On March 1st 2017 an Atlas 5 rocket was launched from the U.S. west Coast (Vandenberg Air Force Base) carrying a classified payload identified only as NROL 79. It was not really classified but rules are rules because the payload belonged to NRO (National Reconnaissance Office). It didn’t take long to gather enough clues (from unclassified sources) to determine what was going on. The classified payload was another pair of Ocean Surveillance satellites called Intruder 12 (or 12A and 12B). Intruder is the third generation ELINT (Electronic Intelligence) satellite that uses two or three identical satellites in a low (1000-1,100 kilometers high) orbit to just listen for radio transmissions from ships at sea or aircraft anywhere. Then using triangulation the location of a ship can be determined and tracked. The first generation of these (called Parcae) went up in 1976 and used three satellites travelling in fixed orbits to locate ships. The second generation replaced the first in 1990 and was called Intruder. The last one (Intruder 4) went up in 1996. When Intruder 5 went up in 2001 it was obviously a third generation because instead of three satellites there were only two. Intruder 5, and subsequent “Binary Intruders” apparently used much improved sensors and electronics because it became clear that this ELINT ocean surveillance was getting better because of, well, the NRO will never go into too much detail about performance but it was obviously becoming harder to have ships disappear (from surveillance) into the high seas and then show up somewhere else much later.
All of these ELINT satellites have been sent up using one of the Atlas launchers with the last five (since Intruder 8 in 2007) using the Atlas 5 This is a 334 ton rocket that can (depending on model) put 29 tons into low orbit and 13 tons into GTO orbit. Intruder 12 consisted of two satellites weighing about three tons each. One potential problem here is that Atlas 5 is dependent on Russian RD-180 rocket motors. That problem is being solved by the American made, and much cheaper, SpaceX launchers.
Since 1978 the United States has also been using Lacrosse satellites, equipped with radar for ocean surveillance. In combination with the Intruders the two systems make it nearly impossible for even small ships to try and hide at sea.