SCR-268 radar
Encyclopedia
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
system. It was developed to provide accurate aiming information and used in gun laying
Gun laying
Gun laying is the process of aiming an artillery piece, such as a gun, howitzer or mortar on land or at sea against surface or air targets. It may be laying for direct fire, where the gun is aimed similarly to a rifle, or indirect fire, where firing data is calculated and applied to the sights...
systems and directing searchlights against aircraft.
The system was already considered out of date by the end of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, having been replaced by the much smaller and more accurate SCR-584 microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
-based system after the Tizard Mission
Tizard Mission
The Tizard Mission officially the British Technical and Scientific Mission was a British delegation that visited the United States during the Second World War in order to obtain the industrial resources to exploit the military potential of the research and development work completed by the UK up...
provided the US with the cavity magnetron
Cavity magnetron
The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...
.
Development
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 MonmouthFort Monmouth
Fort Monmouth was an installation of the Department of the Army in Monmouth County, New Jersey. The post is surrounded by the communities of Eatontown, Tinton Falls and Oceanport, New Jersey, and is located about 5 miles from the Atlantic Ocean. The post covers nearly of land, from the Shrewsbury...
, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
. While most of the Corps' efforts revolved around infra-red detection systems (a popular idea at the time), as well as a newer generation of sound detectors, they also maintained a small program of research on microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radars based on the "beat principle", in which an aircraft would cause two signals to interfere. Low generator efficiency and a lack of ranging capability made these efforts impractical.
In 1935 one of Blair's recent arrivals, Roger B. Colton, convinced him to send an engineer to investigate the US Navy's CXAM radar
CXAM radar
The CXAM radar system was the first production radar system deployed on United States Navy ships. It followed several earlier prototype systems, such as the NRL radar installed in April 1937 on the destroyer ; its successor, the XAF, installed in December 1938 on the battleship ; and the first...
project. The navy's system traced its development from experiments conducted by Albert H. Taylor
Albert H. Taylor
Albert Hoyt Taylor was an American electrical engineer who made important early contributions to the development of radar.-Biography:...
and Leo C. Young at the United States Naval Research Laboratory
United States Naval Research Laboratory
The United States Naval Research Laboratory is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a program of scientific research and development. NRL opened in 1923 at the instigation of Thomas Edison...
in the early 1920s. William D. Hershberger duly went to see what they had, and returned an extremely positive report. They decided to try to find a need for such a unit in order to gain funding, and eventually received a "request" by the Chief of Coast Artillery on February 1, 1936 for a gunlaying system with a range of 15,000 yards through rain, mist, smoke or fog.
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. On May 26, 1937 they were able to show the prototype in a convincing demonstration. After failing to find their target Martin B-10
Martin B-10
The Martin B-10 was the first all-metal monoplane bomber to go into regular use by the United States Army Air Corps, entering service in June 1934...
bomber where it was supposed to be, they started "hunting" for it and found it 10 miles off course. The radar fed pointing data to a team operating a searchlight
Searchlight
A searchlight is an apparatus that combines a bright light source with some form of curved reflector or other optics to project a powerful beam of light of approximately parallel rays in a particular direction, usually constructed so that it can be swiveled about.-Military use:The Royal Navy used...
, and when it was turned on the bomber was seen to be centered in the beam. It was later learned that the target had been blown off course, making the demonstration all the more impressive.
Development of this system was slowed to some degree when a long range early-warning radar became a higher priority and parts of the prototype were salvaged for the SCR-270
SCR-270 radar
The SCR-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...
they were building. Nevertheless the system entered production at Western Electric
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...
about the same time as the -270 in 1939. The radar entered service in 1940, and about 3100 were produced by the end of the war.
Description
The SCR-268 antenna system consisted of a number of dipoleDipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
elements arranged in three groups, each in front of a passive reflector, mounted on a large aimable cross. The cross consisted of a short upright pedestal sitting on top of a large base platform, mounting long cross-arms extending from the mid-point of the upright pedestal. The antenna system was about forty feet wide and ten feet high overall. Both the pedestal and the cross-arm could be rotated around their axis for aiming, in azimuth and altitude respectively.
The left side of the cross arm, as seen from the back, contained a set of dipoles that were set to be sensitive in angle, while almost insensitive to elevation. It was arranged six dipoles wide and four dipoles high, each with its own reflector. On the far right side was a similar, but smaller, arrangement rotated 90 degrees in order to be sensitive in elevation and not angle. This portion was two dipoles wide and six dipoles high with corresponding reflectors. Finally in the "middle" of the cross, between the upright pedestal and the elevation antenna, was the broadcasting array which created a circular beam about 10 degrees wide.
The three radar operators sat at consoles mounted on the pedestal just below the antenna cross-arm, each with their own oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
display. One controlled the azimuth, another the elevation, and the third the range. Pointing the antenna was controlled by rotating large handwheels, the range being reported by a similar wheel.
The accuracy of the antennas themselves was not very high, about 9-12 degrees, so simply rotating the antenna and looking for a maximum would not point it very accurately. To help with this the antennas were deliberately designed to have two directions of high sensitivity, or "lobes". Signals from both lobes were displayed, slightly separated, on the layer's displays. By adjusting the antenna until the returns from both were equally strong, accuracies of about one degree were possible.
Range information was taken from the elevation array, and worked, as it did for most radars of the era, by triggering the trace on an "A-line" oscilloscope and read against a scale at the bottom. A second blip was also generated by equipment attached to the rangefinder's handwheel. By rotating the handwheel until the reference blip overlapped the one returned from the antenna, the timing could be read from the wheel. Range accuracy was about plus or minus 200 yards.
The system also included two sets of "repeaters" that sent the directional information to a searchlight, and both the directional and range (as dialed in on the rangefinder's handwheel) to a gun. The accuracy was not enough for direct gunlaying, but in combination with a searchlight the gun's existing optical equipment could "fine tune" the radar's guidance.
The radar operated at 205 MHz with a PRF of 4098 pulses per second of 6 μS duration, with an inter-pulse time of 240 μS. Radio waves (light) travel at about 0.093 miles/μS round-trip, so the system had a 22 mile maximum range (240 × 0.093). It broadcast about 75 kW of power, which was, in theory, more than enough to offer longer range.
The radar was mobile, requiring four prime movers for support. Two towed the radar base and the antennas themselves, another pulled a K-34 trailer
K-34 trailer
The K-34 trailer was used by the U.S. Army Signal Corps to house electronic equipment, during and after World War II.-Specifications:the fifth wheel type trailer van, was of the single-axle type, but had a Dolly so that it could be moved by other smaller vehicles...
van providing power, and the fourth a van that converted the power to high voltage for the radio equipment. In total, including the trucks, the SCR-268 weighed 82,315 pounds. That the system was mobile at all was more a testament to the US's overwhelming industrial might than any quality of the radar itself.
The SCR-268 was not a particularly advanced design by the standards of the era. Early units were technically behind the times in terms of tracking in particular; while the SCR-268 was being introduced, the Germans were upgrading the Würzburg radar
Würzburg radar
The Würzburg radar was the primary ground-based gun laying radar for both the Luftwaffe and the German Army during World War II. Initial development took place before the war, entering service in 1940. Eventually over 4,000 Würzburgs of various models were produced...
, first to use lobe switching
Lobe switching
Lobe switching is a method used on early radar sets to improve tracking accuracy. It used two slightly separated antenna elements to send the beam slightly to either side of the midline of the antenna, switching between the two to find which one gave the stronger return, thereby indicating which...
, and finally to conical scan
Conical scanning
Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target...
the next year. These sorts of changes were simply not possible on the SCR-268 due to its long wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, which demanded a non-circular antenna.
The only way to "fix" this problem was to move to a microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
radar of their own, an effort that started after the Tizard Mission
Tizard Mission
The Tizard Mission officially the British Technical and Scientific Mission was a British delegation that visited the United States during the Second World War in order to obtain the industrial resources to exploit the military potential of the research and development work completed by the UK up...
. This led to the introduction of the SCR-584, which fit the entire radar system on a single trailer, added conical scanning, and was generally much more accurate and far easier to operate. SCR-268s were removed from service almost immediately with the ending of the war.
The SCR-268 was one of the first radar sets to use lobe switching
of its receiving antennas as a means to locate AAA searchlight beams on aircraft. Since it did not lobe-switch its transmitted signal it
would be classes as one of the first LORO radars.
See also
- List of U.S. Signal Corps Vehicles
- SCR-270 radarSCR-270 radarThe SCR-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...
- Signal Corps RadioSignal Corps RadioSignal Corps Radios were U.S. Army military communications components that comprised "sets". Under the Army Nomenclature System, SCR initially designated "Set, Complete Radio," and later "Signal Corps Radio," though interpretations have varied over time....
- G-numbers
- K-34 trailerK-34 trailerThe K-34 trailer was used by the U.S. Army Signal Corps to house electronic equipment, during and after World War II.-Specifications:the fifth wheel type trailer van, was of the single-axle type, but had a Dolly so that it could be moved by other smaller vehicles...
External links
- http://www.mobileradar.org/radar_descptn_2.html
- http://www.antiaircraft.org/SCR268.htm
- http://www.monmouth.army.mil/historian/photolist.php?fname=Radar%2FSCR-268 pics.
- http://www.history.navy.mil/library/online/radar-5.htm