A tachyon detection grid is a network of ships linked by tachyon beams that can reveal cloaked vessels crossing between them.
During the Klingon Civil War, the Romulans covertly supplied the Duras family. Captain Picard assembled a fleet of Federation ships along the border and linked their sensors with tachyon beams, forming a net that a cloaked ship could not pass through unseen.
Data commanded the USS Sutherland during the operation. When the Romulans tried to confuse the grid, he disregarded orders to rejoin the line and used a torpedo detonation to expose the hidden convoy, prompting the Romulans to withdraw.