Operating Aircraft Transponders
The previous articles discussed mode S transponders only. But still, a lot of aircraft are flying with the familiar mode A/C transponder. We continue here by explaining how to operate an aircraft transponder and which code to set, and which code to set only if in dire need.
During or previous to a flight a pilot is given a code by ATC over the radio, such as in the radio phrase 'Experimental 2909B, squawk 7000'. The pilot inputs these numbers in the transponder box and his dot on the radar scope becomes correctly associated with his identity. Software can add more data to this dot as: call sign, flightplan information and more.
Eurocontrol published a document containing tips for using an aircraft transponder.
Eurocontrol Guidance Notes
Excerpt from the document:
The transponder system is often referred to as Secondary Surveillance Radar (SSR). The secondary (transmitted by the transponder) signal is much stronger than the 'primary' signal reflected from the aircraft. SSR returns therefore show up more clearly on the radar screen, as well as allowing information to be transmitted in the code.
Transponders operate in various 'modes'. Some military modes are not available on commercial transponders. There are 3 modes used in civilian equipment.
To continue reading see the pdf below: