Radar, Anti-Aircraft No. 3 Mk. 7

AA No. 3 Mk. 7

Finnish AA No. 3 Mk. 7 radar used as a counter-battery radar under the name "Yrjö".
Country of origin	UK
Introduced	1952 (1952)
No. built	~365
Type	Gun Laying
Frequency	3 to 3.12 GHz (S band)
PRF	1500 pps
Pulsewidth	0.5 μs
RPM	20 rpm while scanning
Range	950 to 36,000 yd (870–32,920 m)
Diameter	5 ft (1.5 m)
Power	200 kW
Other Names	Blue Cedar

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^[1] Finland ^[2] 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.