China’s announcement Saturday of its establishment of an air defense identification zone in the East China Sea that includes the Japan-controlled Senkaku islets has worsened already tense bilateral ties.

From 10 a.m. Saturday, China said any aircraft entering the ADIZ must obey its rules or face “defensive emergency measures.”

Japan does not recognize the new Chinese claim, and government officials have lodged a protest with Beijing and demanded an end to the zone.

What exactly is an air defense identification zone, and how legitimate is a country’s claim? Here are some details about ADIZs:

What is an ADIZ?

Established beyond national airspace, the zone is a defensive perimeter to monitor incursions by suspicious aircraft.

The zone gives the military time to determine whether a foreign aircraft approaching territorial airspace is friend or foe.

Zones are set up according to a country’s domestic laws. Many countries have established ADIZs, including Japan and the United States. China had none until Saturday.

Japan’s ADIZ was originally set up by the Occupation forces after World War II. Based on its contours, Japan established its own zone in 1969, according to the Defense Ministry.

Because ADIZs are based on domestic law, Japan has no legal grounds for preventing China from establishing an ADIZ over the Senkakus, which it claims, as does Taiwan.

Do foreign aircraft need permission to pass through a country’s ADIZ?

Under international law, aircraft have the right to fly freely in airspace over the high seas, where ADIZs are usually set up, thus no permission is required.

However, because the ADIZ is set up to prevent unknown aircraft from entering its territorial airspace, Japan requests flight plans for all foreign aircraft entering the zone if the intention is to continue flying toward Japanese territory.

What happens if an aircraft enters Japan’s ADIZ without a flight plan?

Fighter jets are scrambled if an unknown aircraft enters the zone without advance notice and is approaching Japanese territory. The Defense Ministry declined to give further details for reasons of national security.

The Air Self-Defense Force scrambled jets 80 times between July and September in response to Chinese aircraft, according to the ministry.

So what’s wrong with China setting up such a zone? Is it any different from Japan’s ADIZ?

One thing is that China’s ADIZ includes Japanese territorial airspace over the Senkaku Islands and that it theoretically forces Japan to obey its rule when flying in the zone.

In a statement issued Sunday, Foreign Minister Fumio Kishida accused China of designating the airspace over the uninhabited islets as if it is China’s territorial airspace.

Beijing is also violating the right to fly freely over the high seas, based on the guidelines it released Saturday, Katsunobu Kato, deputy chief Cabinet secretary, said Monday.

While Japan only “requests” that foreign aircraft submit flight plans in advance if entering its ADIZ to approach its national airspace, China is demanding that all aircraft “must” report flight plans to the Chinese Foreign Ministry or the civil aviation administration of China, a ministry official said in Beijing. The Chinese demand affects even aircraft passing through the zone, heading away from Chinese territory, the official said.

Foreign aircraft must also maintain radio communications and transmit transponder signals and registration ID clearly indicating its nationality, according to the state-run Xinhua News Agency.

Aircraft that fail to obey face “defensive emergency measures” by China’s armed forces, it reported, details of which are unknown.

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