Despite this advantage, Rivet Top experienced 2 problems reducing its effectiveness. Its operators did not have radar scopes to correlate intercepted conversations with specific flights of MiGs and thus could not determine which US aircraft might be under attack. Secondly, like QRC-248, Rivet Gym was an National Security Agency SIGINT asset and subject to even more stringent rules protecting knowledge of its existence. Even when real-time warnings to US aircraft were finally permitted in mid-1972, fighter crews were not made aware of the source of the warnings and because EC-121 radio communications were poor, mandating the use of a radio relay aircraft that often failed, they tended to disregard the credibility of the source...