Radar System Engineering

312 ANTENNAS, SCANNERS, AND STABILIZATION [SEC. 919
may not be line-of-sight stabilized. If this airplane banks, the relative
bearing of ground targets will be, as we shall see, falsely indicated. For
example, let us consider the case of an airplane banking for a right turn,
the relative bearing of the beam being straight ahead. It is clear that
since the fan is perpendicular to the floor of the airplane it is not at the
moment vertical, and that therefore the targets on the straight line on
the ground now being illuminated by the beam are not at zero bearing.
Since such targets are nevertheless displayed as if at zero bearing, the
display is distorted; these targets are shifted by an amount equal to the
altitude of the airplane multiplied by the tangent of the bank angle.
Other targets are less seriously disturbed, those at 90° and 270° being
correctly indicated on a banking airplane.
Two methods have been considered for correcting the airborne decktilt
error. One method is so to stabilize the antenna that the plane of the
fan beam remains indeed vertical. This can be accomplished by stablebase
stabilization or by a three-axis mount. The other method is by
data stabilization, and several circuits have been devised for removing
the distortion if supplied with voltages depending on the angles involved.
None of these circuits has been incorporated in systems now in wide use,
although one radar designed for production incorporates a feature that
accurately corrects the display at the one target designated as the aiming
point.
9.19. Installation of Airborne Scanners.-In discussing antenna
installations we will treat first the airborne case. Here, one of the pressing
problems is the choice of location. The radar must have the unobstructed
“vision” required for its operation, and its antenna should be so
installed that it may be housed in an aerodynamically acceptable radome.
A very common requirement is circular vision of the ground well out
toward the horizon, as in Fig. 9%. The only good location is then below
the fuselage. If rearward vision is not required, an antenna with circular
scan can be mounted just behind and below the nose of the airplane, where
the radome can be completely faircd in.
Scanning of only a forward sector is possible if the antenna is mounted
in the nose. Such a location is impossible in single-engine airplanes, and
external housing becomes necessary. Aerodynamically, perhaps the
best site in this category is at a wing tip, although a blister faired into a
leading edge has been widely used. For maintenance reasons easy
replacement of the set is desirable; this has been accomplished in one
instance by packaging the greater part of the set in a “bomb” suspended
under one wing. In the aircraft-interception radar of nightfighters the
scanner is located for vision in all generally forward directions. Such
scanners can be mounted in any of the locations that are good for sector
scanners. Bomber-borne scanners for protective fire control are located