The problem of North Vietnamese infiltration down the Ho Chi Minh Trail was proving extremely difficult to counter by conventional means.  The lack of timely intelligence was one of the major problems.  Special Forces and indigenous Hmong guerrillas collected intelligence but often sustained heavy casualties.  However, late in 1967 the USAF started to implement a plan to place hidden sensors along the Trail to send information back to orbiting aircraft, which in turn relayed the information back to the Infiltration Surveillance Center at Nakhon Phanom code named Eagle White.  As part of Project Igloo White, large numbers of acoustic and seismic sensors were dropped in Laos to intersect the Trail and to detect movement.  The extensive barrier of sensors led to the project being nicknamed McNamara’s Fence.  The sensors were dropped by a variety of aircraft including A-1s, O-2s and helicopters but the task became the primary mission of VO-67, a special US Navy squadron, which arrived at Nakhon Phanom in November 1967.  The signals from the sensors were picked up by high-flying aircraft which then relayed the information back to the Infiltration Surveillance Center from where orders were issued to strike aircraft to hit potential targets.  The relay aircraft were operated by the 553rd RW and consisted initially of EC-121 Bat Cat aircraft later supplemented by QU-22s.

Twelve SP-2E Neptunes were modified to OP-2E standard for VO-67 by removing the Magnetic Anomaly Detector tail cone and installing an AN/ALE-29 chaff dispenser, a rearward looking camera and an AN/APQ-131 radar.  The sensors had to be dropped at low level by the large aircraft and the attrition rate was expected to be high, despite the fact that the Neptunes were to be escorted by flights of F-4s.  In fact the attrition rate was so high that the Squadron was withdrawn in June 1968, after only eight months of operations.  After the Neptunes were withdrawn, Phantoms flew most of the sensor delivery missions.