Radar

• 1. Introduction

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  In 1940, the United States Navy needed a word for a new secret machine that could see through fog and darkness. They settled on an acronym: radio detection and ranging. Shortened, it became RADAR. The word stuck so completely that it slid into everyday English as a plain noun, shedding its capital letters along the way.

  The idea behind it sounds almost too simple. Send out radio waves, wait for them to bounce off something, and listen for the echo. From that echo a machine can read distance, direction, and speed. Yet getting from that simple principle to a working system took decades of stubborn experiment across many countries.

  Why did a German inventor's working device get rejected by his own military in 1904? How did a question about a death ray lead Britain to a network of towers that helped win a battle in the sky? And how did a machine built to spot enemy bombers end up watching for tornadoes, catching speeding drivers, and steering driverless cars? Those questions run through everything that follows.

• 2. From Sparks To Echoes

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  As early as 1886, German physicist Heinrich Hertz showed that radio waves could bounce off solid objects. The principle was visible decades before anyone built a useful detector from it. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, built an apparatus with a coherer tube to detect distant lightning strikes.

  Popov noticed something strange in 1897 while testing equipment to link two ships in the Baltic Sea. A third vessel passing through the beam created an interference beat. He wrote that the effect might be used to detect objects, then did nothing more with the observation.

  The German inventor Christian Hülsmeyer turned the idea into a device. In 1904, he demonstrated that he could detect a ship in dense fog, though not its distance from the transmitter. He earned a patent in April 1904 and a British patent on the 23rd of September 1904 for a full system he called a telemobiloscope.

  Hülsmeyer's machine operated on a 50 cm wavelength and made its pulses with a spark-gap. It used a horn antenna with a parabolic reflector, the classic arrangement. German military officials watched practical tests in Cologne and Rotterdam harbour, and rejected it.

  In 1922, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young placed a transmitter and receiver on opposite sides of the Potomac River. Ships passing through the beam made the received signal fade in and out. Taylor suggested this might detect ships in low visibility, but the Navy did not pursue it. Eight years later, Lawrence A. Hyland at the Naval Research Laboratory saw the same fading from passing aircraft, and that observation finally pushed the work forward.

• 3. The Death Ray Question

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  In 1915, Robert Watson-Watt used radio technology to warn airmen of approaching thunderstorms. Through the 1920s he led a U.K. research establishment that probed the ionosphere and detected lightning at long distances, making him an expert in radio direction finding.

  Watson-Watt told a junior colleague, the "new boy" Arnold Frederic Wilkins, to review available shortwave receivers. Wilkins picked a General Post Office model after reading a manual note about a "fading" effect, the common term for interference, when aircraft flew overhead.

  In 1935, Watson-Watt was asked to judge reports of a German radio-based death ray. He handed the problem to Wilkins, who calculated that such a weapon was basically impossible. When asked what radio might do instead, Wilkins recalled the earlier note about aircraft causing interference.

  That exchange led to the Daventry Experiment on the 26th of February 1935. A powerful BBC shortwave transmitter served as the source while a bomber flew around a GPO receiver set up in a field. When the plane was clearly detected, Hugh Dowding, the Air Member for Supply and Research, was impressed enough to release funds at once. Watson-Watt's team patented the device under patent GB593017.

• 4. Towers On The Coast

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  By 1936, Watson-Watt had become superintendent of the new Bawdsey Research Station, set in Bawdsey Manor near Felixstowe, Suffolk. Work there produced detection and tracking stations called Chain Home, installed along the East and South coasts of England before war broke out in 1939.

  The first five Chain Home systems were operational by 1936, and by 1940 they stretched across the entire UK, including Northern Ireland. Even for the era, Chain Home was crude. Instead of aiming a beam, it floodlit the whole area in front of it, then used one of Watson-Watt's own radio direction finders to read the direction of the returning echoes.

  That floodlight approach forced Chain Home transmitters to be far more powerful, with better antennas than competing systems. The tradeoff bought speed. The system could be built quickly from existing technology.

  Chain Home gave the Royal Air Force vital advance warning during the Battle of Britain. Without it, large numbers of fighter aircraft that Britain did not have would have needed to stay airborne constantly. The radar fed the "Dowding system," which collected reports of enemy aircraft and coordinated the response.

  Full radar grew up as a pulsed system, and the first elementary version was demonstrated in December 1934 by the American Robert M. Page at the Naval Research Laboratory. Pulsed designs followed in May 1935 from Rudolf Kühnhold and his firm in Germany, and in June 1935 from Watson-Watt's team in Britain. By contrast, the French and Soviet systems of the period used continuous-wave operation that fell short of what modern radar would deliver.

• 5. Research Under Fire

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  In France in 1934, the research branch of the Compagnie générale de la télégraphie sans fil, headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, built an obstacle-locating radio apparatus. Parts of it were installed on the ocean liner Normandie in 1935.

  Soviet engineer P.K. Oshchepkov worked with the Leningrad Electrotechnical Institute to build an experimental apparatus called RAPID, able to detect an aircraft within 3 km of a receiver. The Soviets mass-produced their first radars, RUS-1 and RUS-2 Redut, in 1939. Development slowed after Oshchepkov was arrested and sent to the gulag. Only 607 Redut stations were produced during the war.

  The first Russian airborne radar, Gneiss-2, entered service in June 1943 on Pe-2 dive bombers. More than 230 Gneiss-2 stations were built by the end of 1944.

  A key wartime breakthrough was the cavity magnetron in the UK, which made relatively small systems possible with sub-meter resolution. Britain shared the technology with the United States during the 1940 Tizard Mission.

  Watson-Watt was sent to the U.S. in 1941 to advise on air defense after Japan's attack on Pearl Harbor. Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute of Technology in Cambridge, Massachusetts, which developed microwave radar from 1941 to 1945. In 1943, Page improved radar further with the monopulse technique, which stayed in use across most radar applications for many years.

• 6. How An Echo Speaks

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  Radar signals bounce especially well off materials with high electrical conductivity, such as most metals, seawater, and wet ground. Weak absorption of radio waves by the air lets radar reach far ranges, where visible, infrared, and ultraviolet light would be too strongly attenuated. Fog, clouds, rain, and snow that block sight are usually transparent to radio waves.

  The way waves scatter depends on the size of the wave against the shape of the target. If the wavelength is much shorter than the target, the wave bounces like light off a mirror. If it is much longer, the target may barely show. Early radars used very long wavelengths and got vague signals, while modern systems use a few centimetres or less and can image objects as small as a loaf of bread.

  A corner reflector is three flat surfaces meeting like the inside corner of a cube, and it throws waves straight back to the source. Boats carry them to stay visible and avoid collision or aid rescue. Stealth aircraft do the opposite, avoiding inside corners and perpendicular edges, which is why they look "odd." How much an object reflects is called its radar cross-section.

  If a target moves toward or away from the transmitter, the Doppler effect shifts the frequency of the returning waves. Only the radial part of the velocity matters. A reflector moving at right angles to the beam shows no relative velocity, while one moving along the beam produces the maximum shift.

  Distance comes from time. A short pulse goes out, and the machine measures how long the reflection takes to return. The distance is half the round-trip time multiplied by the speed of the signal, the half accounting for the journey out and back. The radar mile, the time for a pulse to travel one nautical mile and return, runs 12.36 microseconds, since a nautical mile is 1,852 m and light moves at 299,792,458 m/s.

• 7. Fighting The Noise

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  Clutter is the radar operator's word for echoes that do not matter: buildings, ground, sea, precipitation, dust storms, birds, even meteor trails. Disturbances in the ionosphere from geomagnetic storms can add to it, especially near the geomagnetic poles, where the solar wind stirs convection patterns in the ionospheric plasma.

  Clutter tends to sit still between scans, so a system can compare successive sweeps and erase whatever does not move. Sea clutter drops with horizontal polarization, while circular polarization tames rain. The strongest tool is pulse-Doppler radar, which separates clutter from aircraft and spacecraft by their velocity differences using a frequency spectrum.

  Jamming is a different threat: radio signals from outside the radar, sent on its own frequency to mask real targets. It can be a deliberate electronic warfare tactic or an accident of friendly equipment sharing a band. A jammer has an advantage, because its signal travels one way while radar echoes travel two and weaken by the inverse-square law. So jammers can be far weaker than the radars they blind.

  Sidelobe jamming, which sneaks in through the antenna's side lobes, can be cut by better antenna design and by using an omnidirectional antenna to spot and ignore non-mainlobe signals. Mainlobe jamming is harder and cannot be fully removed when the jammer faces the radar with the same frequency and polarization. Frequency hopping and polarization are among the other defenses.

  Noise is the internal limit, the random variation every electronic component generates. It is given by kB T B, where T is temperature, B is bandwidth, and kB is the Boltzmann constant. Push the logic far enough and the limit becomes a single electron at a certain temperature. As the source puts it, radar, like all macro-scale entities, is profoundly impacted by quantum theory.

• 8. A Machine For Many Worlds

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  The first commercial radar fitted to aircraft was a 1938 Bell Lab unit on some United Air Lines planes. From those military roots, radar spread into civilian life across aviation, shipping, and the road. Airports with radar-assisted ground-controlled approach let planes land in fog, with operators reading a precision approach radar screen and giving the pilot landing instructions.

  Meteorologists turned radar into the primary tool for short-term weather forecasting, watching for thunderstorms, tornadoes, and winter storms. The NEXRAD Pulse-Doppler weather radar uses a symmetric antenna to scan the atmosphere in detail. Geologists use ground-penetrating radar to map the composition of Earth's crust.

  Police forces use radar guns to clock vehicle speeds. Automotive radars run adaptive cruise control and emergency braking, measuring moving objects while ignoring stationary roadside clutter that could trigger a false brake. Roadside stopped-vehicle detection radars invert that logic, ignoring moving traffic to find stranded cars and debris. Smaller systems track human movement, from breathing patterns for sleep monitoring to finger gestures for computer interaction.

  The antenna decides where a target sits. Chain Home used two straight antennas at right angles, reading direction from where one display peaked and another dropped. Modern systems favor phased arrays, banks of evenly spaced elements that steer a beam by shifting phase across the array, with no moving parts.

  The first aircraft to use a phased array radar was the B-1B Lancer, and the first fighter was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon was reckoned the world's most powerful fighter radar until the AN/APG-77 active electronically scanned array arrived on the Lockheed Martin F-22 Raptor. As of 2017, NOAA planned a national network of multi-function phased array radars across the United States within ten years, for both weather study and flight monitoring.