Tokyo put spy satellites into operation in the early 2000s after its erratic neighbour North Korea fired a mid-range ballistic missile over the Japanese mainland and into the western Pacific in 1998.
The Japan Aerospace Exploration Agency (JAXA) and Mitsubishi Heavy Industries delayed Thursday's planned launch of the H-2A rocket from Tanegashima Space Center in southwestern Japan, due to the possibility of lightning during lift-off.
The launch at 10:21 am (local time) today was successful, JAXA and Mitsubishi Heavy said, adding: "We confirmed the rocket launched normally."
The backup satellite will supplement the two radar satellites, a government official said earlier.