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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. It is 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.
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 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.
In April 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 Bay. 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-vehicle package including a K-30 operations van for the radio equipment and oscilloscope, a K-31 gasoline-fueled power-generating truck, a K-22B flatbed trailer, and a K-32 prime mover. The antenna folding mount was derived from a well-drilling derrick, and was mounted on the trailer for movement. When opened it was 55 feet (17 m) tall, mounted on an 8-foot (2.4 m) wide base containing motors for rotating the antenna. The 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 three-high four-wide stack. (Later production versions of the SCR-270 used 32 dipoles and reflectors, either eight wide by four high (fixed) or four wide by eight high (mobile)).
In use, the antenna was swung by command from the operations van, the angle being read by reading numbers painted on the antenna mount. The radar operated at 106 MHz, using a pulse width from 10 to 25 microseconds, and a pulse repetition frequency 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, 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 (240 km), and consistently picked up aircraft at that range. A nine-man field operating crew consisted of a shift chief, two oscilloscope operators, two plotters, two technicians, and two electricians.
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":
|Altitude||1,000 ft (300 m)||5,000 ft (1,500 m)||20,000 ft (6,100 m)||25,000 ft (7,600 m)|
|Range||20 mi (32 km)||50 mi (80 km)||100 mi (160 km)||110 mi (180 km)|
Deployment and Incomprehension 
The non portable version, the SCR-271-A, s/n 1 was delivered to the Canal Zone and began operation in October 1940 at Fort Sherman on the Atlantic end of the Panama Canal. It picked up airliners at 117 miles (188 km) in its initial test run. The second set was set up on Fort Grant'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 Ground-controlled interception (GCI) 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 to his former fighter unit. Only when incomprehensible equipment began appearing did the army return Bergquist from his fighter unit and was told his job was 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 gas station some distance away. 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 did not result in changes within the Army structure.
Use of SCR-270 radar at Pearl Harbor 
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. At 7:02 they detected the Japanese aircraft approaching Oahu at a distance of 130 miles (210 km) 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 awful big flight."
The report was passed on to an inexperienced and incompletely trained 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. The officer had little grasp of the technology, the radar operators were unaware of the B-17 flight (nor its size), 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 a day later.
The radar operators also failed to communicate the northerly bearing of the inbound flight. 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, with enormous casualties.
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 Gordon P. 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".
Use of SCR-270 radar elsewhere in World War II 
In the Philippines, the Far East Air Force did not fare much better than the defending air force at Pearl Harbor. Though FEAF had five SR-270Bs, only two were functioning on 8 December 1941, one by a detachment of the 4th Marine Regiment to protect Cavite Naval Base. Even with correct detection of enemy flights from the AAF's operational radar at Iba, command disorganization resulted in 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 (19) outside of the continental US. The Iba set was destroyed in the initial attack on Iba on 8 December. After the first day, the effective striking power of the Far East Air Force had been destroyed, and the fighter strength seriously reduced. The Marine unit was withdrawn to Bataan in January 1942, where it was successfully employed in conjunction with an SCR-268 antiaircraft gun-laying radar to provide air warning to a small detachment of P-40s operating from primitive fields.
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.
At Midway Island in June 1942, an SCR-270 antenna and shack  were located at the western end of Sand Island . During the Battle of Midway, this radar was used to warn the island of incoming Japanese air attacks  and to successfully direct the fighter interception that followed, but the island's radar did not play any significant part in the main carrier-action portion of the battle that followed.
SCR-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 National Electronics Museum near Baltimore.
See also 
- TM 11-1510, 11-1570, 11-1033, 11-1100, 11-1114, 11-1310, 11-1370, 11-1410, 11-1470
- FM 11-25
- SNL G703, antenna trailers, K-22, K-64,
- SNL G511, truck/van K-30, K-31, K-62
|Wikimedia Commons has media related to: SCR-270 radar|
- Witness Testimony regarding Opana Radar
- Information on British Chain Home World War II radar
- Information on World War II 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]http://www.infoage.org/cs-12-08-2005p34.html, Wall, NJ
- http://www.ibiblio.org/hyperwar/USA/ref/FM/index.html FM 11-25
- http://www.monmouth.army.mil/historian/photolist.php?fname=Radar%2FSCR-270 pics of SCR-270
- film of scr 270 at fort bliss/white sands