Its operators did not have radar scopes to correlate intercepted conversations with specific flights of MiGs, thus could not determine which US aircraft might be under attack. Secondly, like QRC-248, Rivet Gym was an NSA 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. The Rivet Top prototype moved to Korat RTAFB along with the College Eye Task Force in October 1967. Originally scheduled to return to the USA in February 1968, because of its value, it remained at Korat until 1969. Flying daily missions through its testing period, it began flying every-other-day missions over the Gulf of Tonkin after 31 March 1968, when Rolling Thunder operations were sharply scaled back. Rivet Gym installations were back-fitted to all College Eye EC-121s by the end of May 1968. Operation Kingpin Two EC-121Ds, newly modified with the Southeast Asia Operational Requirement 62 (SEAOR-62) electronics suite, but not yet operational as EC-121Ts, were ordered to Korat RTAFB in October 1970. Under the guise of being field tested, they were accompanied by a C-121G carrying additional crew members, the most experienced 552nd AEWCW technicians, and equipment necessary to maintain the new electronics. The SEAOR-62 package was supported by a digital data receiver ground terminal and by radio relay equipment transshipped by separate classified airlift. The EC-121Ts arrived in Thailand from McClellan AFB on 12 November. The purpose of the deployment was to provide an integrated tactical data display with real-time inputs (similar to the Navy Airborne Tactical Data System equipping E-1B Tracer platforms of Task Force 77) in support of Operation Kingpin, a mission to rescue US prisoners of war held at Son Tay prison. Once at Korat, some equipment was tested for the first time because of emission restrictions in US airspace and the only available manuals and checklists were notes from early flight tests. Even so, both aircraft were operational by 17 November. On 20 November 1970, 2 Warning Stars, using the callsigns Frog 01 and 02, respectively, took off 10 minutes apart at 22:00 from Korat to take station at the low-altitude Alpha orbit over