Spy ships I

Spy ships in the modern sense have been used at least since the early Cold War, and are in use by all major powers. Their uses, in addition to listening in on communications and spy on enemy fleet movements, were to monitor nuclear tests and missile launches (especially of potential ICBMs).

One of the most important functions for both Cold War spy ship fleets, especially in the 1960s, was the gathering of submarine 'signatures' - the patterns of noise that could often identify the specific type of submarine and were thus valuable in anti-submarine warfare. During that era, the USA fielded about 80 vessels, usually classified as 'environmental research' craft, while the Soviets had around 60 ships, often converted trawlers or hydrographic research ships.