SCR-270

The SCR-270 (Signal Corps Radio model 270) was one of the first operational early-warning radars. It was the U.S. Army's primary long-distance radar throughout World War II and was deployed around the world. Its also known as the Pearl Harbor Radar, since it was a SCR-270 set that detected the incoming raid about half an hour before the attack commenced.

Two versions were produced, the mobile SCR-270, and the fixed SCR-271 which used the same electronics but used an antenna with somewhat greater resolution. An upgraded version, the SCR-289, was also produced, but saw little use. All of the -270 versions were later replaced by newer microwave units after the cavity magnetron was introduced to the US during the Tizard Mission. The only early warning system of the sort to see action was the AN/CPS-1, which was available in late 1944.

The Signal Corps had been experimenting with some radar concepts as early as the late 1920s, under the direction of Colonel William R. Blair, director of the Signal Corps Laboratories at Fort Monmouth, New Jersey. Although the Army focused primarily on infra-red detection systems (a popular idea at the time), in 1935 work turned to radar again when one of Blair's recent arrivals, Roger B. Colton, convinced him to send another engineer to investigate the US Navy's CXAM radar project. William D. Hershberger went to see what they had, and returned a positive report. Gaining the support of James B. Allison, the Chief Signal Officer, they managed to gather a small amount of funding and diverted some from other projects. A research team was organized under the direction of civilian engineer Paul E. Watson.

By December 1936 Watson's group had a working prototype, which they continued to improve. By May 1937 they were able to demonstrate the set, detecting a bomber at night. This demonstration turned out to be particularly convincing by mistake; the Martin B-10 bomber had originally been instructed to fly to a known point for the radar to find it, but could not be located at the agreed upon time. The radar operators then searched for the bomber and located it about ten miles (16 km) from its intended position. It was later learned that winds had blown the bomber off course, so what was to be a simple demonstration turned into an example of real-world radar location and tracking. Development of this system continued as the SCR-268, which eventually evolved into an excellent short-to-medium range gun laying system.

...
Wikipedia