The Echo system was a form of stealth technology developed in the Solarian League. The system tied in a spacecraft's passive sensors with its own active sensors and communications array so that when a wave packet struck the hull and started back, the ship's own active sensors sent out a perfectly matched packet that was shifted a hundred eighty degrees out of phase, thus cloaking the vessel. Charles tried to sell it to the People's Republic of Haven in the early 1910s PD. (HHA5.2: AAoW)