S-Phone

The S-Phone system was a UHF duplex radiotelephone system developed during World War II for use by Special Operations Executive agents working behind enemy lines to communicate with friendly aircraft and coordinate landings and the dropping of agents and supplies.

It was a highly directional unit which required the operator to face the path of the aircraft.

It permitted direct two-way voice communication with an aircraft up to a range of 30 miles (48 km).

[2] The primary purpose of the S-Phone was not navigational as such; rather, it was to provide a reasonably secure channel for coded conversation between staff officers based in London and agents in the field, allowing for the exchange of orders and information.

The security of a circuit was sometimes tested by using an officer who could recognize whether a voice was actually that of the agent who was supposedly speaking.