GEE

In 1935, the British began developing GEE. It was developed by Robert Watson-Watt at the Air Ministry and introduced by the Royal Air Force (RAF). The idea of hyperbolic navigation known in the 1930s, but the first trials didn't happen until 1941.  Starting in 1942 these radio signals were used to guide the WWII carpet bombing of Germany. Germany began it's attempts at jamming the GEE system in 1943 but these were largely ineffectual. So what's hyperbolic navigation?

Hyperbolic navigation is a class of radio navigation systems based which determines position based on the  microsecond difference in timing between the reception of two signals.  While you need three points to triangulate a location in 3 dimensional space, the surface of the earth is effectively a 2 dimensional plane. But hyperbolic navigation requires the plotting all of the potential locations of the receiver for the measured delay. This produces a series of hyperbolic lines (a parabola) on a chart. Taking multiple measurements narrowed this down. GEE was the first such system. 

Imagine two terrestrial radio stations located 186 miles apart. At light speed (the speed radio signals travel) their signals can reach each other in exactly 1 millisecond. So Station A emits a pulse, this pulse is received at Station B and it triggers the transmission of another pulse 1 ms later. But a ship in motion at a third location will receive these at odd intervals because of the distance from each station. But knowing their interval, they can plot a circle around each transmitter, and at their points of intersection (parabolas) they can determine their possible location down to a set of two. Then they had to use traditional navigation to pick which of the two. The source of that problem... is time.

In order to measure the exact time it took for a signals to reach the receiver, the receiver must know the exact time the signal was transmitted. But reliable millisecond time-keeping wasn't possible in the 1930s. The most common clock to use with GEE was a crystal oscillator. But that drifts about 1 to 2 seconds in a month. That may sound trivial, but 2 seconds is a distance of 372 miles... that's the difference between Bermuda and the Carolina coast. That's a big deal. (This problem wasn't overcome until the advent of atomic clocks in the 1960s.) So instead of absolute time, differential time was used. Your distance from the two transmitters was unknown, but the difference between their signals was measurable on an oscillograph and the delay curves were available on a chart.

GEE signals were all sent on the same frequency which made it difficult to distinguish the original signal from the response signal. Chains of GEE stations were built in the UK, France and northern Germany. Ultimately it was replaced by VOR systems and LORAN. Some of the British GEE equipment was used in the later GEE-H system which operated at the 20-80 MHz range. The last GEE chain was shut down in 1970

The irony in all this is that hyperbolic navigation was originally developed by Germany. Meint Harms lectured on the topic as a masters student at Seefahrtschule Lübeck, a navigational school. After becoming a professor of Mathematics, Physics and Navigation at the Kaisertor in Lübeck he began to demonstrate models of the system which he patented in 1932.