Initially, the basic attack flight profile was "high-high-high" at a speed of Mach 1.85, engaging targets up to a maximum radius of 3,500 km (2,175 mi). In the late 1960s, when the threat of surface-to-air missile defences made high-altitude flight too hazardous, the Mirage IVA was modified for low-altitude penetration. Flying low, the maximum attack speed was reduced to 1,100 km/h (680 mph) and the combat radius was also decreased. By 1963, the majority of missions involving the Mirage IV were being planned as low-level flights. By 1964, Mirage IVAs were conducting training penetration runs at an altitude of 200 ft, without the assistance of terrain-following radar, which subjected pilots to considerable workload and those on board to high levels of turbulence.