The role of P-3C Orion patrol planes in the detection earlier this week of an unidentified submarine in Japanese waters has drawn attention to their sub-detection capabilities.

Military experts said this might prompt the government to reconsider its plan to reduce the number of P-3Cs in its inventory in the upcoming revision of the nation's defense policy outline.

With its current 80 P-3Cs, along with destroyers and SH-60 patrol helicopters, the Maritime Self-Defense Force has top-class capability to detect submarines.

The government is considering reducing the number of P-3Cs to 72 under the new National Defense Program Outline to be adopted next month. Japan had about 100 during the Cold War era, mainly to guard against Soviet submarines.

After the Cabinet decided in December to introduce a costly missile defense system, the Ground, Maritime and Air Self-Defense Forces were instructed to come up with plans to free up money by reducing frontline equipment, including destroyers, tanks and fighters.

The P-3Cs are loaded with advance computer systems, sonobuoys and radar. They patrol waters off Hokkaido, the Sea of Japan and the East China Sea at least once a day from bases in Aomori, Kanagawa, Kagoshima and Okinawa prefectures.

Each P-3C patrol lasts about 10 hours and covers a flight distance of over 3,000 km. Each plane carries 11 crew members, two of whom analyze the data picked up by the sonobuoys.

The MSDF searches for submarines by various methods, including dropping sonobuoys into the sea to catch sounds and compare them with the "sound prints" of submarines. Experts say different types of subs can be distinguished because they make distinct noises.

It can also utilize radar or sonar to locate and track submarines.

Each P-3C carries about 84 sonobuoys and is also equipped with detectors for magnetism and infrared rays.