Exterior view of radio set SCR-584, a mobile radar unit.
In: "AAF Manual 105-101-2 Radar Storm Detection," by Headquarters, Army
Air Forces, August 1945.
Radar (radio detecting and ranging) is often called the
weapon that won World War II and the invention that changed the world.
While these claims may be a little hyperbolic, there is no question that
radar was a major development. It has certainly proven to be one of the
most amazingly useful developments of the 20th century and is
vital to aviation. It is also a clear example of a military technology
that had important civilian uses.
To quote Scottish scientist Robert Watson-Watt, one of
the early pioneers of radar technology, radar is "the art of detecting by
means of radio echoes the presence of objects, determining their direction
and ranges, recognizing their character and employing data thus obtained
in the performance of military, naval, or other operations." Radar is used
for navigation, targeting, air traffic control, weather tracking, and a
host of other purposes.
Radio, which was first developed in the late 1800s,
allowed people to communicate over long distances without a physical
connection such as a wire between the transmitter and the receiver. Radio
worked by converting sound into electromagnetic energy that was then
transmitted over a distance. When it was received, it could be converted
back to sound waves. It did not take those who used radio long to realize
that a lot of things could affect its performance. Weather conditions
could reduce the transmission of radio waves, as could physical objects
such as mountains between the transmitter and receiver.
Exactly who gets credit for "inventing" radar is a
topic of some disagreement in historical circles, for many people started
working on the subject in many places at roughly the same time, and many
of their developments influenced each other. In 1934, researchers at the
Naval Research Laboratory (NRL) in Washington, D.C., began work on
bouncing radio signals off of objects after noticing that ships travelling
down the Potomac River interfered with radio signals being transmitted
across the river. Robert Watson-Watt had also heard of reports from the
government post office, which was responsible for shortwave radio
communications, that airplanes flying near post office receivers caused
problems with reception of signals. He wrote a lengthy memo on how this
phenomenon might be used to detect airplanes. In 1936, the U.S. Army
Signal Corps' laboratory for ground equipment at Fort Monmouth, New
Jersey, also started a radar project. In 1934, Robert Page developed a
pulse radar for the detection of aircraft.
The first radar in extensive operational use was the
British Home Chain radar (often referred to as the CH radar), which
entered service in 1937. The CH and other early radars operated in the
"high frequency," or HF portion of the electromagnetic spectrum. But early
radar developers recognized that radars that could operate at frequencies
higher than HF could perform better. In 1936-37, military radar
researchers in the United States developed several devices such as the
resonant cavity circuit, the klystron electron tube, and the coaxial and
waveguide transmission lines and components that allowed the generation of
signals in the microwave region of the electromagnetic spectrum.
(Microwaves operate at a higher frequency than "high frequency.") This
dramatically improved radar performance and was a major military
development. The Americans secretly shared this information with their
counterparts in the United Kingdom and this enabled the British to build
better radars for detecting planes approaching the British Isles. Radar
gave the British warning of approaching German planes during the Battle of
1940 and was instrumental in the outcome of the battle. Britain also
developed airborne radar that helped pilots flying at night to detect
aircraft in the darkness and bomber crews to locate targets at night.
conception of radar beam pattern of a mobile radar unit¢Radio Set SCR-584.
Published in AAF Manual 105-101-2 Radar Storm Detection by Headquarters,
Army Air Forces, August 1945.
The United States Army Air Forces soon established the
Radiation Laboratory at the Massachusetts Institute of Technology, in
Cambridge, Massachusetts. The "Rad Lab" as it became known, worked to
develop numerous radar systems for various uses during the war. These
included radar for aiming anti-aircraft guns, general search radars for
detecting airplanes, ship borne radar, and airborne radars to be carried in
airplanes and used for various purposes, from targeting other airplanes at
night to "weather reconnaissance" to navigation.
Photograph of the radar scope at Orlando, Florida in 1948.
This was the third time that a hurricane had passed sufficiently close to
a radar site to have its structure revealed.
The Germans also made important advances in radar,
particularly with the Würzburg ground radar which entered service in 1940.
They fielded numerous ground-based radars and also developed aerial and
ship borne radars as well, although not nearly in as great numbers as the
Americans or British. But as the war progressed, German radar research
stagnated, which is one reason why some people claim that radar won the
war for the Allies.
After World War II, the Rad Lab closed and research on
radar in the United States and Britain languished for several years. But
although radar technology did not advance much in this period, its use
certainly did. Many military radars were transferred to civilian use where
they were used for Air Traffic Control (ATC) and Ground Controlled
Approach (GCA) to airports.
Not until the Korean War did the U.S. Air Force
recognize the need for more radar research. It created the Lincoln
Laboratory near Boston to research the development of a continental air
defence system for protecting the United States from bomber attack.
Eventually, this led to the SAGE air defence system developed in the
mid-1950s, as well as the Distant Early Warning Line ("DEW-Line") of
radars located along the northern boundary of Canada and Alaska.
For years after World War II, the Soviet Union used
U.S.-built radars it had received during the War. But Soviet engineers
began to modify the radars and improve their range and performance. When
the first American U-2 reconnaissance aircraft flew over the Soviet Union
in the summer of 1956, the Americans expected that the Soviets would not
be able to detect the aircraft using the old U.S. radars. But they were
surprised when the Soviets tracked the plane for almost its entire flight.
This prompted the Americans to seek ways to reduce the radar signature of
controllers watched their scopes for "blips" that indicate the position of
aircraft in early radar systems.
During the 1950s, the quest was for higher and higher
frequencies. Most radars used a rotating dish antenna for transmitting and
receiving the signals. But by the 1950s and 1960s, researchers were
exploring the possibilities of "phased-array" radars that had flat panel
antennas. In these systems, the radar beam is pencil-thin and "steered"
electronically. This eliminates the wasted time when a radar beam is
sweeping across empty space. The most well known of these kinds of radar
was the SPY-1 radar used as part of the U.S. Navy's Aegis weapons system
on cruisers and destroyers starting in the 1980s.
During the 1950s and 1960s, smaller and more robust
radars were developed for use in the nosecones of missiles such as the
Falcon and Sparrow, allowing them to home in on the reflected energy, or
"radar return," from an enemy aircraft. Flat panel antennas were also
developed for airborne use.
By the 1970s, U.S. military aviation experts became
concerned with "low observable" or "stealth" technology that would enable
aircraft to evade radar. By designing aircraft with specific shapes and
coating them in special materials, they dramatically reduced the amount of
electromagnetic energy that the aircraft reflected back to the source. But
these techniques do not work equally well against all frequencies, and
some low-frequency radars can detect stealth aircraft, although they
cannot pinpoint their location.
Improvements in electronics, particularly during the
1970s and 1980s, allowed radar systems to become smaller, lighter and more
capable, and able to achieve even higher frequencies. But a major
improvement in radar capabilities concerned the development of software
for better processing of radar signals. One development was "Synthetic
Aperture Radar" (SAR), which was first explored in the 1970s and later
applied to many different types of radar. SAR electronically stores the
radar returns over a period of time as the radar (mounted on an airplane
or spacecraft) moves. It then combines them into a detailed image of the
ground with picture-like quality. SAR has been used to map the earth from
the Space Shuttle, to provide reconnaissance imagery, and to target
precision weapons from aircraft such as the Boeing F-15E Strike Eagle.
The military has always pushed the boundaries of radar
technology, while civilian needs for Air Traffic Control were far less
demanding. However, by the 21st century, other technologies
were beginning to supplement and in some ways replace radar. In
particular, the Global Positioning System (GPS) and satellite
communications links allow ground controllers to track aircraft without
using radar at all. But radar is such an amazingly useful technology that
it will always be used in aviation.