The United States is sending four Patriot batteries, armed with the PAC-3 anti-missile missiles, to Okinawa. These batteries will defend U.S. bases there against North Korean attack. PAC-3 missiles cost $3.2 million each and are the result of two decades of development. First used during the 1991 Gulf War, the current (PAC-3) version shot down two Iraqi missiles in 2003. During the 2003 operation, 22 Patriot missiles were fired. Two of these took down two coalition aircraft. Electronic and software problems caused the IFF (Identify Friend or Foe) systems to fail. This is less of a problem with incoming missiles, as they are rarely friendly. Chinese war plans are believed to include ballistic missile attacks as well, and one of the targets is said to be Okinawa.
It's uncertain if the Patriot electronics and software have been tweaked to the point where they can shoot down longer range missiles like the ones North Korea, or China, might fire at Okinawa. As a general rule, the longer the range of a ballistic missile, the faster it goes when moving downward towards its target. Longer range missiles approach the ground at over two kilometers per second. The Patriot missiles can reach up about 20 kilometers. This means the Patriot missile has to be fired quickly, and accurately, because it has only seconds to knock down the target. When defending against missiles, the Patriot system is put on automatic. If something resembling a ballistic missile comes within range, a PAC-3 is automatically fired on an intercept course. Often two are launched, to ensure a hit. Development of the Patriot continues, mainly in the area of decreasing response time, and rigging the Patriot system to work with other radar systems (like space based early warning networks that can spot longer range missiles long before the Patriot radar can.)
The U.S. already has PAC-3 equipped Patriot batteries in South Korea and Japan. Taiwan is buying the PAC-3. Japan participates in Patriot missile research, and has a license to build PAC-3 missiles.