Ground-controlled approach

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, including in adverse weather conditions, based on primary radar images.

Originally of the University of California, Berkeley, in 1941 Alvarez was invited to join the recently opened MIT Radiation Laboratory.

The "RadLab" had formed to develop radar systems based on the cavity magnetron, revealed to them by its UK inventors during the Tizard Mission in late 1940.

By the time Alvarez arrived in Boston, the RadLab had already developed a prototype of a new anti-aircraft radar known as XT-1, which had the ability to automatically track a selected target once "locked-on".

In the spring of 1942, XT-1 was moved to Elizabeth City, North Carolina, where the landing path extended out over the Pasquotank River estuary.

[2] XT-1 was based on the conical scanning concept, which greatly increases the angular accuracy of the radar by rotating the beam around a cone-shaped pattern about 15 degrees across.

However, in June 1942 the Office of Scientific Research and Development ordered ten examples anyway, giving the contract to Gilfillan Brothers in Los Angeles.

In November 1942 it was moved to Quonset Point Air National Guard Station where Alvarez began shooting approaches using the system.

The UK had developed their own low-precision approach system based on the Lorenz beam concept, which relied only on a normal audio radio receiver.

[2] In June 1943, Mark I was sent to the UK aboard battleship HMS Queen Elizabeth and emplaced at RAF Elsham Wolds for testing.

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.

Modern ILS and GPS approaches eliminate the possibility of human error from the controller, and can serve many aircraft at the same time.

[1] Ground-controlled approaches have been depicted in several films, including Strategic Air Command, The Big Lift, Airport, Julie, and Skyjacked.

GCA was developed during World War II to enable pilots returning to base to land safely when visibility was poor.