Air/ground communications in air traffic control

One of the vital requirements for safe and efficient control of traffic is the communication link between the controller and pilot. When the US Civil Aeronautics Administration (CAA) first started operating control towers, around 1940, the towers had a single operating poeition which guarded two or three high frequency channels and transmitted on a low frequency channel. Since World War II, most of the air/ground control communications have been transferred to the very high frequencies. In addition to providing vastly greater reliability then the high frequency channe1s which they have largely replaced, the increased spectrum space available for VHF assignments also provides a much greater number of communication channels. These are needed to provide adequate channels for each of the specialized control functionl exercised by controller personnel as well as general use channels for working aircraft with limited communicatlon capabilities. Contrasting with the one man operation of early control towers, some of the large present day installations have six redio equipped positions, all of them sharing the use of the 15 to 20 air/ground channels. The engineering of a communication system of this size has required the solution of a number of design and installation problems. The simultaneous modulation of numerous radio frequency carriers having relatively small frequency separation introduced a serious problem of interterence due to intermodulation, which required the decoupling of transmitting antennae. Desensitization of receivers due to their operation on channels in close proximity to tranemitting frequencies was minimiled by receiver design. These and other similar problems have been investigated extensively in determining the performance requiremente for electronic equipment used in control facities.