BBC - h2g2 - The History of Radar

The early history of radar is closely connected to warfare, and only lately the civilian applications have become trendsetters for the technology and have gained some attention in the public.

The Very, Very Early Days

The oldest radar system was developed millions of years ago and is still in world-wide service: it is the ultrasonic sensor of a bat. Bats emit a short 'cry' from out of their nose, and receive the echo with a set of two antennas which happen to be ears. True, a bat's radar doesn't use electromagnetic rays, but the working principle is the same as that of a modern radar, with a chirped signal, target tracking by Doppler estimation, terrain avoidance function, and fine angle measurement based on the monopulse principle.

The oldest radar warning device also was developed millions of years ago. Tiger moths (which frequently appear on bats' menues) are equipped with ears that can detect the ultrasonic signal of a bat, and they have also developed tactics to evade a bat's attack. Thus, Electronic Combat also came into being a long time ago.

The Early Days: Experiments and Discoveries

Mankind started into the radar business around the beginning of the 20th Century. In 1904, Christian Hülsmeyer (1881-1957) received a German patent (plus others in the UK, the USA and some more countries) for the Telemobiloskop, or Remote Object Viewing Device. The device achieved ranges of 3000m against ships, even before amplifier tubes were invented. It was offered for an application to prevent ship collisions, but didn't find the interest of any customer and fell into oblivion. Thus, radar was invented earlier than Sonar (the acoustic equivalent used in seafaring), which was developed in the aftermath of the Titanic catastrophy of 1912.

R.C. Newhouse of Bell Labs got a patent in 1920, and his experiments performed throughout the decade eventually led to the radio altimeter which became operational in 1937.

In 1922, Guglielmo Marconi held a speech which showed that he had a clear idea of detecting remote objects by radio signals. But it was not before 1933 that he was able to show a first working device.

In 1925/26, the American physicians Breit and Tuve as well as the British researchers Appleton and Barnett performed measurements of Earth's ionosphere, using a pulsed radio transmitter which could be called a radar.

It was 1928 when H.M. Signal School of the UK received the first patent on Radio Location, credited to L. S. Alder.

In 1930, a team of engineers from the US Naval Research Lab performed measurements of a radio antenna, and more or less by serendipity they independently discovered radar. Their radio link happened to stretch across an aircraft landing strip, and the signal quality changed significantly when an aircraft crossed the beam.

In 1933 when Hitler took over power in Germany, the German Kriegsmarine (Navy) started research into what they called Funkmesstechnik, or remote radio measuring technology.

Research in Russia began in 1934, but was somewhat hindered by quarrels between different authorities. However, one of the earlier devices was a success, with 70km detection range against aircraft.

In 1935, Sir Robert Wattson Watt (1892-1973) sucessfully demonstrated the detection of an aircraft by a radio device, during the so-called Daventry Experiment. An order for full scale development of radar was issued later that year, after it was realised that sound locators (which at the time were the only means of detecting inbound bombers) could not provide adequate reaction time. This was the starting point of the world's first operative radar network, called Chain Home or CH in short. The Chain Home became operational in 1937, well before the war broke out. Bombers could be detected at ranges of 150km and more. Sir Wattson Watt is often being addressed as the 'father' and 'inventor' of radar, the latter of which must now be debated.

Also in 1935, a French ship was equipped with a collision avoidance device of local fabrication. A landbased device, the barrage electronique was tested in 1936 and found application in the early days of WW II.

By 1939, the following countries had more or less rudimentary, but operational radars in their inventories: Britain, France, Germany, Hungary, Italy, Japan, the Netherlands, Russia, Switzerland, USA. To some extend the technology behind these devices can be described in terms of todays buzzwords, such as Continuous Wave Doppler, conical scan, Bistatic and Spread Spectrum radars.

World War II

During the Battle of Britain (1939-1941), significant success was attributed to the Chain Home network which detected incoming air raids and provided information to guide interceptors to home in on the bombers. Detection reports were sent via landline to 'filter rooms' which coordinated the efforts of the short supply of interceptors and thus made up a formidable force multiplier. The CH was never really understood by the Germans, and no serious attempt to jam or destroy the whole system has been reported. The radar on the Isle of Wight was a part of the system.

In 1941, radar missed a chance to significantly change history. There was a first system deployed on a hill on the pacific island of Hawaii, close to Pearl Harbour. The operators actually detected the Japanese attack squadrons and reported their observation, but none of their superiors believed them because they were deemed unexperienced¹. Had the reports been acted upon then the whole attack could have been turned into a failure.

On Feb 17, 1942, a raid was undertaken to capture the essential parts of a German Würzburg radar located near Bruneval, Normandie. The parts were evaluated and showed that the device could only work in a narrow frequency range. Further reconnaissance revealed that all the German radars were operating in no more than three major frequency ranges, and thus were prone to jamming. The first operational use of chaff (aluminium strips which are precisely cut to a quarter of the radar's wavelength) took place in July 1943, when Hamburg was subjected to a devastating bombing raid. The radar screens were cluttered with reflections from the chaff and the air defense was, in effect, completely blinded. Chaff (also called 'window', or Düppel as the German term) had been discovered independently by the British and the Germans, with no side knowing of the other side's knowledge. The Germans knew of their vulnerability but found themselves incapable of making their radars tunable within reasonable amounts of time, and they hoped that refraining from using Düppel themselves would keep the secret.

Stealth technology was invented in the 1940s, when German submarines suffered severe losses because they were detected by airborne radars once they were on the surface. Technicians found out that a mixture of graphite and rubber could substantially weaken the radar echoes if it was applied as a 'Schornsteinfeger' ('chimney sweep') coating on the submarine's hull. However, this only worked while the subs were in the dry dock. Once the anti-reflective coating was wet from sea water, it was the water and salt layer which reflected the radar signals.

During the war, higher and higher frequencies of the electromagnetic spectrum were put to use. Researchers started with the first experiments at some 10MHz, the Chain Home operated around 20MHz (with later extensions up to 70MHz), and the bulk of air surveillance and tracking radars worked between 200MHz and 800MHz. The refinement of 1921's cavity magnetron transmitter device by Randall and H. Boot in 1940 was a significant breakthrough. The magnetron was the heart of the american H2S bombing radar which operated at 3GHz. Its plan position indicator (PPI) screen showed a map of the underlying terrain with a resolution that was hitherto unheard of. This type of magnetron wasn't known in Germany, and 3GHz was far beyond the frequency range of their intercept and warning devices. In fact, a conference between Hitler's top military ranks and some researchers had come to the conclusion that 800MHz was enough and that it was questionable whether signals at frequencies above 1GHz could at all propagate through the atmosphere. Rated 'top secret', the H2S radar was not to be used over Germany, for fears that its carrier might be shot down and the magnetron could fall into hostile hands.

But then in spring 1943, an American bomber was shot down near Rotterdam and the H2S radar with its magnetron was a big surprise for the Germans. Significant effort went into repair, study and reproduction of the device, but only a few examples became operational by the end of 1944.

At the end of the war most of today's technologies had already been put to use, although they relied on contemporary technical means. There was a chirp radar in production, the monopulse principle was invented and even a Synthetic Aperture Radar already existed. The Chain Home was used to detect the V2 rockets after they left their launch sites, hence it can also be called the world's first Anti Ballistic Missile (ABM) radar system. Among the few ideas which were born later than 1945 are the phased array antenna technology and the concept of multistatic radar.

After WWII

Having lost the war, Germany was thrown out of the business. Until 1950, any research into the field of radar was forbidden. Lots of researchers emigrated, following the steps of Wernher von Braun and others.

Radar was kept highly secret throughout WW II, and only in 1946 was it published that an American device had successfully measured the distance to the moon, which is a round trip of some 770,000km. Even later it became known that a Hungarian device had already done the same in 1944.

On a sidenote, it is very hard to find any remaining WW II radar equipment in museums today. Which is a pity as radars aren't as impressive as 50ton battle tanks, but every single radar had an influence comparable to a whole battleship. The Imperial War Museum in London has a single exposure of a German Würzburg tracking radar, and the Deutsches Museum in Munich features a Giant Würzburg. The Auto- und Technik-Museum in Sinsheim-Steinfurt (close to the A6 near Heidelberg, Germany) features none, as well as the National Air Museum in Dayton, Ohio. There may be displays in other museums, but this researcher is not aware of them.

The Cold War (1945-1989)

The Vietnam war (1961-1975) saw the first use of Anti Radiation Missiles (ARM) which were carried by dedicated F-4 Phantoms, called Wild Weasel. Once a radar warning receiver inside the aircraft detected the signal from a ground based acquisition or missile guidance radar, the seeker head of the ARM was cued and after lock-on, the missile would home in on the source of the signal (ie: the radar itself) and destroy it.

Desert Storm (1991)

Suppression of Enemy Air Defense, or SEAD was the first mission to be carried out by coalition forces in order to drive out Iraqi forces from occupied Kuwait. These SEAD missions employed the whole spectrum of Electronic Combat measures in order to pave the way for bombers and ground forces.

Modern Days

The layperson may know radar only as an invisible threat when speeding on a motorway, because police radars are employed as a law enforcing means. But radar is also on its way to be used to their advantage as automotive radars are now entering the market. They are used to automatically keep a set distance from the car up front, and to give warning when obstacles are encountered on ones lane.

These days, no major airport can be operated without a radar. Flight controllers in the tower use it to keep track of dozens of aircraft which are circling in the waiting loop and to schedule them for landing. The public is only made aware of this when a radar is defective, hundreds of flights are being cancelled or redirected to other airports and local hotels are unable to accomodate any more guests.

The Future

Many of the first devices that researchers played with were bistatic radars (ie, transmitter and receiver did not share the same location but were set up at two different places). These bistatic radars are drawing attraction again, of course with much higher developed signal processing working behind the scenes. The extension of the principle, using a transmitter plus several interconnected receiving sites, is called Multistatic Radar and is also subject of ongoing research.

At the time of this writing, Passive Radar, imaging radar and Multi-coloured radars are the latest buzzwords that are circling around developers' laboratories. Passive radar relies on background illumination by whatever source happens to be there. Imaging radar is what the name implies ie: the results are displayed as if they had been gathered by a camera and with comparable resolution. Multi-coloured radar is another word for ultra-wideband radar which is able to extract much more information out of the scene it sees - it is much like what colour TV was to a black-and-white seeing world.

However, these current hot-spot activities are also mankind's first steps to imitate the capabilities of sensors that nature developed millions of years ago.