World War II began in September 1939, when Germany invaded—and quickly
defeated—Poland. By the summer of 1940, less than a year later, nearly all
of Western Europe was under Nazi control. The British had been driven from
the shores of Dunkirk and watched France crumble from across the English
Channel. Now, Hitler turned his attention to Great Britain and began
planning an invasion. To do so Hitler needed to gain control of the
English Channel. The German Navy couldn’t do it, so Hitler hoped to
destroy the British Navy, and any coastal resistance, with air
superiority. Hitler had great faith in his undefeated Luftwaffe, and with
good reason. Comprised of more than 2400 fighters and bombers, it would
face Britain’s 900 fighters.
Chain Home radar masts range: 185 miles
With these dismal odds, the British needed an
early warning system, one that would disclose the German air position and
take away their element of surprise. This system would also allow British
pilots to conserve precious time in-flight and remove the necessity of
mounting constant air patrols over the entire English East Coast. It was
through the extraordinary foresight of radio researcher Sir Robert
Watson-Watt that the British had in place in 1940 an entire network of
long-range early-warning radar stations whose reports were tightly
integrated by Fighter Command. This was the Chain Home radar system, the
first radar-based air defense system, which was ready just in time for the
Battle of Britain.
Like all pulsed radars, Chain Home sent a burst of radio energy at a
target, then measured the time it took for the energy to reflect back to
its receiver. The radar calculated the range to the target by multiplying
the time between sending the pulse and its return by the speed of light
and then dividing by two. What made Chain Home unusual was how it measured
the bearing of (or direction to) the target. More conventional radars
measure direction by sending and receiving the radio energy in narrow
beams, like a flashlight, and scanning those beams around in space until a
target is illuminated. The direction to the target is determined by the
pointing angle of the antenna when the received target reflection is
strongest.
A radio goniometer similar to those used in Chain Home
In the late 1930s the technology to generate
high-power pulsed radio energy at wavelengths short enough to make narrow
beams did not exist. Instead, Chain Home relied on antennas that
illuminated a huge area, like a floodlight. These antennas did not move or
scan at all. Rather, Chain Home radar operators chose a target (“blip”) on
their screen and turned the knob of a special coil-like instrument to null
out or minimize the blip. Then they could read the direction to this
target from a scale around the knob. This device (called a radio
goniometer) electronically steered the nulls from a pair of simple fixed
receiving antennas. This idea was invented during the first decade of
radio technology development and was used by Watson-Watt to track radio
static generated by thunderstorms years before the war.
Although it required intense human interaction both to direction-find on
each target and to filter the reports from many radar stations into a
coherent air picture, the Chain Home system was highly effective,
especially during those critical months in the summer and fall of 1940.
Still, the Battle of Britain was a costly conflict. By the end of the
battle, the Luftwaffe had lost some 1700 aircraft and nearly all their
crew. Thanks to the early warning and raid locations provided to Fighter
Command by the Chain Home radar, and the rapid production of replacement
fighters, the British lost just over 900 aircraft and saved about half
their pilots—and most importantly, forced the Germans to abandon their
plans to invade England.