During the Vietnam War, Lockheed P-3 Orions operated primarily in the maritime interdiction role, patrolling the 900-mile coastline of South Vietnam as part of Operation Market Time. Anti-infiltration missions were flown day and night to interdict the supply of arms and supplies to the Viet Cong. In addition, Orions also provided anti-submarine screening for the carrier battle groups operating off the coast of both North and South Vietnam.
But a little known mission of the P-3 Orion during the Vietnam War was that as a "pathfinder". Pathfinder P-3s acted as long range ferry escorts for USAF fighter jets making their way across the Pacific to the Southeast Asian theater. Ferrying aircraft to the war zone was faster than shipping them by sealift and the P-3s would provide navigation and communications relay to the fighter aircraft. During air-refueling, the P-3s loitered in the area in case of an emergency that would result in a pilot ejecting or ditching in the open ocean. The first pathfinder mission was flown on 20 January 1966.
Source: The Age of Orion: The Lockheed P-3 Story by David Reade. Schiffer Publishing, 1998, p23.