aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...

a ground-controlled approach (GCA), is a type of service provided by air-traffic controllers whereby they guide aircraft to a safe landing, including in adverse weather conditions, based on primary radar images. Most commonly a GCA uses information from either a Precision Approach Radar (PAR, for precision approaches with vertical, glidepath guidance) or an Airport Surveillance Radar (ASR, providing a non-precision surveillance radar approach with no glidepath guidance). The term GCA may refer to any type of ground radar guided approach such as a PAR, PAR without glideslope or ASR. When both vertical and horizontal guidance from the PAR is given, the approach is termed a precision approach. If no PAR glidepath is given, even if PAR equipment is used for lateral guidance, it is considered a non-precision approach.

History

Early experiments

The GCA concept was originally developed by nuclear physicist Luis Alvarez. Originally of the

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant u ...

, in 1941 Alverez was invited to join the recently opened

MIT Radiation Laboratory The Radiation Laboratory, commonly called the Rad Lab, was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts. It was first created in October 1940 and operated until 31 ...

. The "RadLab" had formed to develop radar systems based on the

cavity magnetron The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field whi ...

, revealed to them by its UK inventors during the

Tizard Mission The Tizard Mission, officially the British Technical and Scientific Mission, was a British delegation that visited the United States during WWII to obtain the industrial resources to exploit the military potential of the research and development ( ...

in late 1940. By the time Alverez arrived in

Boston Boston (), officially the City of Boston, is the state capital and most populous city of the Commonwealth of Massachusetts, as well as the cultural and financial center of the New England region of the United States. It is the 24th- mo ...

, the RadLab had already developed a prototype of a new anti-aircraft radar known as XT-1. XT-1 had the ability to automatically track a selected target once "locked-on". Production versions of the XT-1 would begin deliveries in 1944 as the famed

SCR-584 The SCR-584 (short for '' Set, Complete, Radio # 584'') was an automatic-tracking microwave radar developed by the MIT Radiation Laboratory during World War II. It was one of the most advanced ground-based radars of its era, and became one of the ...

. Alverez was also a

light aircraft A light aircraft is an aircraft that has a maximum gross takeoff weight of or less.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 308. Aviation Supplies & Academics, 1997. Light aircraft are used as utility aircraft co ...

pilot and was aware of the problems landing aircraft in bad weather. He quickly asked whether the XT-1 could be used for this role; once locked-on to a single aircraft, the radar operator could read the radar displays and give instructions to the pilot to talk them down to a point close to the runway. On 10 November 1941, he was granted time on the XT-1 and successfully measured the position of a landing aircraft with the required accuracy. In the spring of 1942, XT-1 was moved to

Elizabeth City, North Carolina Elizabeth City is a city in Pasquotank County, North Carolina, Pasquotank County, North Carolina, United States. As of the 2020 census, it had a population of 18,629. Elizabeth City is the county seat and largest city of Pasquotank County. It ...

, where the landing path extended out over the

Pasquotank River The Pasquotank River
, from the North Carolina Collection's website at the

conical scanning Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...

concept, which greatly increases the angular accuracy of the radar by rotating the beam around a cone-shaped pattern about 15 degrees across. This caused the beam to periodically sweep across the water when it was pointed near the horizon, which would often be the case as the aircraft approached the ground. A new methodology was developed in May 1942, combining an S-band airport surveillance radar (ASR) that brought the aircraft into the general area of the airport, and a second

X-band The X band is the designation for a band of frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is rather indefinitely set at approxi ...

radar, the precision approach radar (PAR), with separate antennas for vertical and horizontal guidance that were moved in such a way to avoid seeing the ground. The first example of the new system, known as Mark I, began testing in November 1942. A further improved version, Mark II, replaced the mechanical scanning antennas with a

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

"squeeze box" that performed the same scanning without the antennas moving. Mark II also introduced the "expanded-partial-plan-position-indicators", later replaced with the simpler name "beta scan".

First orders

By the time Mark II was ready, the

US Army Air Forces The United States Army Air Forces (USAAF or AAF) was the major land-based aerial warfare service component of the United States Army and ''de facto'' aerial warfare service branch of the United States during and immediately after World War II ...

had already widely deployed the SCS-51

instrument landing system In aviation, the instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to ...

(ILS) for this role, and they displayed no interest in the new system. However, in June 1942 the

Office of Scientific Research and Development The Office of Scientific Research and Development (OSRD) was an agency of the United States federal government created to coordinate scientific research for military purposes during World War II. Arrangements were made for its creation during May 1 ...

ordered ten examples anyway, giving the contract to Gilfillan Brothers in Los Angeles. Meanwhile, testing with the Mark I continued. In November 1942 it was moved to

Quonset Point Air National Guard Station Quonset Point Air National Guard Station is the home base of the Rhode Island Air National Guard 143d Airlift Wing. Naval Air Station (NAS) Quonset Point was a United States Naval Base in Quonset Point, Rhode Island that was deactivated in 1974. ...

where Alvarez began shooting approaches using the system. Navy Ensign Bruce Giffin soaped the windshield of his SNB to demonstrate his trust in the system. On 1 January 1943, a

Consolidated PBY Catalina The Consolidated PBY Catalina is a flying boat and amphibious aircraft that was produced in the 1930s and 1940s. In Canadian service it was known as the Canso. It was one of the most widely used seaplanes of World War II. Catalinas served wi ...

had nearly run out of fuel and was forced to land in spite of bad weather. The Mark I operator talked the PBY down into a successfully landing, the first "save". This story caught the attention of

the Pentagon The Pentagon is the headquarters building of the United States Department of Defense. It was constructed on an accelerated schedule during World War II. As a symbol of the U.S. military, the phrase ''The Pentagon'' is often used as a meton ...

, and a demonstration at Washington National Airport was carried out on 14 February 1943. This resulted in an immediate contract from the Army Signal Corps for 57 examples of what they called the MPN-1A from Gilfillan while the

US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

placed a second contract for 80 MPN-IC from Bendix Radio. Several additional orders followed, including an Army order for 200 from ITT.

UK interest

The UK kept in close contact with their RadLab counterparts, and immediately expressed an interest in the system. The UK had developed their own low-precision approach system based on the

Lorenz beam The Lorenz beam was a blind-landing radio navigation system developed by C. Lorenz AG in Berlin. The first system had been installed in 1932 at Berlin-Tempelhof Central Airport, followed by Dübendorf in Switzerland (1934) and others all over the ...

concept, which relied only on a normal audio radio receiver. This system, the Blind Approach Beacon System, provided horizontal guidance only, and was not accurate enough to use for a primary landing system. ILS offered the required accuracy and vertical guidance, but would require new radios and instruments to be added to every aircraft. As GCA also required only a normal radio receiver to operation, it would be much easier to use with vast bomber fleet. In June 1943, Mark I was sent to the UK aboard battleship HMS ''Queen Elizabeth'' and emplaced at

RAF Elsham Wolds Royal Air Force Elsham Wolds or more simply RAF Elsham Wolds is a former Royal Air Force station in England, which operated in the First World War and the Second World War. It is located just to the north east of the village of Elsham in nort ...

for testing. Over the next months, over 270 approaches were carried out, including the return of 21 Avro Lancasters on an operational mission on the night of 23 August. This led to a Lend-Lease request for a GCA radar for every RAF Bomber Command airfield. This order helped cement US interest in the system, and they agreed to leave the prototype in the UK.

Deliveries and post-war use

The first examples of the production AN/MPN-1A were delivered to the Army in the fall of 1944. The first operational unit was placed in

Verdun Verdun (, , , ; official name before 1970 ''Verdun-sur-Meuse'') is a large city in the Meuse department in Grand Est, northeastern France. It is an arrondissement of the department. Verdun is the biggest city in Meuse, although the capital ...

in December. Units were soon delivered to the Pacific, installed at Iwo Jima. By the end of the war, most airfields in Europe and the Pacific had one. In early 1946, three surplus MPN- 1 were given to the

Civil Aeronautics Board The Civil Aeronautics Board (CAB) was an agency of the federal government of the United States, formed in 1938 and abolished in 1985, that regulated aviation services including scheduled passenger airline serviceStringer, David H."Non-Skeds: T ...

and placed at Washington-National Airport,

LaGuardia Airport LaGuardia Airport is a civil airport in East Elmhurst, Queens, New York City. Covering , the facility was established in 1929 and began operating as a public airport in 1939. It is named after former New York City mayor Fiorello La Guardia ...

, and Chicago-Midway. This led to further orders for more highly developed versions of both the ASR and PAR systems.

Overview

Ground-controlled approach is the oldest air traffic technique to fully implement radar to service a plane. The system was simple, direct, and worked well, even with previously untrained pilots. It requires close communication between ground-based

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...

lers and pilots in approaching aircraft. Only one pilot is guided at a time (max. 2 under certain circumstances). The controllers monitor dedicated precision approach radar systems, to determine the precise course and

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

of approaching aircraft. The controllers then provide verbal instructions by radio to the pilots to guide them to a landing. The instructions include both descent rate (glidepath) and heading (course) corrections necessary to follow the correct approach path. SH-3A of HS-4 makes first GCA-landing c1964

SH-3A of HS-4 makes first GCA-landing c1964

Two tracks are displayed on the Precision Approach Radar (PAR) scope: * Azimuth, showing the aircraft's position relative to the horizontal approach path. * Elevation, showing vertical position relative to the published glidepath. By following controller commands to keep the landing aircraft on both glidepath and approach centerline, a pilot will arrive precisely over the runway's touchdown zone. In order to insure continuous radio communication integrity, controllers are required to make radio transmissions at certain minimum intervals depending on the type of approach flow and phase of the approach. In order to land, pilots must have the runway or runway environment in sight prior to reaching the "decision height," for PAR approaches (usually 100–400 ft above the runway touchdown zone) or prior to the "Missed Approach Point" for non-precision approaches. The published minimum visibility and decision height/minimum descent altitude vary depending upon the approach and runway lighting, obstacles in the approach corridor, type of aircraft, and other factors. Pilots of revenue flights periodically must demonstrate PAR approach proficiency, and GCA controllers must conduct a minimum number of such approaches in a year to maintain competency. Because of their labor-intensive nature—one GCA controller is normally required for each aircraft on final approach—GCAs are no longer in widespread use at civilian airports, and are being discontinued at many military bases. However, air traffic controllers at some locations in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

are required to maintain currency in their use, while the Belgian Air Force still uses the PAR for ground-controlled approaches on a daily basis. NATO has kept GCA active while civil aviation adopted the

(ILS). Global Positioning System (GPS) based approaches that provide both lateral and vertical guidance are coming into widespread use, with approach minima as good as, or nearly as good as, GCA or ILS. Modern ILS and GPS approaches eliminate the possibility of

human error Human error refers to something having been done that was " not intended by the actor; not desired by a set of rules or an external observer; or that led the task or system outside its acceptable limits".Senders, J.W. and Moray, N.P. (1991) Human ...

from the controller, and can serve many aircraft at the same time. The ground-controlled approach is useful when the approaching aircraft is not equipped with sophisticated navigation aids, and may also become a life saver when an aircraft's on-board navigation aids are inoperative, as long as one communication radio works. Sometimes the PAR-based ground-controlled approach is also requested by qualified pilots when they are dealing with an emergency on board to lighten their workload. In the United States, instrument approaches must be monitored by a PAR (if one exists with a coinciding final approach course) during certain condition such as times of darkness or low weather depending upon the controlling agency (USAF, U.S. Army, USN or FAA) or upon pilot request. Ground-controlled approaches have been depicted in several films, including Strategic Air Command,

The Big Lift ''The Big Lift'' is a 1950 American drama war film on location in the city of Berlin, Germany, that tells the story of "Operation Vittles", the 1948–49 Berlin Airlift, through the experiences of two U.S. Air Force sergeants (played by Montgome ...

Airport An airport is an aerodrome with extended facilities, mostly for commercial air transport. Airports usually consists of a landing area, which comprises an aerially accessible open space including at least one operationally active surfa ...

, Julie, and Skyjacked. Arthur C. Clarke's novel '' Glide Path'' fictionalizes the original development of GCA. Clarke contributed to the early application of GCA. GCA was developed during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

to enable pilots returning to base to land safely when

visibility The visibility is the measure of the distance at which an object or light can be clearly discerned. In meteorology it depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time o ...

was poor. It was essential for maintaining the flow of supplies during the

Berlin airlift The Berlin Blockade (24 June 1948 – 12 May 1949) was one of the first major international crises of the Cold War. During the multinational occupation of post–World War II Germany, the Soviet Union blocked the Western Allies' railway, roa ...

in 1948–49.

References

{{reflist

External links

"Radar Becomes Lifeline."
''Popular Science'', July 1946, pp. 82–84, first detailed article for general public on GCA radar. Types of final approach (aviation)