Distant Early Warning (DEW) refers to the line of radar bases in the Arctic of North America. With cooperation from the United States's Canadian neighbors, the DEW Line was constructed in the 1950s. The DEW Line was to be the first indication of air or nuclear missile attack by the Soviet Union over the North Pole. The Distant Early Warning Line stretches from the far West of the US state of Alaska to the farthest East of Greenland. South of the DEW line is the Mid-Canada line. Roughly along the United States-Canada border is the Pinetree System. All these warning systems are connected with the combined North American Air Defense Command (NORAD) headquarters at Ent Air Force Base in Colorado.

It is the Distant Early Warning Line that detects Santa Claus every Christmas eve, and relays that information to NORAD. Also, when your Army/Air Force commanding officer threatens to assign you to some God-forsaken base deep in the Alaskan tundra... this is where he wants to send you.

Distant Early Warning has become a cultural expression in all of America. It, of course, describes a warning of a grave and serious threat. So when you hear that phrase, you know it comes from the chain of radar bases in the far North.

PS- note the DEW Line references in the Rush lyrics.