Excalibur worked during the boost phase and aimed at the booster itself. This meant the X-ray hardening techniques developed for warheads did not protect them. While many of the other SDI weapons had simple countermeasures based on the weapon's required "dwell time", like spinning the booster and polishing it mirror-bright, Excalibur's zero dwell time rendered these ineffective. Thus the primary way to defeat an Excalibur weapon is to use the atmosphere to block the progress of the beams. This can be accomplished using a missile that burns out while still in the atmosphere, thereby denying Excalibur the tracking system information needed for targeting.