SCR-270 radar

SCR-270 radar

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 commenced.

Two versions were produced, the mobile SCR-270, and the fixed SCR-271 which used the same electronics but replaced the antenna with a model 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.

Building of the radar

The Signal Corps had been experimenting with some radar concepts as early at the late 1920s, under the direction of Colonel William R. Blair, director of the Signal Corp 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 duly 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. 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 work on and improve. By May 1937 they were able to demonstrate the set, picking up 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 in order for the radar to find it, but could not be located at the agreed upon time. The radar operators then started a search for the bomber and located it about ten miles from where it was supposed to be. It was later learned that winds had blown the bomber off course, so what was intended to be a simple demonstration of the concept turned into an excellent 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.

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 -268.

Key to the -270's operation was the primary water-cooled 8 kW 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 an 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 roughly 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 consistently picked up aircraft at that range.

The declassified US military document [ "U.S. Radar -- Operational Characteristics of Available Equipment Classified by Tactical Application"] gives performance statistics for the SCR-270-D, namely "maximum range on a single bomber flying at indicated heights, when set is on a flat sea level site":

Deployment and Incomprehension

's Tobaga Island on the Pacific end of the Canal by December 1940, thus giving radar coverage to the vitally important but vulnerable Panama Canal. Westinghouse quickly ramped up production, and produced 100 by the end of 1941.

Operators of sets that were sent to the Panama canal, the Philippines, Hawaii and other strategic locations were all gathered for an air defense school at Mitchel field, New York in April 1941. The school was the culmination of efforts began in 1940, when the War Department created the Air Defense Command headed by Brig. Gen. James E. Chaney. [] Chaney was tasked by Hap Arnold to collect all information on the British air defense system and transfer the knowledge as quickly as possible to the US military. Air Marshal Dowding, one of the designers of the air defense system used during the Battle of Britain, was at the school and discussed with the American generals the design and urgency of establishing the Hawaiian system, in particular emphasizing the need for thorough radar site coverage along the coasts. []

Despite the high level attention and the excellence of the school in training on the use of the SCR-270 and its integration and coordination with fighter intercepts, the army did not follow through on supporting the junior officers who were trained at this session. Army Major Kenneth Bergquist returned to Hawaii after attending the school intending to set up a coordinated system, but when he arrived he found the local Army leadership was uninterested in the system and he was reassigned back to his fighter unit. Only when incomprehensible equipment began appearing did the army return Bergquist from his fighter unit and was told his job was only to assemble the equipment when it arrived. Assigned the defense of Hawaii, General Walter Short had faint grasp of the weapons and tactics that Army technologists led by Hap Arnold were aggressively pushing them to adopt. Except in rare cases, there was little interest in assisting or even cooperating with the goal of setting up the air defense system. On his own initiative, Bergquist along with some other motivated junior officers built a makeshift control center without authorization and only by scrounging.

The first SR-270's became functional in July, 1941 and by November Bergquist only had a small team established and a set of four SCR-270-B's stationed around Oahu with one unit in reserve. They were placed on the central north shore (Haleiwa), Opana Point (northern tip), in the northwest at the highest point- Mount Kaala, and one in the southeast corner at Koko Head. 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. By explicit order by General Short, the radar stations were to only be operated for four hours per day and to shut down by 7AM each day.

Although communications were eventually improved, the chain of command was not. Air defense required direct control of assets spread out over disparate units; anti aircraft guns, radars, and interceptor aircraft were not under a unified command. This had been one of the primary problems identified by Robert Watson-Watt prior to the war, when a demonstration of an early radar system had gone comically wrong even though the radar system itself had worked perfectly. Dowding was well aware of the importance of a unified command, but this knowledge simply did not result in changes within the Army structure.

Use of radar at Pearl Habor

Unit 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 at a distance of 130 miles and Lockard telephoned the information center at Fort Shafter and reported "Large number of planes coming in from the north, three points east". The operator taking his report passed on the information repeating that the operator emphasized he had never seen anything like it, and it was an "an awful big flight."

From that point on the sighting was ignored. The report was passed on to an untrained officer who had arrived only a week earlier. He thought they had detected a flight of B-17s arriving that morning from the US. There were only six B-17s in the group, so this did not account for the large size of the plot. But the officer had little grasp of the technology and the B-17's had no IFF (Identification Friend or Foe) system, nor any alternative procedure for identifying distant friendlies as the British had developed during the Battle of Britain. The raid on Pearl Harbor started 55 minutes later, and signaled the United States' formal entry into World War II.

Additionally the radar operators also failed to communicate their detection of the northerly direction that the departing planes. The US fleet instead was fruitlessly searching to the southwest of Hawaii, believing the attack to have been launched from that direction. In retrospect this may have been fortuitous, since they would have met the same fate as the ships in Pearl harbor had they attempted to engage the vastly superior Japanese carrier fleet. Even if the correct bearing to the fleet was delivered, and the B-17's had survived the attack, it is unlikely that the B-17's would have been able to locate the Japanese fleet, much less survive their fighter defenses.

According to Bergquist during questioning by the Roberts Commission, if the air bases had been on alert the air crews were capable of launching planes within three minutes of an order [] . Had the value of the SR-270s been understood, the bases easily could have had all available squadrons in the air prior to the arrival of the Japanese planes over Pearl Harbor. Another problem reported to the Roberts commission was that turf wars kept the radar units under the direct control of General Short, instead of the Army Air Corps's air defense units.

After the Japanese attack, the RAF agreed to send Watson-Watt to the United States to advise the military on air defense technology. In particular Watson-Watt directed attention to the general lack of understanding at all levels of command of the capabilities of radar- with it often being regarded as a freak gadget "producing snap observations on targets which may or may not be aircraft." General Saville, director of Air Defense at the Army Air Force headquarters referred to the Watson-Watt report as "a damning indictment of our whole warning service".

In the Philippines, the United States Far East Air Force at Clark field did not fare any better than the defending air force at Pearl Harbor. Though they had seven SR-270s, only two of the units were operational and as was the case in Hawaii, there was as little support for their use. Even with ten hours of warning after the attack on Pearl Harbor, and correct detection of enemy flights from an operational radar at Iba, many of the defending fighters in the Philippines were also caught on the ground and destroyed, as was the largest concentration of B-17's outside of the continental US. [] After the first day, the effective striking power of the Far East Air force had been destroyed, and the fighter strength seriously reduced. From this evidence, it is unclear that the results at Pearl Harbor would have been any different were the SR-270 warning from Opana Point taken seriously. Key commanders responsible for the defense of installations vulnerable to air attack did not appreciate the need for and capabilities of the air defense assets they had, and how vital radar was to those defenses. The vulnerability was well demonstrated in war games- in particular those of United States Navy Fleet Problem IX that annihilated the locks on the Panama canal, and Fleet Problem XIII, when the Pearl Harbor fleet was destroyed in a mock attack by 150 planes in 1932. []

CR-270s Today

After its use by the military, the Pearl Harbor unit (s/n 012) was loaned to the University of Saskatchewan in Saskatoon (along with a second unit to the National Research Council in Ottawa), who, unaware of its history, used it to image aurora for the first time in 1949. The technique was published in 1950 in "Nature", and was a field of active research for some time. In 1990, having sat derelict for years, they received a phone call informing them of the historical nature of the radar, and requesting it be sent back to the US for preservation. It is now located at the Historical Electronics Museum near Baltimore.


* TM 11-1510, 11-1570, 11-1033, 11-1100, 11-1114, 11-1310, 11-1370, 11-1410, 11-1470
* FM 11-25

ee also

* List of U.S. Signal Corps Vehicles
* Signal Corps Radio

External links

* [ Witness Testimony regarding Opana Radar]
* [ Information on British Chain Home WWII radar]
* [ Information on WWII radar, including the SCR-270]
* [ U.S. Radar, Operational Characteristics of Radar, Classified by Tactical Application, FTP 217]
* [ Historical Electronics Museum] 1942 view of an SCR-271 at the [Radar Installation and Maintenance School at Camp Evans], Wall, NJ
* FM 11-25

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