How did shore-based radio direction-finding stations assist ships approaching harbors?

Shore-based radio direction-finding stations assisted ships approaching harbors by determining the vessel's position when visibility was limited by fog or darkness. Upon receiving a radio request from a ship, two or more shore stations would identify the direction from which the ship's signal arrived. This directional information was then transmitted back to the vessel, allowing the navigator to fix the ship's position.1

This service was an early application of radio navigation, though it had the significant drawback of being limited to serving only one vessel at a time. To address this, the system evolved beginning in 1921. Instead of relying on shore stations to locate the ship, continuously operating transmitters were placed on shore, and the radio direction finder was moved onto the ship. This allowed navigators to take their own bearings on shore beacons independently, eliminating the bottleneck of the earlier system and providing greater flexibility for safe harbor approaches.1