How Does Radar Work? Radio Echoes That See Through Cloud and Dark

A radar set transmits a pulse of radio energy lasting around a microsecond, then switches to listening. Radio waves travel about 300 metres in a microsecond, so an echo arriving 200 microseconds after the pulse has made a round trip of 60 kilometres, and the target is 30 kilometres away. The set sends thousands of pulses a second and plots each echo at its measured range. Only a tiny fraction of the transmitted energy comes back, since it spreads out on the way to the target and again on the way back, and the returned power falls with the fourth power of the distance, so a target twice as far returns one sixteenth the signal; this is why long-range radars are big, powerful and sensitive.

Direction comes from the antenna. A radar antenna is shaped to send its energy in a narrow beam, and it sweeps, rotating once every few seconds for a search radar or steering electronically in a phased array, so that an echo can be tagged with the direction the beam was pointing when it was sent. Range and bearing together give a position, and a rotating display painting each sweep is the familiar radar screen.

A wave reflected from something moving towards the receiver is squeezed to a higher frequency, and from something moving away is stretched to a lower one, the Doppler effect that raises the pitch of an approaching siren. Radar measures the shift in frequency of each echo and reads off the target's speed along the beam, which is how a police speed gun works and how a weather radar sees the wind inside a storm. Doppler also solves radar's oldest problem, clutter: the echoes from hills, buildings and the sea that would swamp a display. Those do not move, so their echoes have no shift, and the set can be told to ignore anything standing still. The main kinds in use:

• •Air traffic control: primary radar that sees the aircraft's echo, and secondary radar that triggers a transponder on the aircraft to reply with its identity and altitude
• •Weather radar: tuned to the wavelengths that raindrops and hail reflect, mapping precipitation and, with Doppler, the rotation that precedes a tornado
• •Marine radar: on every large ship since the 1950s, for collision avoidance and navigation in fog
• •Ground-penetrating radar: low frequencies that pass into soil and reflect from pipes, graves and buried walls
• •Automotive radar: short-range sets behind the bumper that run adaptive cruise control and emergency braking
• •Space radar: satellites that map the Earth's surface through cloud, and the sets that track the debris in orbit

Heinrich Hertz showed in 1886 that radio waves reflect from metal, and a German engineer, Christian Hulsmeyer, patented a device to detect ships by their echo in 1904 that nobody bought. The military need came with the bomber. In 1935 the British Air Ministry asked the physicist Robert Watson-Watt whether radio could be used as a death ray against aircraft; he replied that it could not, but that it could detect them, and demonstrated it within a month by bouncing a BBC transmitter's signal off a passing bomber. By 1939 a chain of stations along the English coast could see aircraft over France, and the warning they gave was the margin by which the Battle of Britain was won. Germany, the United States, France, the Soviet Union and Japan all had their own programmes; the word radar, for radio detection and ranging, was coined by the American navy in 1940. The British invention of the cavity magnetron in the same year, a device that generated powerful microwaves and could be carried in an aircraft, was taken to America in a black box in September 1940 and has been called the most valuable cargo ever to cross the Atlantic. The same magnetron heats food in a microwave oven.

Because radar sees by reflection, an object that reflects little is hard to see, and stealth aircraft are shaped to bounce radio energy away from the transmitter rather than back to it, with surfaces that absorb what they can. The contest between radar and stealth, between jamming and frequency-hopping, between decoys and the signal processing that sees through them, has run for eighty years. The civilian side is less dramatic and more used. Every landing in bad weather, every shipping lane in fog, every forecast of tomorrow's rain and every car that brakes before its driver does relies on the echo of a pulse that left a moment ago.

A weather radar does more than detect that something is out there. The strength of the echo from a cloud of raindrops rises steeply with the size of the drops, roughly with the sixth power of their diameter, so one drop twice as wide returns about sixty-four times the signal. This means a few large drops can dominate the echo from millions of small ones, and forecasters convert the returned power into a measure called reflectivity, then into an estimate of how heavily it is raining. Very high reflectivity usually means large drops or hail, because nothing else in a cloud reflects so strongly.

Modern weather radars add a second trick, called dual polarisation. They send pulses that vibrate horizontally and vertically and compare the two echoes. Large raindrops are not teardrop shaped as they fall; air pressure flattens them into something like a bun, so they return a stronger horizontal echo. Hailstones tumble and look roughly the same in both directions, and snowflakes have their own signature. When a tornado lifts leaves, roof tiles and other debris, the echoes become jumbled and poorly matched between the two directions, and forecasters can sometimes confirm a tornado on the ground from that jumble alone.

Radar has also been used to measure the solar system. In 1961 radio astronomers in the United States, Britain and the Soviet Union bounced signals off Venus near its closest approach and timed the echoes, which took several minutes to return. Because the speed of the waves was known, the delay gave the distance to Venus directly, and from it the distance between the Earth and the Sun, the astronomical unit, could be calculated far more accurately than by any earlier method. Navigating spacecraft to other planets has depended on distances measured this way ever since.

Radar also revealed things that telescopes had missed. In 1965 the giant dish at Arecibo in Puerto Rico used the spread of frequencies in echoes from Mercury to show that it rotates once every 59 days, not every 88 as had long been assumed, and earlier radar work had shown that Venus spins slowly backwards, once every 243 days. Because radio waves pass straight through Venus's thick clouds, the Magellan spacecraft was able to map almost the whole surface by radar in the early 1990s, revealing volcanoes, lava plains and craters. Radar is now used to measure the shape and spin of asteroids that pass close to the Earth.

Weather radars are built to detect raindrops, but they also pick up anything else in the air, including birds and insects. On clear spring and autumn nights, radar images in many countries show large blooms spreading out from the ground soon after sunset. These are not rain but millions of migrating birds taking off together and climbing to cruising height.

Scientists now use this deliberately. Projects analyse radar data to measure how many birds are migrating, in which direction and at what height, and some produce nightly migration forecasts, similar to weather forecasts, so that people can switch off unnecessary lights in tall buildings on heavy migration nights, when birds are most at risk of being drawn off course by artificial light.

Radar finds objects by transmitting pulses of radio waves and timing their echoes: the delay gives range, the direction of the antenna beam gives bearing, and the Doppler shift in the echo's frequency gives speed and lets stationary clutter be filtered out. Developed in secret by several countries in the 1930s and decisive in the Second World War, it now runs air traffic control, weather forecasting, ship navigation and the sensors in modern cars.