Low Frequency, L/F, Medium Frequency, M/F, and High Frequency, H/F, radio transmissions follow curvature of the earth and can be heard far over the horizon. Very High Frequency (VHF) and Ultra High Frequency (UHF) radio transmissions are line of sight. They travel in a straight line and cannot be heard over the horizon. See "Instrument Flying Handbook," published by the U.S. Department of Transportation, Federal Aviation Administration.

The National Security Agency had the capability of listening to and recording Low, Medium and High Frequency transmissions from the 1967 war zone from a number of communications stations far from Egypt and Israel. Tactical transmissions usually broadcast on VHF or UHF could only be monitored from the surface by a platform within 19 or 20 miles of the source of transmission. An elevated listening platform, such as a Navy EC-121 aircraft or an Air Force C-130 aircraft, could monitor VHF/UHF from greater distances. See 7-4 Line of Sight Transmission, "Instrument Flying Handbook."

It was for this reason that the U.S.S. Liberty and the Navy and Air Force aircraft were positioned so close to Egypt and Israel. It should be remembered that the Liberty's orders were issued in May, approximately two weeks before the war began; and, by June 8, 1967, the orders had been overcome by events that made the presence of the Liberty in the line of sight area no longer necessary. A number of actions, if taken, might have kept the Liberty out of harm's way.