In August 1967, while the College Eye Task Force was still based at Udon RTAFB, another prototype EC-121 variation began operations testing new equipment as Detachment 2 of the Tactical Air Warfare Center.
Known as Rivet Top, this modified EC-121K (later redesignated EC-121M) carried the QRC-248 newly installed in the College Eye aircraft and also had electronic interrogators capable of reading 2 additional Soviet transponders, the SRO-1 and SOD-57.
Its electronics were custom built rather than off-the-shelf.
Its most important upgrade was the top-secret Rivet Gym installation.
This consisted of the addition to the crew of Vietnamese-speaking intelligence specialists manning 4 voice comms intercept stations able to monitor all comms between the MiGs and their GCI controllers.


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...
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 ops were sharply scaled back.
Rivet Gym installations were back-fitted to all College Eye EC-121s by the end of May 1968.
In August 1967, while the College Eye Task Force was still based at Udon RTAFB, another prototype EC-121 variation began operations testing new equipment as Detachment 2 of the Tactical Air Warfare Center. Known as Rivet Top, this modified EC-121K (later redesignated EC-121M) carried the QRC-248 newly installed in the College Eye aircraft and also had electronic interrogators capable of reading 2 additional Soviet transponders, the SRO-1 and SOD-57. Its electronics were custom built rather than off-the-shelf. Its most important upgrade was the top-secret Rivet Gym installation. This consisted of the addition to the crew of Vietnamese-speaking intelligence specialists manning 4 voice comms intercept stations able to monitor all comms between the MiGs and their GCI controllers.

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...
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 ops were sharply scaled back. Rivet Gym installations were back-fitted to all College Eye EC-121s by the end of May 1968.
Lockheed EC-121 Warning Star