar service, he may be asked to "squawk" a certain number e.g. 3423. The pilot will then switch the transponder to standby and then select the number given to him by the controller, in our example 3423. After selecting the number he will the take the set off standby and switch the box to either the On or Alt switch position. Each time the scanning radar beam comes round to our particular aircraft it will trigger the transponder to send the number selected, (3423). This number will appear on the radar screen along side the dot representing our aircraft. This will enable the controller to easily identify each of the aircraft on their screen.
Some light aircraft and all commercial aircraft have what is known as "mode C" or "mode S" transponders. These sets not only transmit the selected squawk number to the controller's radar screen, but also the aircraft's altitude and other details which will appear along side the dot.