Backgrounder: Air defense identification zone

The airforce says it has conducted its first air patrol since the East China Sea Air Defense Identification Zone was announced. Reconnaissance and early-warning aircraft and warplanes were deployed. But what is an Air Defense Identification Zone, and how does it work?

Air Defence Identification Zone is a zone that can extend in some cases up to 300 miles beyond the territorial sea. It's established by some countries off their coasts for security reasons. When entering the zone, all aircraft are required to identify themselves, report flight plans, and inform ground control of their exact position.

Military expert Yin Zhuo said, "Since the 1950s, some countries have demarcated Air Defense Identification Zones on high seas or international waters. It's also called identification belt."

Air Defense Identification Zone is an early-warning air defense concept. It has been implemented in more than 20 countries, including the United States, Canada, Australia, South Korea and Japan.

For example, after Japan's surrender in 1945, the US demarcated an identification zone off Japan's coasts, but it was under the control of the US military in Japan. It was only until 1969 that the US transferred the management of the zone to Japan.

After that, Japan expanded the zone westward twice, once in 1972, the other in 2010. Japan follows a warning sequence for unidentified aircraft: radar detection, emergency calls, fighter emergency launch, requiring forced landing, and bomb warning. Once its own aircraft, land, or vessels are attacked, a defense war will be launched. However, territorial disputes still exist, as the zone is not recognised by Japan's neighbors, Russia and China.