A transponder operates by rebroadcasting a signal it receives. It receives a signal -- called an "interrogation" -- and then transmits that signal back after analyzing the request for information. The transponder receives the signal on its "interrogator," its reception component. The signal goes through a frequency converter and the transponder then automatically transmits back a radio wave at a preprogrammed frequency. This way of operating allows the transponder to receive and transmit signals at the same time.
Transponders operate using codes in the format of a sequence of pulses. These codes are referred to as "interrogation modes." The different modes identify the type of aircraft by its purpose. Mode 1 provides two-digit, five-bit mission code and is only for military use. Mode 2 provides a four-digit octal unit code and is also only for military use, particularly for jet fighters. Mode 3/A provides a four-digit octal identification code that is assigned by the air-traffic controller and is for military and civilian use. Both Mode 4 and Mode 5 use cryptic codes and are for military use. Mode 4 provides a three-pulse reply to crypto-coded "interrogations," and Mode 5 provides a cryptographically secured version of Mode S.
A transponder code can be combined with pressure-altitude information provided by Mode C. Like Mode 3A, C can be used to help air-traffic controllers identify aircraft and maintain distance. Mode S is designed for automatic collision avoidance without the assistance of air-traffic control. It is often required in controlled airspace around the world. Mode S transponders are compatible with Modes A and Mode C and are designed for automatic collision avoidance.
The FAA classifies airspace by the letters A, B, C, D, E and G, each of which has its own rules for aircraft entering the particular space. For example, aircraft entering any airspace designated A, B or C must have a Mode 3/A transponder.
