Radar & Radiofrequency Sensing

His experiments confirming James Clerk Maxwell's theory of electromagnetic waves also demonstrated the physical principle later exploited by radar: that radio waves bounce off objects in predictable ways that reveal their position.

Popov, at the Imperial Russian Navy school in Kronstadt, developed a coherer tube device to detect distant lightning and later added a spark-gap transmitter. While testing ship-to-ship communication in the Baltic in 1897 he observed an interference beat caused by a third vessel passing between them, and reported that the effect might be used to detect objects. He pursued the observation no further.

The German inventor showed that radio waves could reveal the presence of a ship in dense fog, though not its range, and obtained German patents in April 1904 plus a British patent on 23 September 1904 for a full system operating at 50 cm wavelength with spark-gap pulses. The design already used a horn antenna with parabolic reflector. Presented to German military officials in trials at Cologne and Rotterdam harbour, it was rejected.

Watson-Watt applied radio methods to storm detection, and through the 1920s led UK research using radio techniques to probe the ionosphere and detect lightning at long range. This work made him an expert in radio direction finding, the skill set he later turned to aircraft detection.

The two US Navy researchers placed a transmitter and receiver on opposite banks of the Potomac River and observed that passing ships made the received signal fade in and out. Taylor reported that the effect could detect ships in low visibility, but the Navy did not pursue it at the time.

Developed in Japan, the Yagi-Uda antenna later became a standard component of radar systems built by the Allied powers, which made far greater practical use of the design during the Second World War than Japan itself.

Alder's secret provisional patent marked an early British claim on radio detection for naval use. It preceded the breadboard work by Butement and Pollard on pulsed 50 cm equipment.

Hyland's observation of similar radio fading caused by aircraft led to a patent application and a proposal for intensive research into radio-echo signals from moving targets at NRL. This reopened the line of enquiry the US Navy had dropped eight years earlier.

The breadboard unit operated at 50 cm (600 MHz) with pulsed modulation and gave successful laboratory results. Its January 1931 write-up is the first official British record of the technology later used in coastal defence and incorporated into Chain Home Low.

A CSF research team under Maurice Ponte, working from studies of the split-anode magnetron, built a continuous-wave obstacle-detecting set, aspects of which were fitted to the ocean liner Normandie in 1935. As a continuous-wave system it lacked the ranging performance of later pulsed radar.

The Soviet system could detect an aircraft within 3 km of the receiver. Development was slowed by Oshchepkov's arrest and gulag sentence, though the USSR still fielded mass-produced sets later in the decade.

Page's apparatus established the pulsed architecture that full radar would follow, in contrast to the continuous-wave French and Soviet designs. The following year the US Army tested a primitive surface-to-surface radar to aim coastal battery searchlights at night.

Using a powerful BBC shortwave transmitter as the source and a GPO receiver in a field near Daventry, Watson-Watt and Wilkins clearly detected a bomber flying around the site, showing an aircraft could be tracked by the radio waves it reflected. Hugh Dowding, Air Member for Supply and Research, was sufficiently impressed that funds were immediately provided for operational development, and the team patented the device as GB593017; an Air Ministry team under Watson-Watt then demonstrated a pulsed system in June 1935. The demonstration prompted Britain to build the Chain Home network of coastal radar stations before the outbreak of war.

The GEMA demonstration was one of three national pulsed systems shown within months of each other, following Page in the US and preceding Watson-Watt's British team. It marked Germany's independent arrival at the pulsed principle.

Watson-Watt became superintendent of the new establishment at Bawdsey Manor near Felixstowe, where work led to the design and installation of the Chain Home aircraft detection and tracking stations. The station concentrated Britain's radar effort into an operational programme.

The first five Chain Home stations were operational by 1936 and by 1940 the network stretched across the entire UK including Northern Ireland. Crude by the standards of the era, CH floodlit the area ahead of it and used radio direction finders to establish the bearing of echoes, requiring higher power but allowing rapid deployment with existing technology. It fed the Dowding system and gave the RAF the advance warning that helped win the Battle of Britain.

The 1938 Bell Lab unit fitted to some United Air Lines aircraft was the first commercial airborne installation. It marked the technology's move beyond purely military sponsorship.

The two types were the USSR's first serially produced radars, though only 607 Redut stations were built during the war. Soviet progress had been slowed by the arrest of the programme's leading engineer.

The device generated far more powerful microwaves than earlier valves, enabling smaller and more precise centimetric radar; Britain's Tizard Mission handed the technology to the United States, accelerating Allied radar development during the war.

A chain of coastal radar stations let fighter squadrons scramble only when needed rather than maintaining costly standing patrols, demonstrating for the first time how radar could shape the course of an air campaign.

An executive order directed the Department of Defense to design a layered, nationwide missile defence architecture including space-based sensors and interceptors, intended to track and destroy ballistic, hypersonic and cruise missile threats to the United States.

Retaliatory strikes against Iranian military sites reportedly damaged radar components of Iran's Russian-supplied S-300 air defence systems, part of a broader exchange of strikes between the two countries during the year.

The Beriev A-50 airborne early warning aircraft, which carries a large rotating radar dome to track aircraft and missiles at long range, was reported damaged or destroyed, degrading Russia's ability to detect Ukrainian air activity.

Successive tranches of small satellites were placed in low Earth orbit to detect and track ballistic and hypersonic missiles from space, intended to feed targeting data to interceptors as part of a wider layered missile defence architecture.

The German-led scheme for joint European procurement of air defence systems, including radar and interceptors, added further member states, aiming to close gaps in layered air defence exposed by the war in Ukraine.

Additional radar sites continued to be built along the coast and on outlying islands, extending detection range across the Western Pacific and South China Sea, part of a longer-running effort to track aircraft and ships beyond the reach of conventional coastal radar.

Investment continued in the over-the-horizon radar system, which monitors approaches to Australia's north from three outback sites, reflecting concern about surveillance of maritime and air activity across South-East Asia and the Pacific.

Flight tests of the domestically developed radar progressed alongside the wider KF-21 Boramae fighter programme, part of South Korea's effort to build an independent fighter and sensor industry rather than rely on imported systems.

Flight trials continued to integrate the active electronically scanned array radar onto the Tejas fighter, part of a wider push to reduce dependence on imported radar and avionics for India's combat aircraft fleet.

Houthi missile and drone attacks on commercial shipping prompted multinational naval patrols and ship operators to upgrade radar warning systems, as low, slow and small targets proved difficult for radars designed to track larger threats.

NOAA announced plans to deploy phased array radars across the United States within ten years, serving both meteorological study and flight monitoring. The plan reflected the convergence of weather and traffic surveillance functions in one radar type.

Shaped to scatter rather than reflect radar signals back to their source, the aircraft struck targets in Baghdad on the first night of the war, demonstrating that an aircraft could be built to evade radar detection rather than outrun or outmanoeuvre it.

Carrying a rotating radar dome above the fuselage, the aircraft let commanders track and direct large numbers of friendly and hostile aircraft from the air, extending radar's horizon far beyond any ground-based station.

The Soviet physicist's theoretical work, later obtained and applied by American engineers, provided the mathematical basis for shaping aircraft to minimise their radar reflection, underpinning stealth aircraft designed a decade later.

The introduction of new electronics in the 1960s allowed two previously separate techniques to be combined in one set. Early warning radars of the period often operated at peak powers in the tens of megawatts to reach long ranges while maintaining accuracy.

Working at Goodyear Aircraft, Wiley showed that combining radar returns collected as an aircraft moved could simulate a much larger antenna, producing sharp ground images later adapted for reconnaissance and mapping satellites.

Gneiss-2 entered service on Pe-2 dive bombers, giving Soviet aircraft their first onboard detection capability. More than 230 stations had been produced by the end of 1944.

Page's monopulse method greatly improved radar accuracy and remained in use for many years across most radar applications. It was one of several wartime advances driven by demands for better resolution and portability.

The laboratory at Massachusetts Institute of Technology developed microwave radar technology through the war years. Watson-Watt was sent to the US in 1941 to advise on air defence after the attack on Pearl Harbor.

The US Navy formed the term from "radio detection and ranging". It subsequently entered English and other languages as a common noun, losing its capitalisation.