Radar
In 1886, German physicist Heinrich Hertz demonstrated that radio waves could bounce off solid objects. This discovery laid the foundation for all future detection systems. Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube to detect distant lightning strikes in 1895. The next year he added a spark-gap transmitter to his device. While testing this equipment between two ships in the Baltic Sea in 1897, Popov noted an interference beat caused by a third vessel passing through the signal path. He wrote in his report that this phenomenon might be used for detecting objects but took no further action on the idea.
Christian Hülsmeyer became the first person to use radio waves to detect the presence of distant metallic objects in 1904. He demonstrated the feasibility of spotting a ship in dense fog without measuring its distance from the transmitter. Hülsmeyer obtained a patent for his detection device in April 1904 and later secured another patent for estimating the distance to the ship. He also received a British patent on the 23rd of September 1904, for a full radar system which he called a telemobiloscope. His system operated on a 50 cm wavelength and created pulsed radar signals via a spark-gap. It already utilized the classic antenna setup of a horn antenna with parabolic reflector.
Robert Watson-Watt used radio technology to provide advance warning of thunderstorms to airmen in 1915. During the 1920s he led the U.K. research establishment to make many advances using radio techniques including ionosphere probing and long-distance lightning detection. By placing a transmitter and receiver on opposite sides of the Potomac River in 1922, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report suggesting this phenomenon might be used to detect ships in low visibility but the Navy did not immediately continue the work.
Before World War II began, radar research was conducted in the United Kingdom, the United States, France, Germany, Italy, Japan, the Soviet Union, and the Netherlands. In France in 1934, the Compagnie générale de la télégraphie sans fil (CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locating radio apparatus. Aspects of this system were installed on the ocean liner Normandie in 1935. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 but further development slowed following the arrest of P.K. Oshchepkov.
The Daventry Experiment took place on the 26th of February 1935, using a powerful BBC shortwave transmitter as the source. A bomber flew around the site while operators monitored the return signals from their GPO receiver setup in a field. When the plane was clearly detected, Hugh Dowding, the Air Member for Supply and Research, became very impressed with the system's potential. Funds were immediately provided for further operational development. Watson-Watt's team patented the device in patent GB593017.
Development greatly expanded in 1936 when Watson-Watt became superintendent of Bawdsey Research Station located in Bawdsey Manor near Felixstowe, Suffolk. Work there resulted in the design and installation of aircraft detection and tracking stations called Chain Home along the East and South coasts of England. By 1940 these systems stretched across the entire UK including Northern Ireland. This system provided vital advance information that helped the Royal Air Force win the Battle of Britain without it significant numbers of fighter aircraft would have needed to be constantly airborne.
A key development was the cavity magnetron in the United Kingdom which allowed creation of relatively small systems with sub-meter resolution. Britain shared this technology with the U.S. during the 1940 Tizard Mission. Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute Technology in Cambridge, Massachusetts which developed microwave radar technology between 1941 and 1945. In April 1940 Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defense.
The first commercial device fitted to aircraft was a 1938 Bell Lab unit installed on some United Air Lines aircraft. Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems where operators observe the plane's position on precision approach radar screens. They give radio landing instructions to the pilot maintaining the aircraft on a defined approach path to the runway. Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships or navigate when within range of shore references like islands buoys and lightships.
Meteorologists use radar to monitor precipitation and wind making it the primary tool for short-term weather forecasting. It watches for severe weather such as thunderstorms tornadoes winter storms and precipitation types. Geologists use specialized ground-penetrating radars to map the composition of Earth's crust. Police forces use radar guns to monitor vehicle speeds on roads. Automotive radars are used for adaptive cruise control and emergency braking by ignoring stationary roadside objects that could cause incorrect brake application.
As part of Intelligent Transport Systems fixed-position stopped vehicle detection radars are mounted on the roadside to detect stranded vehicles obstructions and debris. Smaller radar systems detect human movement including breathing pattern detection for sleep monitoring and hand gesture detection for computer interaction. Automatic door opening and intruder sensing are also common applications. Modern high tech radar systems use digital signal processing and machine learning capable of extracting useful information from very high noise levels.
A radar system has a transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object they reflect or scatter in many directions though some will be absorbed and penetrate into the target. Radar signals reflect especially well by materials of considerable electrical conductivity such as most metals seawater and wet ground. The reflected radar signals captured by the receiving antenna are usually very weak but can be strengthened by electronic amplifiers.
If the object is moving toward or away from the transmitter there will be a slight change in frequency due to the Doppler effect. This produces information about target velocity during the detection process allowing small objects to be detected in environments containing much larger nearby slow-moving objects. Frequency shift depends upon whether the radar configuration is active or passive with active radar transmitting a signal reflected back to the receiver.
Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal the more difficult it is to discern it from the noise. Signal processing takes advantage of this phenomenon to reduce the noise floor using strategies like signal integration which can improve noise up to 10 decibels for each stage. Pulse-Doppler signal processing includes frequency filtering in the detection process dividing space between transmit pulses into range cells filtered independently.
Early systems tended to use omnidirectional broadcast antennas with directional receiver antennas pointed in various directions. For instance Chain Home used two straight antennas at right angles for reception each on a different display. More modern systems use steerable parabolic dishes to create tight broadcast beams typically using the same dish as the receiver. Parabolic reflectors can be symmetric parabolas producing narrow pencil beams in both X and Y dimensions or spoiled parabolas producing narrow beams in one dimension and wide beams in another.
Phased array radars have been in use since earliest years of World War II including Mammut radar though electronic device limitations led to poor performance initially. Phased array antennas are composed of evenly spaced similar antenna elements such as aerials or rows of slotted waveguide. Each element incorporates discrete phase shift producing phase gradient across array resulting in beam reinforcement or cancellation depending on direction. Almost all modern military radar systems are based on phased arrays where small additional cost is offset by improved reliability of system with no moving parts.
Traditional band names originated as code-names during World War II and remain in military and aviation use throughout world. HF bands from 3 to 30 MHz serve coastal radar systems and over-the-horizon radars while VHF bands from 30 to 300 MHz handle very long range ground penetrating tasks. UHF bands from 300 to 1000 MHz provide very long range ballistic missile early warning and foliage penetration capabilities.
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Common questions
Who invented the first radar system and when was it patented?
Christian Hülsmeyer became the first person to use radio waves to detect distant metallic objects in 1904. He obtained a patent for his detection device in April 1904 and secured another patent for estimating distance to ships on the 23rd of September 1904.
When did the Daventry Experiment take place and what was its outcome?
The Daventry Experiment took place on the 26th of February 1935 using a powerful BBC shortwave transmitter as the source. Hugh Dowding became very impressed with the system's potential after operators detected a bomber, leading to immediate funding for further operational development.
What role did radar play during the Battle of Britain in 1940?
Radar systems called Chain Home stretched across the entire UK including Northern Ireland by 1940 to provide vital advance information. This technology helped the Royal Air Force win the Battle of Britain without requiring significant numbers of fighter aircraft to be constantly airborne.
How does the Doppler effect improve radar detection capabilities?
If an object moves toward or away from the transmitter there will be a slight change in frequency due to the Doppler effect. This produces information about target velocity allowing small objects to be detected in environments containing much larger nearby slow-moving objects.
Which countries conducted radar research before World War II began?
Before World War II began, radar research was conducted in the United Kingdom, the United States, France, Germany, Italy, Japan, the Soviet Union, and the Netherlands. The Soviets produced their first mass production radars RUS-1 and RUS-2 Redut in 1939 while France installed aspects of its obstacle-locating apparatus on the ocean liner Normandie in 1935.