The principle was devised by the French engineer Lorin in 1913. Currently, ramjets are used operationally only to power cruise missiles, like ASMP cruise missiles, around Mach 3....
Hedge-HoppingCruise Missile Terrain-Following Navigation. Flight testing of the Apache cruise missile (MATRA Defence source). Principle of terrain following over hilly ground. In this mode, the missiles fly at very low altitude (100 m and less) completely independently in order to avoid detection by enemy defences....