up localized deviations in the Earth's magnetic field normally used to detect submerged submarines. The Black Crow system was slaved into the targeting computers of the AC-130A/E/H, enabling the detection of the unshielded ignition coils of North Vietnamese trucks hidden under dense jungle foliage along the Ho Chi Minh Trail. It could also detect hand-held transmitter signals of air controllers on the ground to identify and locate targets. The PGM-38/U enhanced 25 mm high-explosive incendiary round was created to expand the AC-130U gunships' mission in standoff range and survivability for its 25 mm GAU-12/U gun. This round is a combination of the existing PGU-25 HEI and a M758 fuze designated as FMU-151/B to meet the MIL-STD-1316. The FMU-151 has an improved arming delay with multisensitive range. Operational history Vietnam War The AC-130 gunship first arrived in South Vietnam on 21 September 1967 under the Gunship II program and began combat operations over Laos and South Vietnam that year. In June 1968, AC-130s were deployed to Tan Son Nhut AB near Saigon for support against the Tet Offensive. By 30 October 1968, enough AC-130 Gunship IIs arrived to form a squadron, the 16th Special Operations Squadron of the 8th Tactical Fighter Wing, at Ubon Royal Thai Air Force Base, Thailand. At this time, the C-130A gunship was designated the AC-130A. On 18 August 1968, an AC-130 gunship flying an armed reconnaissance mission in Vietnam's III Corps was diverted to support the Katum Special Forces Camp. The ground commander quickly assessed the accurate fire and capabilities of this weapons system and called for fire on his own perimeter when the Viet Cong attempted to bridge the wire on the west side of his position. By December 1968, most AC-130s flew under F-4 Phantom II escort (to protect the gunship against heavy and concentrated antiaircraft fire) from the 497th Tactical Fighter Squadron, normally three Phantoms per gunship. On 24 May 1969, the first Spectre gunship was lost to enemy fire. In late 1969, under code name "Surprise Package", 56-0490 arrived with solid-state, laser-illuminated, low light-level TV with a companion YAG laser designator, an improved forward-looking infrared (FLIR) sensor, video recording for TV and FLIR, an inertial navigation system, and a prototype digital fire-control computer. The remaining AC-130s were refitted with upgraded similar equipment in the summer of 1970, and then redeployed to Ubon RTAFB. On 25 October 1971, the first "Cadillac" gunship, the