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1886: Heinrich Hertz (1857 - 1894), German physicist: showed that radio waves could be reflected from solid objects.
1897: Alexander Popov (1859 - 1906), Russian physicist and radio pioneer: while testing a spark-gap transmitter communicating between two ships in the Baltic Sea, he took note of an interference beat (change in signal amplitude) caused by the passage of a third vessel. Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation
1904: Christian Huelsmeyer (1881–1957), German engineer: was the first to use radio waves to detect the presence of distant metallic objects. In 1904 he demonstrated the detection of a ship in dense fog. His device (which he called telemobiloscope) consisted of a simple spark gap used to generate a radio signal that was aimed using an antenna consisting an array of wires and a simple cylindrical parabolic reflector to enhance the signal. When a signal reflected from a ship it was picked up by a similar antenna attached to a coherer receiver and a bell sounded. The apparatus detected presence of ships up to 3 km away and it did not provide range information, only warning of a nearby object so that collisions could be avoided.
1917: Nikola Tesla (1856 - 1943), Serbian-American inventor and electrical engineer: suggested a primitive radar system working by analyzing changes in standing electromagnetic waves caused by interferences and frequency shifts in order to determine the relative position, course, distance or the speed of a moving object, such as a ship at sea - a simple continuous-wave detection concept (see below). He also suggested a fluorescent screen monitor or indicator for his device.
1922: Albert H. Taylor (1879 - 1961), American electrical engineer and Leo C. Young (1891 - 1981), American radio engineer were researchers working with the U.S. Navy: discovered that when radio waves were broadcast at 60 MHz it was possible to determine the range and bearing (direction) of nearby ships in the Potomac River. Despite Taylor's suggestion that this method could be used in darkness and low visibility, the Navy was skeptical.
1922: Guglielmo Marconi (1874 - 1937), Italian inventor and prominent radio pioneer: suggested a ship transmitting radio waves in any desired direction and when coming across a metallic object, such as another ship, the waves would be reflected back to a receiver on the transmitting ship, and thereby immediately reveal the presence and bearing of the other ship in fog or thick weather.
1928: Abram Slutskin (1881–1950), Russian scientist: developed an early version cavity magnetron (a high-powered vacuum tube that generates microwaves) and used these devices in early radio-location (radar) systems (see below).
1931: W. A. S. Butement (1904 - 1990) and P. E. Pollard: developed the first experimental technology, in Great Britain, that would eventually become radar. The system operated at 600 MHz using pulsed modulation and gave successful laboratory results, but was not of interest to War Office officials.
1933: Rudolf Kühnhold (1903–1992), experimental physicist and Scientific Director at the German Navy Experimental Institute of Communication Systems in Kiel: concluded that the desired accuracy in measuring distance to targets outside of water (under water are used ultrasonic waves - SONAR) could be attained only by using pulsed centimeter electromagnetic waves (see below).
1934: Emile Girardeau (1882–1970), French engineer: headed the team which developed the first RADAR system in France - a continuous-wave (CW) system, based on Tesla's ideas (see above), which was not as efficient as the would be coming pulsed systems. In 1939, Girardeau built radar installations for the defense of Paris against plane raids. A similar apparatus was developed by the Russian Pavel Oshchepkov (see below) in 1935 - the RAPID capable of detecting an aircraft 3 km away.
1934: Robert Morris Page (1903 - 1992), American physicist: The first pulsed radar system was demonstrated by Page, working at the Naval Research Laboratory. In 1943, Page greatly improved radar with the monopulse (conical scanning) technique that was used for many years in most radar applications. (Full radar evolved as a pulsed system because it enabled transmitter and receiver to work alternately for short periods facilitating the use of the same antenna for both jobs. Pulsed radar systems made it possible to overcome some limitation of continuous-wave (CW) radar systems like limitations in detecting stationary targets).
1934: Harold Zahl (1905–1983), American physicist: developed a technique, at the SCL (U.S. Signal Corps Laboratories), of locating an aircraft by detecting thermal radiation emitted from its engines.
1934: Hans-Karl von Willisen and Paul-Günther Erbslöh, two German engineers started a company called GEMA that built the first radar for naval use. It used a 50cm wave-length (600 MHz) and could locate ships up to 10 km away.
1935: Robert Watson-Watt (1892 - 1973), Britsh scientist: introduced passive radar detection - a passive radar does not have its own transmitter and instead it receives reflections transmitted by others like commercial radio stations. Since the distance of the radio station transmitter from the radar receiver is known it is possible do deduce range, velocity, and location of different objects of interest. Besides some limitation, passive radar enables stealth operation since it does not transmit detectible waves.
1936: William Blair (1874–1962), American scientist and Army Officer who led the U.S. Signal Corps Laboratories (SCL) at Fort Monmouth, New Jersey during its formative years: under his command the laboratory developed the U.S. Army’s first system for Radio Position Finding (RPF) later called radar. The experimental pulsed transmission apparatus detected an aircraft at 7 miles distance.
1936: Paul Watson, American civilian engineer: developed the SCR-270 (Signal Corps Radio model 270) that 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.
1937: Edward Bowen (1911 - 1991), British physicist: developed of the first airborne radar; considered a notable achievement since installing a radar system in an aircraft was difficult because of the size and weight of the equipment and the antenna and that the equipment had to operate in a vibrating and cold environment.
1937: Pavel Oshchepkov (1908 - 1992), Russian physicist: invented a pulsed radio-location system that detected an aircraft at a range of about 17 km (11 mi). The system, however, could not yet directly measure range (distance) to the target, a firm requirement for detection systems that would later be called radar.
1939: Abram Ioffe (1880 - 1960), a prominent Soviet physicist and director of the Leningrad Physical-Technical Institute (LPTI): a mobile radar system was developed under Ioffe’s direction and designated Redut (Redoubt) giving 50KW peak-power, 10-μs pulse-duration, 150Km range detection and Yagi antennas that were used for both transmitting and receiving.
1940: The name RADAR (Radio Detection And Ranging) was coined in 1940 by the U.S. Navy for public reference to their highly classified work. The term has since entered the English language as a standard word - radar.
1940: John Randall (1905 - 1984) and Harry Boot (1917 - 1983), British physicists: developed the modern cavity magnetron (a high-powered vacuum tube that generates microwaves). The compact cavity magnetron tube drastically reduced the size of radar sets so that they could be installed in anti-submarine aircraft and small escort ships. At present, cavity magnetrons are commonly used in microwave ovens and in various radar applications. The cavity magnetron shorter wavelength and high power pulses allows detection of smaller objects.
Radio & Radar Technology
What is Electronic Surveillance - ScienceGumShoes.com
Echo the Bat - NASA
RADAR - ology - The Franklin Institute Online
Radio Astronomy for Kids
The Electromagnetic Spectrum - NASA
How Radar Works - HowStuffWorks
How Radio Works - HowStuffWorks
The Shuttle Radar Topography Mission (SRTM) - NASA
Science Fair Projects and Experiments
Radar & Sonar Fair Projects
Radio Waves Propagation: Experiments, Labs, Studies and Background Information