As it was expensive to operate and it exposed a large crew to enemy fire, it was replaced in December 1970 by the much smaller (and eventually unmanned drone) QU-22 Pave Eagle sensor monitor.

From July 1970 to January 1971, they rotated on 30- to 90-day temporary-duty deployments to Korat Royal Thai Air Force Base, Thailand, under the name Commando Buzz.

At the onset of Rolling Thunder, the North Vietnamese had an advantage in that their radar coverage could detect most US strike aircraft flying at or above 5,000 feet (1,500 m) virtually anywhere in the country, using a system that was difficult to jam.

US forces countered with radar ships (Crown) in the Gulf of Tonkin and a ground site at Nakhon Phanom RTAFB, Thailand, but both systems were line-of-sight and suffered coverage gaps.

[15] The EC-121s were designed for detection of aircraft flying over water, so ground clutter (spurious signal returns off of terrain features such as mountains) caused interference with their radar pictures.

[4] This provided a practical detection range of 100 miles (160 km), enough to cover the Hanoi urban area and the main MiG base at Phúc Yên.

A major disadvantage of this arrangement, however, was that most MiG contacts were beyond the 70 miles (110 km) range of the Big Eye's APS-45 Height Finder radar, so that they were unable to provide this data to USAF strike forces.

Air-conditioning systems aboard the EC-121 were virtually useless in this profile, and the heat produced by the electronics, combined with the threat of being shot down, made Alpha orbit missions in particular very stressful.

[4] The Big Eye Task Force remained at Tan Son Nhut until February 1967, when the threat of Viet Cong ground attacks prompted a move to Thailand.

[19] From April 1965 to early 1966 and beginning again in late 1967, the EC-121Ds also controlled a flight of MiGCAP fighters for unarmed support aircraft operating over the Tonkin Gulf.

The EC-121Ds also served as an airborne communications relay center for strike aircraft to transmit mission results and position reports to Danang Air Control Center; directed operations of fighter escorts, MiGCAPs, Lockheed C-130 Hercules flare ships and A-26 strike aircraft along the North Vietnamese-Laotian border; provided radar and navigational assistance for combat search and rescue missions; and assisted fighters in finding tankers for emergency refueling.

In May 1966, the government of China formally protested an incursion by a Republic F-105 Thunderchief pursuing a North Vietnamese MiG it subsequently shot down 25 miles (40 km) inside Chinese territory.

The SRO-2 transponders installed in Soviet export MiGs enabled Cuban ground-controlled interception (GCI) radars to identify and control their fighters.

A testbed EC-121 called Quick Look had flown with College Eye in January 1967 to test QRC-248 and found that North Vietnamese MiGs used the same transponder.

Ethan Bravo's mission was changed from that of a backup for Ethan Alpha to being the primary QRC-248 listener, but College 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.

[20] 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.

Ethan 03 (Laotian orbit) began "positive control" (airborne direction) of C-130 flareship flights and A-26 Invader night interdiction missions along the Ho Chi Minh Trail in Laos on 19 April 1968.

[4] The task force was scaled back in July 1968, to four EC-121Ds and the Rivet Top testbed aircraft to allow for the basing of another College Eye detachment at Itazuke AB, Japan.

[22] 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,[N 1] 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 two additional Soviet transponders, the SRO-1 and SOD-57.

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 17-man crews were advised in the air of the nature of the rescue mission and their role, providing MiG warning and directing USAF F-4 Phantom CAP intercepts.

[29] In October 1971, North Vietnamese MiGs, operating from forward bases opened after the end of Rolling Thunder, began a campaign to intercept Boeing B-52 Stratofortress missions over southern Laos.

When Operation Linebacker began on 10 May 1972, Disco was one of two principal GCI radars used by US forces,[N 2] although it continued to be handicapped by poor radio communications.

In addition, its slow-turning radar limited its value as a controller of fighters during MiG engagements, while the size of USAF raids during Linebacker nearly saturated its capabilities.

At the end of the month, Disco was also integrated into Teaball control center, a highly classified system established to collate all signal intelligence on North Vietnamese air activity gathered by all sources, including nonmilitary.

[14] During the Vietnam War, some 40 EC-121s were modified from USN WV-2 and WV-3 early warning Constellations for use with ground sensors to detect enemy troop movements along the Ho Chi Minh Trail and 25 were deployed to Korat RTAFB as a part of Operation Igloo White.

The wing activated in December 1970 and the 554th RS relocated to Nakhon Phanom RTAFB to fly QU-22 sensor monitors nicknamed "Baby Bats".

In April 1969, North Korean Air Force MiG-21 fighter-interceptors shot down an EC-121 in international airspace off the country's east coast, killing all of the crew of 31 on board.