At the onset of Rolling Thunder, the N-Vietnamese had an advantage in that their radar coverage could detect most US strike aircraft flying at 5.000 ft or above 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 had serious gaps in coverage.

To increase coverage the Seventh Air Force (7 AF) requested airborne radar support and the Air Defense Command (redesignated Aerospace Defense Command in 1968) was directed to set up the Big Eye Task Force.
 5 EC-121Ds and 100 support personnel of the 552nd AEWCW at McClellan AFB were deployed to Tainan Air Station, Taiwan, with 4 of the EC-121s sent on to a forward operating location at Tan Son Nhut Air Base, near Saigon, Republic of Vietnam.

The EC-121s were designed for detection of aircraft flying over water and the 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 50 to 300 ft 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 150 miles!
Operating in pairs, one Big Eye EC-121 flew a 50-mile race track pattern approximately 30 miles offshore (Alpha orbit), with the orbit's center at 19°25′N 107°25′E. The second one flew a track at 10,000 ft (Bravo orbit) farther from the coast, acting as a spare for the Alpha EC-121.

This provided a practical detection range of 100 miles, 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-mile 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 5 hours.
Because of the threat of MiG interception, EC-121s were protected by a Lockheed F-104 StarfighterMiGCAP.
If for any reason the MiGCAP could not rendezvous, the EC-121s cancelled their mission.
Airco 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 2 MiG-17s.

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.
At the onset of Rolling Thunder, the N-Vietnamese had an advantage in that their radar coverage could detect most US strike aircraft flying at 5.000 ft or above 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 had serious gaps in coverage.
To increase coverage the Seventh Air Force (7 AF) requested airborne radar support and the Air Defense Command (redesignated Aerospace Defense Command in 1968) was directed to set up the Big Eye Task Force.  5 EC-121Ds and 100 support personnel of the 552nd AEWCW at McClellan AFB were deployed to Tainan Air Station, Taiwan, with 4 of the EC-121s sent on to a forward operating location at Tan Son Nhut Air Base, near Saigon, Republic of Vietnam.
The EC-121s were designed for detection of aircraft flying over water and the 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 50 to 300 ft 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 150 miles! Operating in pairs, one Big Eye EC-121 flew a 50-mile race track pattern approximately 30 miles offshore (Alpha orbit), with the orbit's center at 19°25′N 107°25′E. The second one flew a track at 10,000 ft (Bravo orbit) farther from the coast, acting as a spare for the Alpha EC-121.
This provided a practical detection range of 100 miles, 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-mile 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 5 hours. Because of the threat of MiG interception, EC-121s were protected by a Lockheed F-104 StarfighterMiGCAP. If for any reason the MiGCAP could not rendezvous, the EC-121s cancelled their mission. Airco 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 2 MiG-17s.
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.
Lockheed EC-121 Warning Star