Ground-controlled approach
Encyclopedia
In aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

 a ground-controlled approach (GCA), is a type of service provided by air-traffic controllers whereby they guide aircraft to a safe landing in adverse weather conditions based on radar images. Most commonly a GCA uses information from either a Precision Approach Radar
Precision Approach Radar
Precision approach radar is a type of radar guidance system designed to provide lateral and vertical guidance to an aircraft pilot for landing, until the landing threshold is reached. After the aircraft reaches the decision height or decision altitude , guidance is advisory only...

 (PAR, for precision approaches with vertical, glide path guidance) or an Airport Surveillance Radar
Airport Surveillance Radar
An airport surveillance radar is a radar system used at airports to detect and display the position of aircraft in the terminal area.-Digital Airport Surveillance Radar :...

 (ASR, providing a non-precision Surveillance Radar Approach
Surveillance radar approach
In aviation, Approach Surveillance Radar is a type of radar instrument approach provided with active assistance from Air Traffic Control. The only airborne radio equipment required for radar approaches is a functioning radio transmitter and receiver. The radar controller vectors the aircraft to...

 with no glide path guidance). Technically, the term GCA applies specifically to the precision radar approach with glide path guidance.

Overview

Ground-controlled approach is the oldest air traffic technique to fully implement radar to service a plane - it was largely used during the Berlin airlift in 1948-49. It requires close communication between ground-based air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

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
Precision Approach Radar
Precision approach radar is a type of radar guidance system designed to provide lateral and vertical guidance to an aircraft pilot for landing, until the landing threshold is reached. After the aircraft reaches the decision height or decision altitude , guidance is advisory only...

 systems, to determine the precise course and altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often 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 (glide path) and heading (course) corrections necessary to follow the correct approach path.

Two tracks are displayed on the GCA or Precision Approach Radar (PAR) scope:
  • Azimuth, showing the aircraft's position relative to the extended runway centerline, and
  • Elevation, showing vertical position relative to the ideal glide path.

By following both tracks a landing aircraft will arrive precisely over the runway's touchdown zone. Controllers issue position information and/or correction for both of them at least every five seconds. The guidance is stopped over the approximate touchdown point, but to continue the approach to a landing, pilots must be able to see the runway environment before reaching the published "decision height," usually 200-400 ft above the runway touchdown zone and 1/4 to 3/4 miles from the touchdown point (the published minimum visibility and decision height vary depending upon 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 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 for a long period while civil aviation adopted the instrument landing system
Instrument Landing System
An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

 (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 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, or to "back up" ILS or other approach guidance.

Ground-controlled approaches have been depicted in several films, including Strategic Air Command
Strategic Air Command (film)
Strategic Air Command is a 1955 American film starring James Stewart and June Allyson, and directed by Anthony Mann. Released by Paramount Pictures, it was the first of four films that depicted the role of the Strategic Air Command in the Cold War era....

, The Big Lift
The Big Lift
The Big Lift is a 1950 drama film shot on location in the city of Berlin, Germany, that tells the story of "Operation Vittles", the 1948-1949 Berlin Airlift, through the experiences of two U.S...

, Julie, and Skyjacked.

Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

's novel Glide Path
Glide Path
Glide Path is a novel by Arthur C. Clarke, published in 1963. Clarke's only non-science fiction novel, it is set during World War II, and tells a fictionalized version of the development of the radar-based ground-controlled approach aircraft landing system, and includes a character modeled on...

fictionalizes the original development of GCA.
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