In December 2017 Russia put a third Lotos S spy satellite into orbit using a Soyuz rocket launched from the Plesetsk Cosmodrome in the far north. Lotos S is a six ton electronic surveillance satellite which an elliptical orbit that brings the satellite as low as 200 kilometers to earth. The first Lotos went up in 2009 while the second went up in late 2014. The first Lotos was classified as “experimental” and had several problems that limited its effectiveness. The second and third ones apparently worked as intended and the third one was designated Losot S1 indicating substantial upgrades. Lotos is part of the Russian effort to catch up with the United States in space-based ELINT (electronic reconnaissance, also called SIGINT for signals intelligence).
Meanwhile in June 2016 the U.S. launched its seventh (since 1995) Mentor ELINT satellite which Lotos is competing with. The six ton Mentor deploys the largest antenna array (over 110 meters in diameter) ever used in a satellite. Details of how Mentor satellites operate is highly classified, but they are known to pick up a large number of electronic signals from ships, aircraft and ground stations, as well as other satellites. This data undergoes some processing on the Mentor satellite, is then encrypted and transmitted to American ground stations for further analysis. The American satellite ELINT program began in 1985 with a Space Shuttle putting two Magnum satellites (each 2.5 tons) into orbit. The seven Mentor satellite appear to have maintained the same general design but have grown a bit in heft (from 5.2 to six tons) and apparently even more in capabilities.
Little is said about ELINT satellites because, more than photo satellites, these electronic listening birds can potentially pick up anything (radar, radio, whatever) that is broadcast from anywhere. This is an alarming possibility for producers and users of military electronics. Not knowing exactly what those enemy ELINT satellites are picking up is very disturbing. China, for example, has launched Cyber War type hacking attacks on American companies involved with collecting and analyzing Mentor satellite data. If the Chinese have reached the Mentor database, it has made Chinese electronics much less likely to encounter unpleasant surprises in wartime. In the meantime China has not built anything comparable to Lotos or Mentor but has obtained similar capabilities in the last decade by adding ELINT capabilities to their dozen or more radar satellites which tend to operate in groups.