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AN/CPN-4 Ground Controlled Approach Facility

DAYTON, Ohio -- AN/CPN-4 Ground Controlled Approach Facility at the National Museum of the United States Air Force. (U.S. Air Force photo)

DAYTON, Ohio -- AN/CPN-4 Ground Controlled Approach Facility at the National Museum of the United States Air Force. (U.S. Air Force photo)

Note: This item is currently in storage.

The AN/CPN-4 is a complete facility, which is used to locate an approaching aircraft and guide it to a safe landing when visibility is poor. It is made up of three major systems: a two-way radio system for communication between the pilot and the controller, a search radar for locating the aircraft within a 45-mile radius, and a precision radar system for tracking the aircraft during the 11-mile final approach to the runway. After radio contact is established with the approaching aircraft, the radar controller gives the pilot his initial instructions along with information on weather conditions, other air traffic, etc. When the aircraft comes within range of the search radar, the controller directs the pilot to the area covered by the precision radar system. Communication with the pilot then is transferred to the GCA final controller. By watching azimuth-elevation indicators, which show the position of the aircraft with respect to the desired glide path and course, the final controller can tell the pilot what corrections are needed to hold the proper course and glide path as the aircraft descends toward the runway. When the aircraft reaches the runway threshold, the pilot should be able to see well enough to land visually. If not, he must abandon the approach and try again, or proceed to an alternate airfield where there is better weather.

The two units -- the power trailer and operations trailer -- are placed close to the runway and carefully aligned to cover the proper approach to the runway threshold. Although stationary when in use, the GCA facility can be towed, shipped by railroad or transported to a new location by cargo aircraft.

This AN/CPN-4 (Serial No. 1) on display spent most of its career of more than 20 years at Keesler Air Force Base, Miss., as a maintenance training set. Then it was sent to the Sacramento Air Logistics Center at McClellan Air Force Base, Calif., where it was restored for the museum by the Communication-Electronic-Meteorological Division. It was shipped to the museum in 1980.

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