T H E A R M Y A I R F O R C E S I N W O R L D W A R I 1
which could generate considerable power a t very short wave lengths.
No one nation enjoyed a monopoly of these discoveries, but the most
rapid progress in devising practical military applications was registered
by the British.
The basic problem of providing early warning to defending aircraft
had been solved in Britain by 1939. A chain of radar stations along the
coast provided reasonably effective information on planes approach-
ing from the continent."' Thus, when the Germans began large-scale
daylight raids against England in August 1940, the RAF was able to
conserve its limited fighter strength by keeping the planes on the
ground until an attack actually impended and thus to achieve a more
efficient concentration of force.lo3 When the Germans switched to
indiscriminate night attacks, British defenses for a time proved less
effective,lo4 for advance warning of the enemy's approach did little
good unless he could be found in the dark. Fortunately, the RAF,
magnificently served by such advisers as Robert Watson-Watt,
rapidly went beyond the original conception of radar as merely a
device for early warning. Under the compulsion of war and the special
lash of the night blitz, the British perfected and put into use a whole
new series of electronic devices which guided every step in the proc-
ess of intercepting enemy planes.
T h e original radar equipment installed along the coasts of Britain
had been well suited to handling daytime raids, for it had a long range-
up to 2 0 0 miles. But these sets did not permit the fine discrimination
between targets which was needed to guide night fighters to the
immediate vicinity of enemy A new type of radar known
as GCI (ground controlled interception) transformed the British sys-
tem from one of crude warning into a machine for dependable control
of interception. GCI equipment made it feasible to plot, virtually
instantaneously, the positions of a large number of planes. Moreover,
it displayed the information on an ingenious PPI (plan position indi-
cator) scope, which was a cathode ray tube on the face of which a map
of the region had been drawn. Aircraft within range of the GCI set
were shown on the map as spots of light.lo6The British also developed
equipment known as IFF (identification friend or foe) which indicated
to a ground observer which of the targets in the PPI scope were
friendly. The identification was accomplished by installing in friendly
planes an electronic device-a simple short-wave radio transmitter-
which sent coded signals when interrogated by a ground station.lo7
288