It was used extensively by the British Army and was exported to countries such as Holland, Switzerland, Sweden[1] Finland [2] and South Africa.

The unit was housed in an air-conditioned trailer that was significantly smaller and more portable than the World War II-era SCR-584 and GL Mk.

III radars it replaced; the Blue Cedar weighed about 5 short tons, compared to about double that for the SCR-584.

It could be emplaced and operational in under an hour, automatically feeding data though synchros to the gunnery computer and then directly to the guns.

It remained in service with field units, notably the British Army of the Rhine, until 1957 when large AA guns began to be replaced by the Thunderbird missile.

The British Army's first radars were intended to measure the distance to aircraft as an aiming aid for anti-aircraft artillery.

In the 1930s, measuring the angle to the target was easily accomplished with optical instruments, but rangefinding remained a time-consuming and inaccurate process.

Since the radar's signal spread over about 20 degrees, it observed a wide area of the sky and was able to detect targets before the crews on the optical instruments could.

II design which also measured angles with enough accuracy to directly lay the guns, removing the need for the optical instruments entirely.

In the case of the GL radars, the antennas were supported on large steel frameworks about 10 m across, rather less portable than desired.

A simple device the size of a fist generated tens of kilowatts of radio energy, rivalling some of the most powerful room-filling broadcasters.

More importantly, it operated at wavelengths that were much shorter than any existing system; at ~10 cm, the antennas were only a few centimetres long, making them very easy to fit on aircraft and small vehicles.

They were so small that it became practical to use a parabolic reflector to focus them, producing beams only a few degrees wide from an assembly perhaps a meter across, or smaller.

Initially, most work with the magnetron was focused on airborne roles, where its small size was an enormous advantage.

However, as the winds of war changed, there was increasing demand for a new anti-aircraft radar that could replace the existing GL's with something that was far more practical, and in particular, much more mobile.

In November 1940 the magnetron had been demonstrated to Canadian and US researchers, both of whom had begun developing their own versions of GL based on it.

The Canadian version reached service first, with early production examples being shipped to the UK in November 1942.

Using two reflectors was not going to work on aircraft, and the Air Ministry teams, now known as the Telecommunications Research Establishment (TRE), continued looking for solutions.

By comparing the height of the blips, their operators could see which antenna was closer to the target, and turn the light in that direction to track it.

This used simple electronics that were fed the paired signals and output a current whose magnitude depended on the amount of difference between the two.

IIIs to use auto-follow, but as this system swung the entire cabin around to track, the power required to drive the pointing motors was huge.

This all proved to be lucky timing, as the arrival of production SCR-584s was coincident with the start of the V-1 flying bomb campaign that summer.

Estimates that another war would be at least ten years off suggested no major production should take place, as developments in the interim would render any radars built in that period obsolete.

However, various events in 1949, notably the first Soviet atomic test, led to sweeping upgrades, including the first contract for production Mk.