The opening of higher frequencies (those of the microwave region) to radar, with its attendant advantages, came about in late 1939 when the cavity magnetron oscillator was invented by British physicists at the University of Birmingham. In 1940 the British generously disclosed to the United States the concept of the magnetron, which then became the basis for work undertaken by the newly formed Massachusetts Institute of Technology (MIT) Radiation Laboratory at Cambridge. It was the magnetron that made microwave radar a reality in World War II.

The successful development of innovative and important microwave radars at the MIT Radiation Laboratory has been attributed to the urgency for meeting new military capabilities as well as to the enlightened and effective management of the laboratory and the recruitment of talented, dedicated scientists. More than 100 different radar systems were developed as a result of the laboratory’s program during the five years of its existence (1940–45).

One of the most notable microwave radars developed by the MIT Radiation Laboratory was the SCR-584, a widely used gunfire-control system. It employed conical scan tracking—in which a single offset (squinted) radar beam is continuously rotated about the radar antenna’s central axis—and, with its four-degree beamwidth, it had sufficient angular accuracy to place antiaircraft guns on target without the need for searchlights or optics, as was required for older radars with wider beamwidths (such as the SCR-268). The SCR-584 operated in the frequency range from 2.7 to 2.9 GHz (known as the S band) and had a parabolic reflector antenna with a diameter of nearly 6.6 feet (2 metres). It was first used in combat early in 1944 on the Anzio beachhead in Italy. Its introduction was timely, since the Germans by that time had learned how to jam its predecessor, the SCR-268. The introduction of the SCR-584 microwave radar caught the Germans unprepared.

Postwar progress

After the war, progress in radar technology slowed considerably. The last half of the 1940s was devoted principally to developments initiated during the war. Two of these were the monopulse tracking radar and the moving-target indication (MTI) radar (discussed in the section Doppler frequency and target velocity). It required many more years of development to bring these two radar techniques to full capability.

New and better radar systems emerged during the 1950s. One of these was a highly accurate monopulse tracking radar designated the AN/FPS-16, which was capable of an angular accuracy of about 0.1 milliradian (roughly 0.006 degree). There also appeared large, high-powered radars designed to operate at 220 MHz (VHF) and 450 MHz (UHF). These systems, equipped with large mechanically rotating antennas (more than 120 feet [37 metres] in horizontal dimension), could reliably detect aircraft at very long ranges. Another notable development was the klystron amplifier, which provided a source of stable high power for very-long-range radars. Synthetic aperture radar first appeared in the early 1950s, but it took almost 30 more years to reach a high state of development, with the introduction of digital processing and other advances. The airborne pulse Doppler radar also was introduced in the late 1950s in the Bomarc air-to-air missile.

The decade of the 1950s also saw the publication of important theoretical concepts that helped put radar design on a more quantitative basis. These included the statistical theory of detection of signals in noise; the so-called matched filter theory, which showed how to configure a radar receiver to maximize detection of weak signals; the Woodward ambiguity diagram, which made clear the trade-offs in waveform design for good range and radial velocity measurement and resolution; and the basic methods for Doppler filtering in MTI radars, which later became important when digital technology allowed the theoretical concepts to become a practical reality.

The Doppler frequency shift and its utility for radar were known before World War II, but it took years of development to achieve the technology necessary for wide-scale adoption. Serious application of the Doppler principle to radar began in the 1950s, and today the principle has become vital in the operation of many radar systems. As previously explained, the Doppler frequency shift of the reflected signal results from the relative motion between the target and the radar. Use of the Doppler frequency is indispensable in continuous wave, MTI, and pulse Doppler radars, which must detect moving targets in the presence of large clutter echoes. The Doppler frequency shift is the basis for police radar guns. SAR and ISAR imaging radars make use of Doppler frequency to generate high-resolution images of terrain and targets. The Doppler frequency shift also has been used in Doppler-navigation radar to measure the velocity of the aircraft carrying the radar system. The extraction of the Doppler shift in weather radars, moreover, allows the identification of severe storms and dangerous wind shear not possible by other techniques.

The first large electronically steered phased-array radars were put into operation in the 1960s. Airborne MTI radar for aircraft detection was developed for the U.S. Navy’s Grumman E-2 airborne-early-warning (AEW) aircraft at this time. Many of the attributes of HF over-the-horizon radar were demonstrated during the 1960s, as were the first radars designed for detecting ballistic missiles and satellites.

Radar in the digital age

During the 1970s digital technology underwent a tremendous advance, which made practical the signal and data processing required for modern radar. Significant advances also were made in airborne pulse Doppler radar, greatly enhancing its ability to detect aircraft in the midst of heavy ground clutter. The U.S. Air Force’s airborne-warning-and-control-system (AWACS) radar and military airborne-intercept radar depend on the pulse Doppler principle. It might be noted too that radar began to be used in spacecraft for remote sensing of the environment during the 1970s.

Over the next decade radar methods evolved to a point where radars were able to distinguish one type of target from another. Serial production of phased-array radars for air defense (the Patriot and Aegis systems), airborne bomber radar (B-1B aircraft), and ballistic missile detection (Pave Paws) also became feasible during the 1980s. Advances in remote sensing made it possible to measure winds blowing over the sea, the geoid (or mean sea level), ocean roughness, ice conditions, and other environmental effects. Solid-state technology and integrated microwave circuitry permitted new radar capabilities that had been only academic curiosities a decade or two earlier.

Continued advances in computer technology in the 1990s allowed increased information about the nature of targets and the environment to be obtained from radar echoes. The introduction of Doppler weather radar systems (as, for example, Nexrad), which measure the radial component of wind speed as well as the rate of precipitation, provided new hazardous-weather warning capability. Terminal Doppler weather radars (TDWR) were installed at or near major airports to warn of dangerous wind shear during takeoff and landing. Unattended radar operation with little downtime for repairs was demanded of manufacturers for such applications as air traffic control. HF over-the-horizon radar systems were operated by several countries, primarily for the detection of aircraft at very long ranges (out to 2,000 nautical miles [3,700 km]). Space-based radars continued to gather information about the Earth’s land and sea surfaces on a global basis. Improved imaging radar systems were carried by space probes to obtain higher-resolution three-dimensional images of the surface of Venus, penetrating for the first time its ever-present opaque cloud cover.

The first ballistic missile defense radars were conceived and developed in the mid-1950s and 1960s. Development in the United States stopped, however, with the signing in 1972 of the antiballistic missile (ABM) treaty by the Soviet Union and the United States. The use of tactical ballistic missiles during the Persian Gulf War (1990–91) brought back the need for radars for defense against such missiles. Russia (and before that, the Soviet Union) continually enhanced its powerful radar-based air-defense systems to engage tactical ballistic missiles. The Israelis deployed the Arrow phased-array radar as part of an ABM system to defend their homeland. The United States developed a mobile active-aperture (all solid-state) phased-array called Theater High Altitude Area Defense Ground Based Radar (THAAD GBR) for use in a theatrewide ABM system.

Advances in digital technology in the first decade of the 21st century sparked further improvement in signal and data processing, with the goal of developing (almost) all-digital phased-array radars. High-power transmitters became available for radar application in the millimetre-wave portion of the spectrum (typically 94 GHz), with average powers 100 to 1,000 times greater than previously.