Radar System Engineering

SEC. 6.14] PRECISION TRACKING OF A SINGLE TARGET 203
satisfactory way by the AIA equipment and its redesigned successor, the
AN/APS-6. This 3-cm equipment has an 18-in. reflector which is
housed in a wing nacelle in the same fashion as that of the AN/APS-3.
As this reflector is rotated at 1200 rpm about an axis passing through its
feed and parallel to the line of flight of the aircraft, the paraboloid is
slowly tilted from a position in which its axis coincides with the axis of
rotation to a position in which there is a 60° angle between these axes.
Because of the pattern thus traced out in space by the beam (Fig. 6. 12f)),
this is called a spiral scan; a complete scan takes 4 sec. The display
used for search is the “double dot” indicator described in Sec. 6.7
(Fig. 612cz).
In the last stages of a horning operation, the assumption is made that
the pilot can safely concentrate all his attention on his target. Turning
a switch then changes both the scan and the indication. The tilt of the
antenna with respect to the axis of its rotation is fixed at 3°, resulting in a
conical scan. Two voltages, respectively linear with the azimuth error
and with the elevation error in pointing the axis of the conical scan
toward the target, are applied to a spot error indicator of the type
described in Sec. 6.8. The pilot completes the interception by keeping
the spot centered on his scope, and closing the range until the wings of
the spot have grown to the desired size.
6.14. Precision Tracking of a Single Target. -During the war, considerable
effort wx expended in the design of radar for highly precise
position-finding on one target at a time. This tracking was ordinarily
intended to permit the control of fire against such a target, but as the
war drew to a close, it was also used for detailed control of the maneuvers
of the target by radio instructions from a controller at a ground station.
This was done to permit blind bombing by fight-bombers too small to
carry a radar set and its operator, or to enable an airc#t not equipped
with radar to make a blind landing approach on instructions from a controller
at a ground radar. The latter use has, and will probably continue
to have, a considerable peacetime importance.
Lobe-sw”tching and Pip-matching-The first radar set intended for
precision tracking of a single target was the SCR-268, a laboratory prototype
of which was formally demonstrated to the Secretary of War in
May 1937. The problem of accurately tracking a moving target with an
antenna that can be elevated and trained is similar to the problem of
homing with the help of. fixed antennas on a movable vehicle, and the
same techniques are useful. Lobe switching, which has been described
in the last Section, is employed by the SCR-268 for angle-error determination;
in fact, the SC R-268 was the earliest production radar to use
this technique. The general appearance of the set is shown in Fig. 6.34,
1By E. C. Pollard and L. N. Ridenour.