 In Patriot's early days, the system was used exclusively as an anti-aircraft weapon, with no capability against ballistic missiles.This was remedied during the late 1980s when Patriot received its first major system overhaul with the introduction of the Patriot Advanced Capability missile and concurrent system upgrades.Patriot Advanced Capability (PAC-1), known today as the PAC-1 upgrade, was a software-only upgrade.