In 1940, less than a year after blitzkrieging its way through Poland and the rest of Western Europe, the Nazi army set its sights on the UK and commenced a vicious bombing campaign targeting the nation's major cities. Luckily, the British Royal Air Force saw this threat coming and cleverly deployed a series of radio stations to counter the Luftwaffe's legions in what would become one of the world's first practical radar early warning systems.
Code-named Chain Home, this ring of coastal radar receivers served as the backbone of the world's first integrated air defense. Throughout WWII it provided the RAF with nearly a half hour of lead time in which to scramble squadrons of fighters to intercept threats. It was so powerful, in fact, that it could detect German formations before they even reached the English Channel from their bases in occupied France.
The original iteration of Chain Home consisted of 21 AMES Type 1 radar stations running from from Southampton to the Tyne, each comprised of a fixed antenna array strung across a series of 360-foot-tall steel pylons. By beaming radio waves out across the channel and measuring the lag of return pings between various stations, the system could identify and track incoming squadrons within five degrees of accuracy. And with the advent of IFF technology two years prior, British radar operators could easily differentiate their own troops from the enemy's.
This system did have its limitations. While aircraft at 2,000 foot elevations could be spotted from 235 miles out, elevations above or below that had severely limited range—roughly 25 miles for anything under 1,000 feet and just 50 miles for planes at 5,000 feet. This left the UK desperately vulnerable to low-flying squadrons, which is why the system was quickly augmented with a secondary line of radar stations—dubbed Chain Home Low—specifically tuned to catch low-altitude insurgents.
These two arrays were tethered to one another by telephone wires, as well as linked to RAF's central Fighter Command. They provided invaluable and unsurpassed tactical data to the RAF Observer Corp, allowing them 20 precious minutes to best position their defenses ahead of the German raids. [IEEE - Wiki - Britannica - History Learning Site]