Radar System Engineering

SEC. 69] IiARLY AIRCRAFT-WARNING RADAR 175
suited for use with a spiral scan. Arangesweep moves frornthe center
of the tube in a direction corresponding to the projection of the antenna
beam onaplane perpendicular totheaxis ofscanning (Fig. 6l2c). The
echoes therefore appear at radial distances corresponding to their range.
If the target is on the symmetry axis of the scan, it is equally illuminated
at all “spin” angles and the echo appears as a full circle. If it is far off
axis it is illuminated only through a narrow spin angle (target B) and
the arc on the display is short. Target C indicates an intermediate
case. The display is surprisingly easy to interpret after a short period of
observation.
6.8. Error Indicators.—A cathode-ray tube may be used as a radar
indicator in a quite different. way from those so far described; that is, as a
meter on which to display various forms of intelligence. One common
use of an error indicator is to indicate accuracy of pointing in connection
with a conical scan. The signal intensity is combined electrically with
the scanning information to provide voltages proportional to (or at least
increasing with) the pointing error in
both azimuth and elevation. These
voltages are used to deflect the spot.
Elevation
A departure of the spot from the tube
+
error
center indicates the direction and, to /“i
d=J
a qualitative degree at least, tbe mag-
1
nitude of the pointing error. The
radar signal is sometimes used to intensify
the spot in order to distinguish ~irnuth
o
error
between perfect pointing and no tar-
FIG. 6.13.—Spot. error indicator with
get. Range can be indicated by caus- wingsto indicatetmgetrange.
ing the spot to grow “wings” whose
length is some rough inverse measure of the range. In this form (Fig.
6.13) the indication gives to a surprising degree the illusion of an actual
aircraft which apparently grows larger as it approaches.
EXAMPLES OF THE MAJOR OPERATIONAL REQUIREMENTS
6.9. Early Aircraft-warning Radar. l—The British Home Chain.—The
first radar to have actual combat use was the British CH (Chain, Home)
equipment. Work was begun in 1936 toward setting up five stations,
about 25 miles apart, to protect London and the Thames estuary, and by
the time war broke out in 1939 this Chain had been extended to cover the
greater part of the south and east coasts of England and Scotland.
During the next few years the Chain was extended and its equipment and
performance were improved. In spite of the introduction of more modern
and efficient equipment, the Chain provided the basis of the British
1By B. V. Bowdenand L. N. Ridenour.