The JCG is wary of such vessels, believing they aim to strengthen Beijing’s arguments to the international community in order to gain effective control of the islands, reports The Japan News.

An AIS is a radio device that automatically transmits and receives information on a vessel’s position, course, speed and so on. Under the International Convention for the Safety of Life at Sea, all passenger ships and other vessels engaged in international voyages are required to carry this equipment.

Radio transmissions are received by surrounding ships and satellites, and are used to facilitate safe navigation in heavily trafficked sea areas.

AIS information is publicly available online and can easily be checked on the “Marine Traffic” website, which provides information on vessel operations around the world.

Since March, according to sources related to the JCG, China Coast Guard vessels have been operating AIS devices in the territorial waters and contiguous zone — a band of sea extending 22 kilometers beyond territorial waters — around the Senkaku Islands, The Japan News reported.

CCG vessels usually sail in fleets of four. One such vessel, with the vessel number “1302,” entered the contiguous zone on May 16.

It appeared to be moving around Uotsuri Island and Kuba Island and entered Japan’s territorial waters on May 20 and 21.