GENERALLY, UNTIL THE 1980S, radio communication between Pilots and ground stations such as Air Traffic Control (ATC) or the Airline Operations Department (AOD) only took the form of the voice communications that took place in the Very High Frequency (VHF) and High Frequency (HF) ranges. As VHF frequencies are broadcast over line-of-sight (LOS), aircraft flying over open water faced problems. In optimal conditions, LOS waves travel in a direct path from the source to the receiver-and there have to be no obstacles between them. But things get complicated on land, even when ocean-going planes approach near land. Back then, if an aircraft was about 200 miles offshore, it needed to switch to the lower HF frequency range to maintain radio communications with ground stations.
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