The ACARS allows air/ground datalink communications for Airline Operational and Air Traffic Communications (AOC and ATC). It stands for Aircraft Communications, Addressing and Reporting System and is included in the aircraft Management Unit (or MU or CMU). ACARS messages are transmitted from the ground to the aircraft and vice-versa using several radio communication media such as VHF, HF, SATCOM. In the early days of the ACARS it was using only VHF for transmission, but nowadays advanced technology allows the ACARS to choose itself the transmitting method depending on the current coverage and flown airspace.