Radar, Anti-Aircraft Number 3 Mark 7, also widely referred to by its development rainbow code Blue Cedar, was a mobile anti-aircraft gun laying radar designed by British Thomson-Houston (BTH) in the mid-1940s. It was used extensively by the British Army and was exported to countries such as Holland, Switzerland, Sweden Finland and South Africa. In British service, it was used with the 5.25 inch and QF 3.7 inch AA guns, as well as the Brakemine missile.
Mk. 7 developed from experiments in the mid-WWII era on auto-follow radar systems on the GL Mk. III radar and Searchlight Control radar systems. Production was not taken up at that time due to the imminent arrival of the SCR-584. The concept was studied again in the immediate post-war era, further improvements made, and put into production starting in 1952. About 365 were made in three major production runs, the last ordered in 1954.
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 was normally towed by an AEC Matador artillery tractor unit, as opposed to requiring a semi-trailer. 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.
Blue Cedar was the primary gun-laying system for the Army through the 1950s. Beginning in 1953, the air defence mission began to move from the Army to the Royal Air Force, and from anti-aircraft artillery to surface-to-air missiles, which had their own radars. 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. Some were converted into counter-battery radars known as the Mk. 7(F). These, and other modifications for roles like weather balloon tracking, kept numbers of Blue Cedar in service well into the 1970s.
History
GL Mk. I & II
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. This led to the creation of the GL Mk. I radar which was introduced in 1939.
It quickly became clear that a range-only system was wasting considerable utility inherent to the design. 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. Additionally, it worked at night or in bad weather. This led to the GL Mk. II design which also measured angles with enough accuracy to directly lay the guns, removing the need for the optical instruments entirely. However, as this was not immediately available, a modification of the existing Mk. I's, the Mk. I/EF, was used until the Mk. IIs became more widely available in 1941.
While these systems worked, they were unwieldy. The size of an antenna needed to efficiently broadcast and receive a signal is a function of the wavelength, so with the GL's ~4 m wavelength, antenna elements several meters across were needed. Focussing such a signal requires several such antennas, or one antenna and a suitable reflector, making the complete antenna system much larger. In the case of the GL radars, the antennas were supported on large steel frameworks about 10 m across, rather less portable than desired.
GL Mk. III
The introduction of the cavity magnetron in 1940 led to a revolution in radar design. 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.