Air-to-ground communication was first made possible by the development of two-way aerial telegraphy in 1912, soon followed by two-way radio. By the Second World War, radio had become the chief medium of air-to-ground and air-to-air communication. Since then, transponders have enabled pilots and controllers to identify planes automatically, greatly improving air security. Most recently, in addition to sophisticated radio and GPS systems, the unmanned aerial vehicle, or drone, has revolutionised aerial surveillance and combat.