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Automatic Direction Finding
From SKYbrary Wiki
Automatic direction finding (ADF) is an electronic aid to navigation that identifies the relative bearing of an aircraft from a radio beacon transmitting in the MF or LF bandwidth, such as an NDB or commercial radio broadcast station.
Essentially, ADF consists of a loop aerial which is rotated physically or electronically and detects the direction of minimum reception from the beacon relative to the aircraft direction. This information can be used directly to home to the beacon; alternatively, the aircraft equipment combines the relative bearing with heading information from the aircraft compass to provide bearing position lines which may be plotted on a chart.