Performance Analysis of a Data Link System

Because of the increased traffic densities and aircraft speeds contemplated in the near future, the present air-trafficcontrol (ATC) system must be improved by a far more efficient and partially automatic system. To prevent further burden on alreadyburdened voice-communication facilities and frequencies, there is a requirement for more efficient means of transferring information between air and ground. This paper deals with the modulation and detection characteristics of a time-division multiplexed, digital radio link intended as an initial step in providing relief to the ATC communication problem. As a result of both analytical and experimental work, two preferred modulation types have been chosen for further investigation. These types are frequency-shift-keyed amplitude modulation (FSK-AM) and frequency-shift-keyed carrier modulation. This analysis shows that there is little difference in performance of the FSK-AM and FSK systems for system frequency instabilities as low as 5 kc. Improvement of the system frequency instability to values below 5 kc shows a rapid improvement of the FSK system over the FSK-AM system. RCA is developing both systems for the Bureau of Research and Development, Federal Aviation Agency, in its AGACS (Experimental Automatic Ground/Air/Ground Communication System) project so that the Bureau can establish, in an operational environment, a comparison of the experimental and theoretical results.