The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
The early history of radar1 is closely connected with warfare - only relatively recently have civilian applications stimulated development of radar technology and attracted some public attention.
The Very, Very Early Days
The oldest known precursor to modern radar systems evolved in bats millions of years ago, and is known to us today as sonar2. Bats emit a short 'cry' from their noses, receiving the echo with a set of two antennae 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, PRF agility, 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' menus) are equipped with ears that can detect and jam 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 first became involved in this radar business around the beginning of the 20th Century. In 1904, Christian Hülsmeyer (1881 - 1957) received a German patent (as well as others in the UK, the USA and elsewhere) for the Telemobiloskop, or Remote Object Viewing Device. The device achieved ranges of 3,000m against ships, even before amplifier tubes were invented. It was offered as an application to prevent ship collisions, but didn't attract any customer interest 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 catastrophe of 1912.
RC 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 gave a speech which demonstrated that he had a clear idea that it was possible to detect 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 the Earth's ionosphere, using a pulsed radio transmitter which could be described as a radar.
It was 1928 when HM 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 a 70km detection range against aircraft.
In 1935, Sir Robert Watson-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 Watson-Watt has often been credited with the invention of radar - something which is obviously, at best, debatable.
Also in 1935, a French ship was equipped with a collision avoidance device of local fabrication. A land-based device, the barrage electronique was tested in 1936 and was used in the early days of the Second World War.
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 and the USA. To some extent, the technology behind these devices can be described in terms of today's 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 co-ordinated 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 an initial 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 inexperienced3. Had the reports been acted upon then the whole attack could have been turned into a failure.
On 27 February, 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 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 defence was, in effect, completely blinded. Chaff (also called 'window' by the RAF, or Düppel in German) had been discovered independently by the British and the Germans - neither side knew of the other side's knowledge. The Germans were aware 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 enable it to be kept 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 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 interceptor 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 propagate through the atmosphere at all. Rated 'top secret', the H2S radar was not to be used over Germany, for fears that its carrier might be shot down, letting the magnetron fall into hostile hands.
But then in spring 1943, a bomber was shot down near Rotterdam and the H2S radar with its magnetron was a big surprise to the Germans. Significant effort went into the repair, study and reproduction of the device, but only a few examples of what they called the 'Rotterdamgerät' 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.
Having lost the war, Germany was ejected from the radar business. Until 1950, any research into the field of radar was forbidden. Lots of researchers emigrated, following in the footsteps of Wernher von Braun and others.
Radar was kept highly secret throughout the Second World War, 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 thing in 1944.
It is very hard to find any remaining Second World War radar equipment in museums today. Even though radars aren't as impressive as 50-ton battle tanks, every single radar had an influence comparable to a whole battleship, so some sense of the impact and significance of radar has been lost. The Online Air Defense Radar Museum features a good selection of American radars. The National Electronics Museum, Linthicum, Maryland (close to Baltimore Washington International Airport) has inter alia an SCR-584 tracker like the ones used to down the majority of V1 flying bombs. It also has an SCR-270 like the one that detected the Japanese assault force on Pearl Harbour. The Imperial War Museum in London has a single exposure of a German Würzburg tracking radar. The Deutsches Museum in Munich as well as the Musée Radar at Douvres, France, feature a Giant Würzburg. The Auto and Technik Museum in Sinsheim-Steinfurt (close to the A6 near Heidelberg, Germany) features one, and so does the National Air Museum at Wright Patterson, 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 the F-105 Thunderchief, also affectionately known as 'Thud' or 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 (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.
The layperson may know radar only as an invisible threat when speeding on a motorway, because police radars are employed as a means to enforce law. However, radar is also on its way to being used to their advantage as automotive radars are now entering the market. They are used to automatically keep a set distance from the car in front, and to warn the driver when obstacles are encountered in his lane.
These days, no major airport can be operated without a radar. Flight controllers in the tower use radar 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 and hundreds of flights are cancelled or redirected to other airports, and local hotels are unable to accommodate any more guests as a result.
Many of the first devices that researchers played with were bistatic radars (this is when the transmitter and receiver do not share the same location but are set up at two different places). These bistatic radars are getting new attention, obviously with much more highly-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 the subject of ongoing research.
At the time of writing, Passive Radar, imaging radar and Multicoloured 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 - the results are displayed as if they had been gathered by a camera and with comparable resolution. Multicoloured radar is another word for ultra-wideband radar which is able to extract much more information out of the scene it sees - it is rather like how colour television must have seemed to a world familiar with only black-and-white.
However, these current cutting-edge activities are also mankind's first steps to imitate the capabilities of sensors that nature developed millions of years ago.
History: The History of Radar | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft