SCR-270

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

An upgraded version, the SCR-289, was also produced, but the set was later replaced by newer microwave units after the cavity magnetron was introduced to the US during the Tizard Mission.

Building of the radar

The Signal Corps had been working on radar systems as early at the late 1920s, under the direction of the US's "father of radar", William R. Blair, director of the Signal Corp Laboratories at Fort Monmouth, New Jersey. Although the Army focussed 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 visit the US Navy's CXAM radar efforts. William D. Hershberger duely went to see what they had, and returned an extremely positive report. Gaining the support of James B. Allison, the Chief Signal Officer, they managed to gather a small amount of funding and "stole" some more from other projects.

By December 1936 they had a working prototype, which they continued to work on and improve. By May 1937 they were able to demonstrate the set, picking up a bomber at night. Development of this system continued as the SCR-268, which eventually evolved into an excellent short-to-medium range gun laying system.

In April of 1937 a LtC. Davis, an officer in an Army Air Corps Pursuit Squadron in the Panama Canal Zone (CZ), sent a request for a "Means of Radio Detection of Aircraft" to the US Army's Chief Signal Officer (CSig.), bypassing normal channels of command. The SCR-268 was not really suited to this need, and after its demonstration in May they again received a request for a long-range unit, this time from "Hap" Arnold who wrote to them June 3, 1937.

Shortly thereafter the Signal Corps became alarmed that their radar work was being observed by German spies, and moved development to Sandy Hook at Fort Hancock, the coast artillery defense site for lower New York harbor. After the move work immediately started on the Air Corps request for what was to become known (in 1940) as the "Radio Set SCR-270". Parts of the SCR-268 were diverted to this new project, delaying the completion of the SCR-268.

Key to the -270's operation was the primary water-cooled 8kW continuous/100 kW pulsed transmitting tube. Early examples were hand-built, but a contract was let to Westinghouse in October 1938 to provide production versions under the Westinghouse designation "WL-530" and the Signal Corps type number "VT-122". A pair of these arrived in January 1939, and were incorporated into the first SCR-270 in time to be used in the Army's maneuvers that summer. Several improved components followed as the Army offered additional contracts for eventual production.

The original -270 consisted of a four-truck package. The antenna was mounted on a folding mount derived from a well-drilling derrick, and could be mounted on a flatbed trailer for movement. When opened it was 55 feet tall, mounted on a 8 foot wide base containing motors for rotating the antenna. The actual antenna itself consisted of a series of 36 half wave dipoles backed with reflectors, arranged in three bays, each bay with twelve dipoles arranged in a four-high three-wide stack. The antenna mount required another truck of its own (although it had its own wheels and didn't need a trailer), the generator and broadcast amplifier were placed in another truck, and finally the operations van was the last.

In use, the antenna was swung by command from the operations van, the angle being read by looking at numbers painted on the antenna mount. The radar operated at 106 MHz, using a pulse width from 10 to 25 microseconds, and a PRF of 621 Hz. With a wavelength of about nine feet, the SRC-270 was comparable to the contemporary Chain Home system being developed in England, but not to the more advanced microwave systems in Germany. This frequency did turn out to be useful, however, as it is roughtly the size of an airplane's propeller, and provided strong returns from them depending on the angle. Generally it had an operational range of about 150 miles, and consistantly picked up aircraft at that range.

Deployment

A fixed version of the '270, SCR-271-A, s/n 1 was delivered to the Canal Zone and "Went to Radiate" (in operation) by October of 1940 at Fort Sherman on the east end of the Panama Canal. It picked up airliners at 117 miles in its initial test run. The second set was set up on Tobaga Island on the west end of the Canal by December of 1940, thus giving radar coverage to the vitally important but vunurable Panama Canal.

Production at Westinghouse quickly ramped up, and by the end of 1941 100 had already been delivered.

Six SCR-270's were sent to Hawaii and provided almost continuous coverage of the main islands. However there was no real communications system or reporting chain set up. At one point the operators of one of the sets were instructed to phone in reports from a local gas station some distance down the road. Although communications were eventually improved, the chain of command was not.

Use of radar at Pearl Habor

SCR-270-B s/n 012 was at Opana Point, Hawaii on the morning of the seventh of December, 1941 manned by two privates, Elliot and Joseph Lockard. That morning the set was supposed to be shut down, but the soldiers decided to get in additional training time in since the truck scheduled to take them to breakfast was late in arriving. At 7:02 they saw the Japanese aircraft approaching Oahu and Lockard telephoned the information center at Fort Shafter and reported the incoming planes. The operator taking his report passed on the information, but from that point on it was ignored, reportedly as the commanders thought they had detected a flight of B-17's arriving that morning from the US.

The raid on Pearl Harbor signaled the United States' formal entry into World War II.