Automatic direction finder
An automatic direction finder (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station. ADF receivers are normally tuned to aviation or marine NDBs (Non-Directional Beacon) operating in the longwave band (LW) between 190 – 535 kHz.
Source: Wikipedia — Automatic direction finder (CC BY-SA 4.0)