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Ground Controlled Approach

General Procedure

Ground Controlled Approach familiarly called G.C.A. is a radar talkdown system of aiding a pilot to a landing under conditions of re-duced visibility and ceiling. It is the only system which has in practice demonstrated ability to bring in safety under adverse condi-tions any reasonably competent pilot in any type of aircraft equipped with two way communication facilities. No airborne aircraft equipment is re-quired. Pilot check out and familiarization with the gear while highly desireable, is not absolutely essential.

G.C.A. equipment consists of a (usually) mobile ground radar and communication system providing facilities for directing the movement of aircraft on a predetermined air path for a safe approach to a run-way. No equipment other than standard communication is carried in the aircraft. The radar portion on the ground provides its operators accurate and continuous information regarding the location of incoming aircraft with respect to the correct approach to the runway. This information is in the form of range azimuth and elevation and is inter-preted by the operators as lateral and vertical deviations from the pre-selected approach or glide path.

The communication section of the gear is employed to instruct the pilot verbally as to the course he must fly in order to make the proper approach to the runway. Through such instructions, the pilot is directed to bring his plane in on a three-degree glide path and in line with the runway, thus enabling him to touch down under visual control in the usual mannor. Facilities are provided over the 2000-9000 kilo-cycles and 100-156 megacycles bands.

As well as providing range, azimuth, and elevation data on a particular plane that is being brought in for a landing, Ground Con-trolled Approach also supplies range and azimuth information within a radius of 30 miles, provided such craft are not above 5000 feet in altitude. This is done by means of the Search System, a microwave radar operating at a frequency of approximately 3000 megacycles, which has a maximum range of thirty miles. The information obtained from this system is presented on Plan Position Indicators (PPI) in the form of a polar-coordinate map with mobile station as the center, and with all detected targets located in coordinates of range and azimuth. Coverage of 360 degrees in azimuth is obtained by using an antenna which rotates at a speed of 30 revolutions per minute. Headings relayed by the search system operator direct the aircraft into an area scanned by a second or "Precision" radar system.

In the Precision System, the transmitting magnetron operates at a frequency of approximately 10,000 megacycles, and is pulled at a rate of 2000 per second in synchronism with the Search System transmitter. The peak power delivered is approximately 20 kw, and the system has a maximum range of 10 miles. It is subdivided into two portions, one con-cerned with azimuth and range data and the other with elevation and range data. To obtain this three-dimensional information, tow antennas are necessary, one whose beam scans through a vertical plane to provide