A radio direction finder (RDF) is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance from their destination (see Radio navigation). The distinct technology Range and Direction Finding was the abbreviation used to describe the predecessor to radar.
Read more about Radio Direction Finder: History, Operation, Usage in Maritime and Aircraft Navigation, Automatic Direction Finder (ADF)
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