Radar System Engineering

310 ANTENNAS, SCANNERS, AA’D STABILIZATION [SEC. 917
sphere with the antenna imagined at its center. All lettered points are
on the sphere: Z is the zenith, M is the prolongation of the mast, and H
is the ship’s heading. If the antenna is not stabilized, its pencil beam will
at some instant be directed toward A below the horizon. The indicator
will then show an angle measured by the arc HA. If the beam is stabilized
by a two-axis mount, it will at that instant be at the point B where
the great circle AM intersects the horizon; since HB exceeds HA the
indicator will report the position of the beam falsely. If the beam is
stabilized by a three-axis mount, it will be directed toward C; the indication
is again false since HC is less than HA. Thus either of these types
of stabilization results in a distortion of the bearing of targets on the
horizon.
A recent successful device for the stabilization of shipborne antennas
includes computers and an associated gyroscope, and is located wi ‘~in the
ship. The principal components of the stable vertical gyroscope are a
gimbal system so supported that the outer and inner gimbal rings tilt
respectively about the roll and pitch axes of the ship, a second gimbal
system supported by and within the first, and the gyroscope wheel supported
in the second gimbal system. The wheel axis seeks the vertical
because of a pendulous magnet carried by a third gimbal system, and
compensation is provided in the gyroscope for the effect of ship’s turns or
changes of speed.
The second gimbal system is held parallel to the plane of the gyro
wheel, that is, horizontal, by means of a servomechanism; and from this
gimbal system two push-rods extend upward and provide roll and pitch
inputs to the mechanical computer located immediately over the gyro
unit. The force to move these rods is provided by the servomechanism
rather than by the gyro itself. In addition to roll and pitch, tim desired
angular position of the beam in space is fed to the co. {.puter as another
mechanical input provided by a servomechanism. The cc nputer is
cinematically equivalent to a small replica of the antenna mount which
is to be stabilized. The outputs arc the deck-tilt correction and elevation
angle for a two-axis mount, or deck-tilt correction and level and crosslevel
angles for a three-axis mount. They are mechanical but are converted
to voltages by means of synchros, and control the servomechanisms
which actuate the motors for the three axes of the mount.
310re than one computer located on the gyro unit can be actuated
simultaneously, making possible the simultaneous stabilization of more
than one antenna. No computer is needed in stabilizing stable-base
mounts, since roll and pitch data are furnished directly from the stable
vertical.
Returning for the moment to generalities, t\vo alternative methods
are at hand bv which the deck-tilt correction ran be effected, One is to