Gee, sometimes written GEE, was a radio-navigation system used by the Royal Air Force during World War II. It measured the time delay between two radio signals to produce a fix, with accuracy on the order of a few hundred metres at ranges up to about 350 miles (560 km). It was the first hyperbolic navigation system to be used operationally, entering service with RAF Bomber Command in 1942.

Gee was devised by Robert Dippy as a short-range blind-landing system to improve safety during night operations. In the course of development by the Telecommunications Research Establishment (TRE) at Swanage, the range was found to be far better than expected. It then developed into a long-range, general navigation system. For large, fixed targets, such as cities that were attacked at night, Gee offered enough accuracy to be used as an aiming reference without the need to use a bombsight or other external references. Jamming reduced its usefulness as a bombing aid, but it remained in use as a navigational aid in the UK area throughout and after the war.

Gee remained an important part of the RAF's suite of navigation systems in the postwar era, and was included in aircraft such as the English Electric Canberra and the V-bomber fleet. It also had civilian use, and several new Gee chains were set up to support military and civil aviation across Europe. The system started to be shut down in the late 1960s, with the last station going off the air in 1970. Gee inspired the original LORAN ("Loran-A") system.

The basic idea of radio hyperbolic navigation was well known in the 1930s, but the equipment needed to build it was not widely available at the time. The main problem involved the accurate determination of the difference in timing of two closely spaced signals, differences measured in milli- and microseconds.

During the 1930s, the development of radar demanded devices that could accurately measure these sorts of signal timings. In the case of Chain Home, transmission aerials sent out signals, and any reflections from distant targets were received on separate aerials. An oscilloscope (or oscillograph as it was known in the UK) was used to measure the time between transmission and reception. The transmitter triggered a time base generator that started a "trace" moving quickly along the oscilloscope display. Any received signals caused the beam to deflect downward, forming a blip. The distance that the trace had moved from the left side of the display could be measured to accurately calculate the difference in time between sending and receiving, which, in turn, could be used to calculate the slant range to the target.

Radar can also be used as a navigation system. If two stations are able to communicate, they could compare their measurements of the distance to a target, and use basic trilateration to determine the location. This calculation could then be sent to the aircraft by radio. This is a fairly manpower-intensive operation, and while it was used by both the British and Germans during the war, the workload meant it could generally only be used to guide single aircraft.

In October 1937, Robert (Bob) J. Dippy, working at Robert Watson-Watt's radar laboratory at RAF Bawdsey in Suffolk, proposed using two synchronized transmitters as the basis for a blind landing system. He envisaged two transmitting antennas positioned about 10 miles (16 km) apart on either side of a runway. A transmitter midway between the two antennas would send a common signal over transmission lines to the two antennas, which ensured that both antennas would broadcast the signal at the same instant.

A receiver in the aircraft would tune in these signals and send them to an A-scope-type display, like those used by Chain Home. If the aircraft were properly lined up with the runway, both signals would be received at the same instant, and thus be drawn at the same point on the display. If the aircraft were located to one side or the other, one of the signals would be received before the other, forming two distinct peaks on the display. By determining which signal was being received first, pilots would know that they were closer to that antenna, and would be able to recapture the proper direction by turning away from it.