It incorporates active radar with an inertial reference unit and micro-computer system, which makes the missile less dependent upon the fire-control system of the aircraft.

它把主动雷达和一个被动参考单元以及微电脑系统结合在一起，可使得导弹更少的依靠飞机上的发射控制系统。

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When the control computer has decided the best time to launch, it tells a launcher to fire a missile or missiles - typically two might be fired at a target in rapid succession because one alone does not guarantee a hit.

BBC: Fact file: Patriot missile defence