DEW line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A line of radar stations near the 70th parallel across the North American continent, maintained by the United States and Canada and intended to give advance warning of approaching enemy aircraft and missiles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military The Distant Early Warning Line, a series of radar stations, located north of the Arctic Circle and extending from Alaska to Baffin Island, operated by American and Canadian authorities from the late 1950s to the early 1990s to help protect North America by detecting incoming hostile missiles and aircraft.