There’s apparently some misunderstanding out there that the Japanese government warned that it would shoot down drones in the air Japanese defense identification zone around the Senkaku Islands. Since the new Chinese ADIZ raises the chances of some incident blowing up into a more serious clash, the following memo may be of some interest to anyone who chances upon this blog.
First, the Japanese authorities were referring to territorial air space, not just any "air defense [identification] zone(s)." In fact, the western part of Yonakuni Island, an undisputed (and well-inhabited) part of Okinawa had not been included in any Japanese ADIZ until the latter was expanded during the very recent DPJ administration. Most of the Japanese and Chinese ADIZs cover non-territorial air space, where aircraft are in principle free to traverse. On the other hand, all international air flight requires reporting to the proper international authorities. I do not know the details of this requirement, but sovereign states have access to this flight information, which means that all normal civilian flights in the new ADIZ (or any non-territorial airspace in any ADIZs for that matter) will continue unmolested by the Japanese or Chinese military. Of course any unreported aircraft--which likely will include most Japanese and Chinese aircraft in the area--will be subjected to proper scrutiny, i.e. demand for self-identification, warnings and the like, and ultimately attacked in self-defense in the case where there is an clear and present danger--a decision that may be made with regard to the non-territorial airspace within the ADIZ. It is unlikely that a JSDAF fighter will shoot down a Chinese drone/aircraft in the ADIZ or even in Senkaku Islands airspace just because the latter is recalcitrant or unable to identify itself. A Chinese PLA aircraft may be more gung-ho, though, and may decide to attack a JSDAF or Japanese Coast Guard aircraft that happens to fly over the new Chinese ADIZ, territorial air space in particular. (The Japanese authorities do not, to the best of my knowledge, have drones in operation in that neighborhood.) Far more likely, though, is that a Chinese pilot might close in on a JSDAF or JCG aircraft as a form of warning (or just to put a scare into the Japanese) and cause an accident. (Remember the Hainan Island Incident.)
My second caveat is that the Japanese authorities did not explicitly state that they would use weapons. It was in response to a hypothetical that the Chinese authorities directly and somewhat belligerently addressed the matter. The relevant comments from Japanese defense minister at the Sept. 10 press conference, when he addressed the matter in response to questions from the media, follow:
Based on the current Self Defense Forces Act, my understanding is that necessary measures can be taken in order to make aircraft invading Japan's territorial airspace withdraw. However, in the theoretical case in which an unmanned aircraft invades Japan's territorial airspace and it is left alone, the situation may threaten the sovereignty of Japan and the lives and assets of Japanese people. Under such circumstances, do you think shooting the aircraft down can be an option as a last resort?
In any case, we will take necessary actions in order to defend Japan's territorial land, sea and airspace as well as protecting lives and assets of the people."
Concerning countermeasures against invasion of Japan's territorial airspace, not only by an unmanned aircraft but in any form, I presume that necessary countermeasures exclude the use of weapons that can harm the invaders, unless they are used for emergency evacuation or self-defense based on the conventional interpretation. When you said earlier, "We will take necessary actions," did you mean that the use of weapons that are harmful to the invaders is one of the options, too?
I will refrain from answering about each specific countermeasure, since doing so may reveal Japan's strategies. In any case, we will take necessary measures."