Installation of a new missile defense radar began at a US military base in the Japanese city of Kyotango on the coast of the Sea of Japan on Tuesday.
Such early warning systems are deployed in Japan to warn first of all about missile launches from North Korea and may be used to monitor launches from Russia and China.
The radar will begin operating this year. It will detect launches and distinguish real from false warheads at a range of more than 4,000 km.
Another American mobile radar of such a type has been already operating in the prefecture of Aomori in northern Japan since 2006.