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.