What is the purpose of a transponder?
Transponders are typically used for detecting, identifying and locating objects, but they can also be used in other technologies, such as in satellites to relay communications signals. Transponders are commonly found in both civilian and military aircraft and in objects, such as car keys. Active transponders continually broadcast their own signal, which makes them particularly useful in situations where continuous monitoring or tracking is required. They are commonly used in location, identification and navigation systems for commercial and private aircraft, among other applications.
Does every plane need a transponder?
Here’s a breakdown: Class A, B, and C airspace: All aircraft must be equipped with an operable Mode C transponder. Class D airspace: No transponder is required unless otherwise specified by ATC (Pilots only require two-way radio communication in this class of airspace). However, if you wish to operate in class A, B, or C airspace, or at an altitude of over 10,000′ MSL, or within a 30 nautical mile radius of the primary airport in class B airspace, you will need a transponder and altitude encoder (commonly referred to as mode C).