as to not violate territorial borders. From there, it will use its electronic equipment to monitor sea and air traffic, radio and radar frequencies and also try to intercept and decrypt coded radio or phone communications. This is mostly done via passive means such as radio receivers or passive sonar. Sometimes however, active measures such as radar or sonar may also be used to detect the movement of aircraft, missiles, ships or other vehicles or troops. However, this risks revealing the ship's purpose. As it is located much closer to the surveilled area than a fixed installation (given a close by shoreline), the monitoring is usually much more efficient and in some respects better than even that of spy satellites. Tracking vessels also have some of the capabilities of spy ships, and as they are controlled by their national governments, they are also intermittently used for similar purposes, such as tracking enemy missile tests. Soviet AGI trawlers As the United States Navy began deploying ballistic missile submarines in 1960, the Soviet Union attempted to obtain more information about the capabilities of the UGM-27 Polaris missile and the locations of the submarines capable of launching them. While the Soviet Navy requested more sophisticated ships, they were allocated trawlers (called tra-ou-lery) from the fishing fleet equipped with more sophisticated sensors and communication equipment. Very capable crews were assigned to these trawlers of unremarkable appearance. They were assigned to patrol stations off United States naval bases to photograph and report arrival and departure of United States warships and auxiliaries. Other trawlers of similar appearance would patrol weapons firing ranges used by the United States Navy to observe practice firings of modern weapons and record the acoustic and/or electromagnetic signature of the sonar, search radar, fire-control radar, guidance, and/or command electronics of each weapons system. The United States Navy officially designated these trawlers as Auxiliary, General Intelligence or AGI, and they were informally known as "tattletales". In 1972, as the U.S. and U.K. partners started operating radar station Cobra Mist, it garnered attention from many Soviet spy trawlers. A year later, the radar station was shut down due to interference. The source of the interference was never confirmed and some theories still hold Soviet countermeasures responsible. An AGI might be assigned to a single patrol station for as long as six months. These ships were not fast enough to keep up with most warships,