US-A satellites (Russian: Управляемый Спутник Активный) is a family of Soviet military satellites launched between 1970 and 1988 that feature active radar. They were used to locate the largest NATO ships to guide transhorizon missiles launched from Soviet ships, aircraft or submarines in wartime. To provide the large amount of energy needed to operate the radar, the designers chose to install a small nuclear reactor. At the end of its operational life, the reactor was to be separated and placed on a higher orbit to delay the date of re-entry into the atmosphere by several hundred years. But several of these satellites failed and their reactors made a premature atmospheric re-entry.

US-A satellites are placed in circular orbits at an altitude of about 255 km with an inclination of 65°. Once their mission is complete, an engine propels them into a higher orbit. Once this orbit is reached, it takes five or six centuries before friction causes them to decay. This time is sufficient to neutralize the nuclear material on board. US-As are equipped with an automated ejection system that ensures the placement of the nuclear reactor in high orbit, even if the satellite's systems malfunction. A US-A satellite has a mass of 3800kg, a diameter of 1.3m and a length of 10m. The radar operates at 8.2 GHz.

Topaz is the first orbital nuclear reactor to have been developed and launched. It was tested on US-A satellite prototypes from 1971. It has equipped all operational US-A satellites since then. The fuel it uses is uranium 235. It is equipped with a lithium hydride radiation shield.

During flight tests, Topaz-1 provided 5kW for 344 days. During ground tests, Topaz-2 provided 6kW during a year and a half. In 1991, the United States bought two copies of Topaz-2 from its manufacturer, TsKB Mach.