HARP meaning in Military and Defence ?

Project HARP, short for High Altitude Research Project, was a joint venture of the United States Department of Defense and Canada's Department of National Defence created with the goal of studying ballistics of re-entry vehicles and collecting upper atmospheric data for research. Unlike conventional space launching methods that rely on rockets, HARP instead used very large guns to fire projectiles into the atmosphere at extremely high speeds.

A 16-inch HARP gun operated by the U.S. Army's Ballistic Research Laboratory (now called the U.S. Army Research Laboratory) at Yuma Proving Ground currently holds the world record for the highest altitude (180 km) a gun-fired projectile has achieved.