In the context of aviation and aerospace, "Automatic Direction Finding" (ADF) refers to a navigation system used by aircraft to determine their bearing (direction) from a known radio transmitter, typically a Non-Directional Beacon (NDB). The system is called "automatic" because it uses an automatic antenna system to determine the direction of the signal.

Here's how it works:

1. Non-Directional Beacon (NDB): An NDB is a radio transmitter located on the ground that broadcasts a continuous signal. The NDB's signal is not directional, meaning it radiates equally in all directions.
2. Aircraft's ADF system: The aircraft is equipped with an ADF system, which consists of a special antenna system and a receiver. The antenna system is designed to rotate to find the direction of the NDB's signal.
3. Signal detection: When the aircraft's ADF system detects the NDB's signal, it uses the rotating antenna to determine the direction of the signal.
4. Bearing indication: The ADF system displays the bearing (direction) from the aircraft to the NDB on a navigation instrument, such as a compass or a dedicated ADF indicator.

The ADF system is used for navigation, particularly in instrument meteorological conditions (IMC) or when visual references are limited. By knowing the bearing from the aircraft to the NDB, pilots can determine their position and navigate to a specific location, such as an airport or a waypoint.

ADF systems have been widely used in aviation for many decades, but they are being gradually replaced by more modern navigation systems, such as GPS (Global Positioning System) and RNAV (Area Navigation). However, ADF systems are still used as a backup or auxiliary navigation aid, especially in areas where GPS signal coverage is limited or unreliable.