The Defense Agency believes that keeping two Aegis warships equipped with Standard Missile 3 interceptor missile systems constantly deployed is sufficient to shield Japan from ballistic missiles, according to agency sources.

Under the missile defense program for the period to 2010, the interceptor missile systems aboard the current fleet of four Aegis warships -- the Kongou, Choukai, Myoukou and Kirishima -- are being upgraded in stages.

All four will eventually be equipped with SM-3 interceptors and Block IV Aegis radar systems. The warships are currently equipped with SM-2 interceptors.

The upgrade will provide a significant boost to the Maritime Self-Defense Force's ability to intercept ballistic missiles, both in terms of analyzing the course of an approaching missile and in destroying it, an MSDF official said.

The improved version of the Aegis system is designed to monitor and track incoming missiles and automatically fire SM-3 interceptor missiles, which will be guided by the Aegis system toward their targets.

The Aegis system with the SM-3 interceptor has an improved guidance system. In addition, the SM-3 has a far longer range than the SM-2.

The Defense Agency believes the new interceptor system needs to have the capability to shoot down an enemy ballistic missile with a 1,300-km range.

"The SM-3 missile, with a longer range (than the SM-2), would be able to intercept a missile at a high-altitude location outside the Earth's atmosphere, so it would enable Japan to defend wider areas," a Defense Agency source said, suggesting the new system would put the entire Japanese archipelago under its umbrella.

The agency believes all four Aegis warships need to be equipped with the new system because the ships must dock on a rotating basis for such reasons as routine maintenance or additional crew training.