In the movies, radar screens show incredible detail about everything. In real life, radar is easily confused, doesn’t see small planes, and may have trouble determining altitudes. Transponders solve this by reporting an aircraft’s altitude, speed, directional heading and identification code to air traffic controllers and nearby aircraft, using an electronic format that syncs with radar. And the identification codes tell controllers which blip is which flight, something radar has no way to detect.
— The Atlantic’s Gregg Easterbrook, making the case that all passenger aircraft should be equipped with transponders that can’t be turned off.