Eye was prohibited by the Joint Chiefs of Staff from actively "interrogating" MiG transponders, following a National Security Agency security policy protecting its "intelligence sources" (of which the QRC-248 was one), thus was restricted to waiting for North Vietnamese GCI to interrogate its aircraft. QRC-248 began regular use in July 1967, but by then North Vietnam's MiG force, which had suffered serious losses in May, had suspended its combat ops. In the last week of August, however, after a period of intensive training and revision of tactics, the MiGs began engaging US strike forces again, scoring a number of kills. Then, Seventh Air Force finally obtained permission for the new Ethan Bravo mission EC-121 to actively interrogate with QRC-248 on 6 October. By 4 December, its success outweighed any value in flying the Ethan Alpha orbit, which was discontinued until July 1972. On 1 March 1968, College Eye callsigns changed to Ethan 01, 02, 03, and 04 in conformity with USAF SOPs. Ethan 03 (Laotian orbit) began "positive control" (airborne direction) of C-130 flare ship flights and A-26 Invader night interdiction missions along the Ho Chi Minh Trail in Laos on 19 April 1968. The task force was scaled back in July 1968, to 4four EC-121Ds and the Rivet Top testbed aircraft to allow for the basing of another College Eye detachment at Itazuke AB, Japan. The name of the task force was discontinued in October 1968, when it was redesignated a final time as Detachment 1 (Rotational), 552nd AEWCW. EC-121 deployments to Southeast Asia were discontinued in June 1970 in the expectation that they would no longer be used. Rivet Top 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 two problems, reducing its effectiveness.