|This article does not cite any references or sources. (June 2009)|
In aviation 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 (PAR, for precision approaches with vertical, glide path guidance) or an Airport Surveillance Radar (ASR, providing a non-precision Surveillance Radar Approach with no glide path guidance). Technically, the term GCA applies specifically to the precision radar approach with glide path guidance.
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 controllers 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 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 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 (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.
- "Radar Becomes Lifeline." Popular Science, July 1946, pp. 82–84, first detailed article for general public on GCA radar.