EC-121s were used extensively in Southeast Asia between 16 April 1965, and 1 June 1974, particularly in support of
Operation Rolling Thunder and
Operation Linebacker/
Linebacker II, providing radar early warning and limited airborne control of USAF fighter forces engaging MiG interceptors. 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. The crews, however, were experienced in tracking
Soviet aircraft over Cuba and had developed a technique whereby an EC-121 flying at to above water could bounce a signal from its bottom-mounted APS-95 Search radar off the surface of the water and detect aircraft at medium altitudes out to . Operating in pairs, one Big Eye EC-121 flew a
race track pattern approximately offshore (Alpha orbit), with the orbit's center at . The second one flew a track at (Bravo orbit) farther from the coast, acting as a spare for the Alpha EC-121. This provided a practical detection range of , 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 range of the Big Eye's APS-45 Height Finder radar, so that they were unable to provide this data to USAF strike forces. Furthermore, technical shortcomings in the EC-121D's systems precluded either controlling a fighter intercept or identifying a specific flight under attack. The missions from Tan Son Nhut AB began 21 April 1965, using callsigns Ethan Alpha and Ethan Bravo, which became standard. After refueling at
Danang Air Base, Ethan Alpha made a wave-top approach to its orbit station, where it remained for five hours. Because of the threat of MiG interception, EC-121s were protected by a
Lockheed F-104 Starfighter MiGCAP. If, for any reason, the MiGCAP could not rendezvous, the EC-121s cancelled their mission. 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. On 10 July 1965, in its first airborne-controlled interception, an EC-121 provided warning to a pair of USAF
F-4C fighters, resulting in the shooting down of two
MiG-17s. Seven of 26 EC-121s deployed from Otis AFB and arrived at Korat, on the 19th. 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 inside Chinese territory. A US board of inquiry recommended that College Eye also had to monitor the "
no-fly zone" inside the North Vietnamese border with China, to provide alerts to US aircraft nearing the buffer zone and to report border crossing violations by US aircraft. 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. 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.
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 two 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 four voice communications intercept stations able to monitor all communications between the MiGs and their GCI controllers. Despite this advantage, Rivet Top experienced two problems, reducing its effectiveness. 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, two 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 the Gulf of Tonkin, with Frog 02 as a backup. 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. As Frog 01 began its climb to the higher Bravo orbit, it experienced a ruptured oil line forcing shutdown of one engine. As planned, Frog 02 became the primary aircraft when Frog 01 made an emergency landing at Danang. The new equipment failed to function properly aboard Frog 02. The ground receivers at the command post in Danang failed to receive data and the
APX-83 IFF processors would not display aboard the aircraft, despite repeated repairs. Their own radar monitors experienced excessive electronic noise and the jamming of North Vietnamese radars by nearby
EKA-3B Skywarriors hindered efforts of the radar technicians to correct the problems. While unable to provide vectoring information to the F-4s escorting the mission, Frog 02 remained on station and relied on its Rivet Top voice intercept capability to provide supplementary data.
Disco 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. On 20 November, a MiG-21 launched air-to-air missiles at a B-52 that evaded by dropping flares. As a result, Warning Stars of Det. 1 returned to Korat RTAFB to provide radar support by flying the Laotian orbit again, using callsign
Disco. Improvements made in the systems since 1968 enabled the operators to distinguish MiG types and a color code system for them entered the air operations vernacular: "Red Bandits" (Mig-17s); "White Bandits" (
MiG-19s); "Blue Bandits" (MiG-21s), and "Black bandits" (MiGs low on fuel). On 6 July 1972, as the result of seven F-4 Phantoms shot down in a two-week period, a second Disco track was initiated. Flown near the former Alpha orbit over the Gulf of Tonkin, its purpose was to gain better low-altitude coverage in the Hanoi area. 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. Disco was used as a conduit through which warnings and control vectors were given, but the delay in Teaball acquiring the information and relaying it through Disco (often using an unreliable radio relay
KC-135A Combat Lightning aircraft operating under callsign Luzon) cancelled out its value for use in "real time", and the fact that its existence was kept from US aircrews damaged its credibility. Teaball received direct communications capability, but experienced transmission failures with frustrating frequency. Disco remained the primary backup controller, but its usefulness remained limited because it directly controlled only MiGCAP missions and could only provide its information to strike, chaff, and escort forces via the "
Guard" frequency. On 15 August 1973, Disco EC-121s flew their final combat mission and on 1 June 1974, Det.1 was permanently withdrawn from Southeast Asia. Between 1965 and 1973, the EC-121s flew 13,921 combat missions, more than 98,000 accident-free flying hours, assisted in the shoot-down of 25 MiGs, and supported the rescue of 80 downed flyers. No Big Eye, College Eye, or Disco aircraft were lost.
Batcat 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 resulting
EC-121R configuration was nicknamed the
Batcat. Two Batcats were lost during the war, with the loss of 22 crewmen, one in a takeoff accident during a thunderstorm in April 1969, the other in September 1969, in a landing accident which also killed four Thai civilians. Batcat EC-121s were camouflaged in standard tree-color Southeast Asia scheme, while the College Eye/Disco early warning aircraft were not. BatCat missions were 18 hours in length, with 8 hours on station at one of 11 color-coded orbits used during their five-year history, three of which were over South Vietnam, six over Laos, one over Cambodia, and one over the Gulf of Tonkin. EC-121Rs were operated by the
553d and
554th Reconnaissance Squadrons of the 553d Reconnaissance Wing, between October 1967 and December 1970, with about 20 Batcats on hand at any time. The wing activated in December 1970 and the 554th RS relocated to Nakhon Phanom RTAFB to fly
QU-22 sensor monitors nicknamed "Baby Bats". Initially with 11 aircraft, the 553rd RS continued operations for another year, gradually returning aircraft and crews to the USA. The final Batcat mission was in December 1971. The last remaining administrative and support personnel returned to Otis AFB in January 1972. ==Variants==