When first proposed, the plan was to place enough Excaliburs in orbit so at least one would be over the Soviet Union at all times. But it was soon noted that this allowed the Excalibur platforms to be directly attacked; in this situation, the Excalibur would either have to allow itself to absorb the attack or sacrifice itself to shoot down the attacker. In either case, the Excalibur platform would likely be destroyed, allowing a subsequent and larger attack to occur unhindered.
