Technically, the satellites are in violation of a 1969 Japanese law, which mandated Japan only use space for non-military purposes. To get around this these satellites are technically non-military and are not controlled by the military. Japan had long refrained from launching military satellites but this changed when North Korea fired a ballistic missile over Japan in 1998. Japan promptly set out to get eight surveillance satellites in orbit by 2006, in order to keep an eye on North Korean nuclear weapons and ballistic missile efforts. This proved impossible to do. While two Japanese satellites were launched in early 2003, another two were destroyed during late 2003, when the rocket malfunctioned.
Japan has long relied on commercial photo satellites and whatever they could get from the Americans. But for high resolution shots, on demand, of North Korea, and electronic eavesdropping from space, they need their own spy satellites. It is believed that the Japanese spy satellites are also being used to watch military developments in China and Russia.
The Japanese program has cost over three billion dollars. Much of this was spent to develop rockets large and reliable enough for satellite work. The latest launch was the 16th consecutive successful use of the locally designed and manufactured H-2A rocket. These rockets operate from a launch complex in southern Japan.
The Japanese optical satellites weigh about a ton, while the radar one weighs about a third more. The United States provided a lot of technical assistance on the design and construction of the satellites. Japan built its own rockets to launch them. Like most spy satellite users, Japan does not report on how effective they are. It is known that Japan could get more detailed photos from commercial satellites. But those are not controlled by the Japanese government.