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. To improve survivability, the French Air Force began dispersing Mirage IVs to pre-prepared rough strips during the 1960s; while the use of hardened bunkers had been assessed, it was found to be financially impractical. By the 1970s, it had become clear that vulnerability of the Mirage IV to air defences, even while flying at low altitudes, had made the delivery of gravity bombs such as the AN-11 or AN-22 impractical. Thus, it was decided to pass a greater share of the deterrent role onto land-based missiles and submarine-launched ballistic missile alternatives; as a result, a single wing of Mirage IVs was stood down in 1976, partially due to fleet-wide attrition losses. In 1973, it was reported that a force of 40 Mirage IVs would continue to perform as a part of France's nuclear deterrent until the 1980s, and that steady improvements were to be undertaken. In 1975, all Mirage IVs were progressively painted in a new green/grey camouflage scheme. In 1979, in response to the decreasing effectiveness of free-fall bombs used by both its strategic and tactical nuclear forces, development of the ASMP stand-off missile was initiated; the ASMP would possess a range of up to 400 km (250 mi) and was alternative armed with either a single 150 or 300 kiloton nuclear warhead. Various test launches of dummy and later live ASMPs were performed using the Mirage IV as the launch platform between 1981 and 1983. In July 1984, a contract was formally issued for the upgrade of a total of 18 Mirage IVAs to carry the ASMP missile in the place of traditional bombs; these aircraft were redesignated Mirage IVP (Penetration). The conversion of Mirage IVAs to IVPs involved a large number of modifications and re-workings of the aircraft. A deep centerline pylon was added, which could accommodate either a single ASMP missile or a CT52 reconnaissance pod. The main