Automatic Direction Finding System
a navagation system used by aircraft and sea-going ships. It consists of one or more ground stations that transmit a signal in the 225 to 500 kHz (.225 to .500 MHz) range. Because of the low-frequnecy of these signals, their range is very good and not limited by the line-of-sight range of the VOR system. The recevier in the airplane/boat uses a loop antenna to "home-in" on the station. There is also a long-wire receiving antenna installed on many aircraft. Most ADF receivers can also recieve the AM broadcast band (530 to 1710 kHz) so pilots can also home in on powerful broadcast stations.
ADF signals should not be used in thunderstorms because the high amount of electro-magnetic interferance that lightening causes.
Many pilots can't even use the ADF system, which is a shame, becuase of better systems such as VOR and the GPS systems.