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 vehicle s , weather formations , and terrain . The radar dish or antenna transmits pulses of radio waves or microwaves which bounce off any object in their path . The object returns a tiny part of the wave ' s energy to a dish or antenna which is usually located at the same site as the transmitter . Radar was secretly developed by several nations before and during World War II . The term " RADAR " itself , not the actual development , was coined in 1940 by the United States Navy as an acronym for " RAdio Detection And Ranging " . The term " radar " has since entered English and other languages as the common noun " radar " , losing all capitalization . The modern uses of radar are highly diverse , including air traffic control , radar astronomy , air - defense systems , antimissile systems ; marine radar s to locate landmarks and other ships ; aircraft anticollision systems ; ocean surveillance systems , outer space surveillance and rendezvous systems ; meteorological precipitation monitoring ; altimetry and flight control system s ; guided missile target locating systems ; and ground - penetrating radar for geological observations . High tech radar systems are associated with digital signal processing and are capable of extracting useful information from very high noise levels . Other systems similar to radar make use of other parts of the electromagnetic spectrum . One example is " lidar " , which uses visible light from lasers rather than radio waves . History . As early as 1886 , German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects . In 1895 , Alexander Popov , a physics instructor at the Imperial Russian Navy school in Kronstadt , developed an apparatus using a coherer tube for detecting distant lightning strikes . The next year , he added a spark - gap transmitter . In 1897 , while testing this equipment for communicating between two ships in the Baltic Sea , he took note of an interference beat caused by the passage of a third vessel . In his report , Popov wrote that this phenomenon might be used for detecting objects , but he did nothing more with this observation . The German inventor Christian Hülsmeyer was the first to use radio waves to detect " the presence of distant metallic objects " . In 1904 he demonstrated the feasibility of detecting a ship in dense fog , but not its distance from the transmitter . He obtained a patent for his detection device in April 1904 and later a patent for a related amendment for estimating the distance to the ship . He also got a British patent on September 23 , 1904 for a full system , that he called a " telemobiloscope " . In 1922 A . Hoyt Taylor and Leo C . Young , researchers working with the U . S . Navy , had a transmitter and a receiver on opposite sides of the Potomac River and discovered that a ship passing through the beam path caused the received signal to fade in and out . Taylor submitted a report , suggesting that this might be used to detect the presence of ships in low visibility , but the Navy did not immediately continue the work . Eight years later , Lawrence A . Hyland at the Naval Research Laboratory observed similar fading effects from a passing aircraft ; this led to a patent application as well as a proposal for serious work at the NRL ( Taylor and Young were then at this laboratory ) on radio - echo signals from moving targets . Before the Second World War , researchers in France , Germany , Italy , Japan , the Netherlands , the Soviet Union , the United Kingdom , and the United States , independently and in great secrecy , developed technologies that led to the modern version of radar . Australia , Canada , New Zealand , and South Africa followed prewar Great Britain , and Hungary had similar developments during the war . In France in 1934 , following systematic studies on the magnetron , the research branch of the Compagnie Générale de Télégraphie Sans Fil ( CSF ) , headed by Maurice Ponte , with Henri Gutton , Sylvain Berline , and M . Hugon began developing an obstacle - locating radio apparatus , a part of which was installed on the Normandie liner in 1935 . During the same time , the Soviet military engineer P . K . Oshchepkov , in collaboration with Leningrad Electrophysical Institute , produced an experimental apparatus , RAPID , capable of detecting an aircraft within 3 km of a receiver . The French and Soviet systems , however , had continuous - wave operation and could not give the full performance that was ultimately at the center of modern radar . Full radar evolved as a pulsed system , and the first such elementary apparatus was demonstrated in December 1934 by the American Robert M . Page , working at the Naval Research Laboratory . The following year , the United States Army successfully tested a primitive surface - to - surface radar to aim coastal battery search lights at night . This was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and the firm GEMA in Germany and then one in June 1935 by an Air Ministry team led by Robert A . Watson Watt in Great Britain . Development of radar greatly expanded on 1 September 1936 when Watson - Watt became Superintendent of a new establishment under the British Air Ministry , 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 in time for the outbreak of World War II in 1939 . This system provided the vital advance information that helped the Royal Air Force win the Battle of Britain . The British were the first to fully exploit radar as a defence against aircraft attack . This was spurred on by fears that the Germans were developing death ray s . The Air Ministry asked British scientists in 1934 to investigate the possibility of propagating electromagnetic energy and the likely effect . Following a study , they concluded that a death ray was impractical but that detection of aircraft appeared feasible . Robert Watson Watt ' s team demonstrated to his superiors the capabilities of a working prototype and then patented the device . It served as the basis for the Chain Home network of radars to defend Great Britain , which detected approaching German aircraft in the Battle of Britain in 1940 . In April 1940 , " Popular Science " showed an example of a radar unit using the Watson - Watt patent in an article on air defence , but not knowing that the U . S . Army and U . S . Navy were working on radars with the same principle , stated under the illustration , " This is not U . S . Army equipment . " Also , in late 1941 " Popular Mechanics " had an article in which a U . S . scientist speculated about the British early warning system on the English east coast and came close to what it was and how it worked . Alfred Lee Loomis organized the Radiation Laboratory at Cambridge , Massachusetts which developed the technology in the years 1941 - 45 . Later , in 1943 , Page greatly improved radar with the monopulse technique that was used for many years in most radar applications . The war precipitated research to find better resolution , more portability , and more features for radar , including complementary navigation systems like Oboe used by the RAF ' s Pathfinder . Applications . The information provided by radar includes the bearing and range ( and therefore position ) of the object from the radar scanner . It is thus used in many different fields where the need for such positioning is crucial . The first use of radar was for military purposes : to locate air , ground and sea targets . This evolved in the civilian field into applications for aircraft , ships , and roads . In aviation , aircraft are equipped with radar devices that warn of aircraft or other obstacles in or approaching their path , display weather information , and give accurate altitude readings . The first commercial device fitted to aircraft was a 1938 Bell Lab unit on some United Air Lines aircraft . Such aircraft can land in fog at airports equipped with radar - assisted ground - controlled approach systems in which the plane ' s flight is observed on radar screens while operators radio landing directions to the pilot . Marine radar s are used to measure the bearing and distance of ships to prevent collision with other ships , to navigate , and to fix their position at sea when within range of shore or other fixed references such as islands , buoys , and lightships . In port or in harbour , vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters . Meteorologists use radar to monitor precipitation and wind . It has become the primary tool for short - term weather forecast ing and watching for severe weather such as thunderstorms , tornadoes , winter storm s , precipitation types , etc . Geologists use specialised ground - penetrating radar s to map the composition of Earth ' s crust . Police forces use radar gun s to monitor vehicle speeds on the roads . Principles . A radar system has a transmitter that emits radio waves called " radar signals " in predetermined directions . When these come into contact with an object they are usually reflected or scattered in many directions . Radar signals are reflected especially well by materials of considerable electrical conductivity — especially by most metals , by seawater and by wet lands . Some of these make the use of radar altimeter s possible . The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work . If the object is " moving " either toward or away from the transmitter , there is a slight equivalent change in the frequency of the radio waves , caused by the Doppler effect . Radar receivers are usually , but not always , in the same location as the transmitter . Although the reflected radar signals captured by the receiving antenna are usually very weak , they can be strengthened by electronic amplifiers . More sophisticated methods of signal processing are also used in order to recover useful radar signals . The weak absorption of radio waves by the medium through which it passes is what enables radar sets to detect objects at relatively long ranges — ranges at which other electromagnetic wavelengths , such as visible light , infrared light , and ultraviolet light , are too strongly attenuated . Such weather phenomena as fog , clouds , rain , falling snow , and sleet that block visible light are usually transparent to radio waves . Certain radio frequencies that are absorbed or scattered by water vapor , raindrops , or atmospheric gases ( especially oxygen ) are avoided in designing radars , except when their detection is intended . Radar relies on its own transmissions rather than light from the Sun or the Moon , or from electromagnetic wave s emitted by the objects themselves , such as infrared wavelengths ( heat ) . This process of directing artificial radio waves towards objects is called " illumination " , although radio waves are invisible to the human eye or optical cameras . Reflection . If electromagnetic waves traveling through one material meet another , having a very different dielectric constant or diamagnetic constant from the first , the waves will reflect or scatter from the boundary between the materials . This means that a solid object in air or in a vacuum , or a significant change in atomic density between the object and what is surrounding it , will usually scatter radar ( radio ) waves from its surface . This is particularly true for electrically conductive materials such as metal and carbon fiber , making radar well - suited to the detection of aircraft and ships . Radar absorbing material , containing resistive and sometimes magnetic substances , is used on military vehicles to reduce radar reflection . This is the radio equivalent of painting something a dark color so that it cannot be seen by the eye at night . Radar waves scatter in a variety of ways depending on the size ( wavelength ) of the radio wave and the shape of the target . If the wavelength is much shorter than the target ' s size , the wave will bounce off in a way similar to the way light is reflected by a mirror . If the wavelength is much longer than the size of the target , the target may not be visible because of poor reflection . Low - frequency radar technology is dependent on resonances for detection , but not identification , of targets . This is described by Rayleigh scattering , an effect that creates Earth ' s blue sky and red sunset s . When the two length scales are comparable , there may be resonance s . Early radars used very long wavelengths that were larger than the targets and thus received a vague signal , whereas some modern systems use shorter wavelengths ( a few centimeter s or less ) that can image objects as small as a loaf of bread . Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass . The most reflective targets for short wavelengths have 90° angles between the reflective surfaces . A corner reflector consists of three flat surfaces meeting like the inside corner of a box . The structure will reflect waves entering its opening directly back to the source . They are commonly used as radar reflectors to make otherwise difficult - to - detect objects easier to detect . Corner reflectors on boats , for example , make them more detectable to avoid collision or during a rescue . For similar reasons , objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions , which leads to " odd " looking stealth aircraft . These precautions do not completely eliminate reflection because of diffraction , especially at longer wavelengths . Half wavelength long wires or strips of conducting material , such as chaff , are very reflective but do not direct the scattered energy back toward the source . The extent to which an object reflects or scatters radio waves is called its radar cross section . Radar equation . The power " P " returning to the receiving antenna is given by the equation :