Transponders are used to help air traffic control radar to identify aircraft and in collision avoidance systems.
Upon interrogation the transponder transits information dependant on the mode. Below is information on the possible modes a transponder can be or be set to.
| Squawk Code | Pressure Altitude | Flight Identifier | |
|---|---|---|---|
| Mode A | YES | NO | NO |
| Mode C | YES | YES | NO |
| Mode S | YES | YES | YES |