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In the aviation realm of today, radar and its associated technologies have become a common tool of safety and navigation. However, the radar tracking systems we use today have evolved over decades of application. During this blog, we'll trace the development of radar along with its civil aviation use.

Radar, as defined, stands for Radio Detection and Ranging. Experiments conducted by Heinrich Hertz, a notable German physicist, proved the theory that electric and magnetic fields could travel through space in the form of an electromagnetic wave in the 1880's, as well as reflecting from metallic objects. Though Hertz's experiments showed great potential, it wasn't until the early twentieth century that radio technology advances made the transmission of electromagnetic signals more practical. However, Hertz's test results were studied by another German physicist, Christian Hulsmeyer. Hulsmeyer built upon Hertz's success by designing a device called a Telemobiloscope. The Telemobiloscope was basically a spark-gap transmitter connected to an array of dipole antennas, and a coherer receiver with a cylindrical parabolic antenna which could rotate a full 360 degrees. While the transmitted signal had a broad coverage, the receiving antenna was narrowly focused. When a reflected signal reached the receiver, a relay was activated, which in turn rang an electric bell. Though the Telemobiloscope could not directly indicate range, a separate device was developed utilizing vertical measurements and trigonometry to calculate the approximate range. Several tests of the Telemobiloscope system in 1904 and 1905 only achieved mixed results. A study conducted after the tests indicated the probable cause of Telemobiloscope test failures were due to the system using wireless technology from the late 1890's, which did not include tuning circuits for frequency selection. Virtually all shipboard radios were installed with tuning circuits in the early 1900's and could interfere with the signals of the Telemobiloscope.

While aviation as a whole progressed dramatically during World War I, there was little effort in radio detection technology. However, this began to change during the 1920's as scientists in several countries began to study the behavior of radio signals under different atmospheric conditions. In Britain, Robert Watt, a meteorologist, developed the use of radio signals to determine the position of thunderstorms. The difficulty in plotting the direction of these fleeting signals led to the use of rotating directional antennas, followed by the use of oscilloscopes in 1923 from which to display the signals. At about the same time as Watt was conducting his experiments, Albert Taylor and Leo Young were conducting communication experiments at the U.S. Naval Aircraft Radio Laboratory when they discovered a wooden ship in the Potomac River was interfering with their signals. Taylor and Young then prepared a memorandum stating that radio signals might be used for the detection of ships in harbor defense. Though their suggestion was not acted upon by the Navy, by 1930 Taylor and Young were working with Lawrence Hyland at the Naval Research Laboratory, which replaced the Naval Aircraft Radio Laboratory, developed a similar arrangement of radio equipment to detect passing aircraft. The results of these experiments then led to a formal proposal to the government and the granting of a patent using radio equipment to detect passing ships and aircraft. Though a simple wave-interference device could detect an object, it could not determine its speed or location. Once pulsed radar and additional coding became practical, this information could be extracted from a continuous wave signal. Later tests conducted by Taylor, Young and Robert Page with a pulsed radio signal system in 1934 proved both the location and speed of a detected aircraft could be plotted, making it the world's first true radar system. Page developed a device called a duplexer, which allowed both the transmitter and receiver to use the same antenna without damaging the sensitive receiver circuitry. The duplexer also solved the problem associated with synchronization of separated transmitter and receiver antennas, which is critical to the positioning of long-range targets. By 1936 the Naval Research Laboratory was able to track planes at distances of up to 25 miles.

The development of pulse systems was perhaps the key advance that enabled the operation of modern radar systems. By timing the pulses on an oscilloscope, the range of a target could be determined, as well as its angular location. The advancement of radar capabilities took on added significance during World War II, as both axis and allied nations sought a military advantage by its use and applications. The first operational radar system in extensive use was the British Chain Home radar, which was completed in 1937. Chain Home and several early radars operated in the high frequency portion of the electromagnetic spectrum. Even as the high frequency radars entered service, their designers recognized that radars that could operate at frequencies higher than the high frequency segment could perform better. In the mid 1930's, military radar scientists in the United States developed several devices such as the klystron electron tube, the resonant cavity circuit, in addition to the coaxial and waveguide transmission lines and components that allowed the generation of signals in the microwave region of the electromagnetic spectrum. This microwave technology offered a quantum leap in efficiency and performance over previous radar frequencies. The United States shared this technology with Britain, enabling them to detect aircraft approaching the British Isles more rapidly. This gave the Royal Air Force a distinct advantage over the Luftwaffe during the Battle of Britain, contributing to its outcome. Britain also developed airborne radar systems using the microwave spectrum that assisted pilots flying at night to detect aircraft in the darkness, as well as allowing bomber crews to locate targets at night. The United States Army Air Forces (USAAF) established a radar research facility at the Massachusetts Institute of Technology in Cambridge, Massachusetts. This program applied radar technology to a number of weapons systems to include airborne radars, used to target aircraft at night, in addition to ships and submarines, weather reconnaissance and navigation, shipborne radar, search radars for detecting aircraft, as well as aiming anti-aircraft guns. The Germans also made major advances in radar, such as the Wurzburg ground radar, which entered service in 1940. While the Germans operated a number of ground-based radar systems in addition to airborne and shipborne radars, they could not keep pace with the allied research and development effort as the war progressed.

Ground Control Approach Radar was first used by the USAAF in 1943. Ground Controlled Approach or GCA aids pilots to land safely in conditions of poor visibility. GCA systems utilize a pair of oscillating radar antennas with narrow beams, one scanning azimuth (directional) with the other scanning elevation (altitude). An air traffic controller observes the precise angle returns from these azimuth and elevation radars, then radioing landing instructions to the pilot. GCA systems also include a 360 degree rotating radar dish for monitoring the entire terminal area. This radar utilizes a narrow azimuth coupled with a high elevation and is capable of detecting aircraft out to twenty miles from the airport up to an altitude of 10,000 ft. The first civil use of military GCA equipment was at LaGuardia Airport in 1945, tripling the landing rate from five planes per hour to fifteen. Though Ground Control Approach systems experienced a few problems, the system was effective enough that it was utilized at the majority of airports in the continental United States by 1952. A few years later, GCA systems began to be replaced by a new landing aid called the Instrument Landing System (ILS). ILS uses similar course guidance principles, but uses receivers in aircraft that display course deviation directly to the cockpit.

Though safe landings are of paramount importance, it is of equal importance that they have a safe flight. When the ILS system replaced the GCA scanning pencil beams, advanced rotating radars with faster scan rates and larger coverage areas began to replace the GCA terminal area surveillance radars. With air traffic increasing during the 1950's, it also became important to track planes in high-altitude airspace. As a result, the coverage of air traffic radar grew throughout the 1960's as long-range radars were deployed along the major air routes. Initially, these aircraft surveillance radars had no automatic tracking capability. Controllers pushed plastic markers called shrimp boats around the screen to track the movement of an aircraft. While manual tracking was adequate for routes with a low traffic volume, the rapid expansion of long-range routes required an automatic tracking system. By the end of the 1960's radar surveillance of civil aircraft routinely included automatic aircraft tracking. With today's technology, air controllers are now able to track both aircraft and hazardous weather. Modern air traffic control systems now use Doppler radar from which to distinguish moving aircraft from fixed targets, in addition to measuring storm velocities. Another development in civil air tracking was the use of the Identification Friend or Foe (IFF) system. Conceived during World War II, IFF utilized special radar receiver/transmitter units located in friendly aircraft. The radar transponder units responded to coded radar transmissions (interrogations) to confirm they were friendly aircraft. By the late 1950's, the application of IFF technology to track civil aircraft was under consideration. The use of transponders increases the detection range of the radar, eliminates clutter interference, as well as offering a means of aircraft identification and altitude reporting. The Federal Aviation Administration (FAA) mandated a national standard for air traffic control transponders and interrogators in the early 1960's. By 1970 more than 200 ground-based interrogators were in use in the U.S. with the FAA requiring all civil aircraft flying near major airports in controlled airspace to be equipped with transponders.

During the 1970's the FAA upgraded the original air traffic control radar beacons to improve their surveillance capacity in crowded airspace, in addition to expanding the radar system capacity to receive and transmit a greater number of coded signals, which were discrete addressed, utilizing ground sensors and airborne transponders that can operate with the original system, yet achieving the capability and functionality of the new. In the 1980's the FAA developed an enhanced transponder signal apparatus, capable of air-to-air surveillance using the same transponders as the ground to air units. This collision avoidance system proved to be highly successful and is now required on all civil aircraft operating in the United States and Europe. With the availability of the Global Positioning System (GPS) in the 1990's, aircraft navigation and control was further advanced. The Global Positioning System is a navigation system using satellites, a receiver and algorithms to synchronize location, velocity and time data for air, sea and land travel. The satellite system consists of a constellation of 24 satellites in six earth-centered orbital planes, each with four satellites, orbiting at 13,000 miles above the earth and traveling at a speed of 8,700 mph. While only three satellites are necessary to produce a location on the earth's surface, a fourth satellite is often used to validate data from the other three, as well as calculate the altitude of a device. In 2003 the U.S. Congress directed the FAA to transition to NextGen, a satellite-based air traffic control system employing GPS technology. The new satellite technology will allow planes to take off, land and fly closer together and take more direct routes between airports. Other improvements planned as part of NextGen include digital communications between pilots and controllers, more precise weather information and new tools to help planes land and take off in inclement weather. NextGen will also improve the ability of the FAA to restart or continue air traffic control operations after a disruption. While NextGen is an impressive air control system, it hasn't been without problems. Several key features of the program have been delayed from the original completion date of 2025 to 2030, while others have been delayed indefinitely. Costs are another factor in NextGen implementation, with the initial cost of 18 billion increasing to more than 22 billion, although design changes accounted for part of the increase. Perhaps the most critical issue facing NextGen is one of cybersecurity, with the General Accounting Office and outside agencies questioning whether the FAA has done enough to reduce cyber threats to air traffic control operations.