▁ 1 9 6 4 ▁the ▁Air ▁Force ▁issued ▁a ▁contract ▁to ▁TR W ▁to ▁consider ▁the ▁problem ▁of ▁IC B M ▁surv iv ability ▁under ▁the ▁name ▁" G old en ▁Ar row ", ▁looking ▁for ▁ways ▁to ▁make ▁their ▁own ▁miss iles ▁as ▁inv ul ner able ▁as ▁the ▁Navy ' s . ▁Ide ally , ▁the ▁US ▁IC B M ▁fleet ▁should ▁be ▁able ▁to ▁ride ▁out ▁an ▁attack ▁and ▁then ▁launch ▁a ▁response , ▁which ▁would ▁require ▁as ▁many ▁as ▁ 1 , 0 0 0 ▁war head s , ▁one ▁for ▁each ▁Soviet ▁IC B M ▁sil o . ▁Not ▁only ▁did ▁they ▁have ▁to ▁surv ive , ▁but ▁they ▁had ▁to ▁have ▁the ▁ability ▁to ▁be ▁quickly ▁ret arget ed ▁so ▁the ▁surv iving ▁US ▁miss iles ▁could ▁be ▁aim ed ▁only ▁at ▁the ▁remaining ▁Soviet ▁miss iles , ▁not ▁empty ▁sil os . ▁ ▁One ▁early ▁conclusion ▁was ▁that ▁it ▁was ▁possible ▁to ▁build ▁miss ile ▁sil os ▁that ▁would ▁surv ive ▁any ▁conce iv able ▁war head ▁expl oding ▁at ▁a ▁distance ▁of ▁. ▁It ▁was ▁assumed ▁that ▁Soviet ▁war head s ▁during ▁the ▁late ▁ 1 9 7 0 s ▁would ▁have ▁accuracy ▁better ▁than ▁one ▁mile , ▁so ▁the ▁question ▁became ▁how ▁to ▁ensure ▁the ▁war head s ▁would ▁not ▁approach ▁within ▁that ▁distance . ▁Among ▁the ▁many ▁solutions ▁studied , ▁one ▁quickly ▁rose ▁to ▁the ▁top , ▁the ▁idea ▁of ▁valley ▁bas ing . ▁This ▁would ▁rely ▁on ▁the ▁physical ▁bar rier