What did the Constellations do in Vietnam's Operation Igloo White?

Igloo White was a covert US electronic warfare operation that ran from late January 1968 to February 1973, during the Vietnam War. It combined electronic sensors, computers and relay aircraft to automate the collection of information on enemy logistical traffic in southern Laos and then guide strike aircraft to their targets. Over the course of the operation, some 20,000 sensors were deployed in Laos, including seismic, acoustic and gasoline-engine detectors.

The sensors' transmitters sent data to an EC-121R of the Air Force's 553rd Reconnaissance Wing, based at Korat, Thailand, which circled over the area. The aircraft relayed the information over a 2200–2300 MHz radio link to Nakhon Phanom, home of Task Force Alpha, where it was entered, cross-checked and stored together with data from other sources. The Navy supplied another piece of the system with the VO-67 unit, which flew heavily modified OP-2E Neptunes. The EC-121R belonged to the EC-121 Warning Star family, the military branch of the Lockheed Constellation.

The EC-121Rs and their crews proved too vulnerable, and in 1969 and 1970 they were partly replaced by QU-22Bs, designed to fly unmanned. Mechanical problems with these aircraft kept them from ever flying an operational mission with no one on board, and in December 1971 C-130B aircraft took over in turn. In 1972 dollars, an ADSID sensor cost $619, an ACOUSID $1,452, and an engine detector $2,997. In bad weather or at night, strikes could be guided by radar using the MSQ-77 Combat Skyspot system or by LORAN radio navigation.