Armored Troop Carriers (ATC), often called Tangos from the phonetic alphabet for T, were LCM-6 landing craft modified for riverine patrol missions.
They were used by the Mobile Riverine Force (MRF) of the United States Army and Navy in the Vietnam War. They were also used by Republic of Vietnam Navy (RVNN) and Khmer National Navy.
The chief difference between the Monitor and the ATC could be seen in the well deck area. Monitors had a rounded bow as opposed to a drop-down ramp, making them slightly longer than the ATC (60.5 feet vs 56 feet). They also mounted additional weaponry
—a 40mm gun turret forward and an 81mm mortar amidships—and carried four additional sailors to help man those weapons and operate the boat.
The 81mm mortar was the only indirect-fire weapon fielded by River Flotilla 1 and, with the assistance of an artillery observer, could hit targets up to 4,000 yards (3,700 m) away.