Radar System Engineering

SEC. 9.17] STABILIZATION OF THE BEAM 309
which the seamer has two degrees of freedom, correspondhg to the train
(azimuth) and elevation axes. In shipborne radar it has become evident
that for certain applications a third axis should be added to ensure a true
and undistorted display of the radar signals. The nature of these distortions
will be clear from the following considerations. When a ship
and its two-axis antenna mount are rolled to the right and the elevation
axis momentarily set athwartship
so that the antenna is looking forward,
z
if the elevation angle of the M
beam is varied the beam thereupon
describes on the sky the arc of a
great circle which is not vertical
but which is inclined toward the
right. All airplanes detected on
this arc will be indicated as dead
ahead, since the train axis is oriented
to this position. This indication
is of course false, and the
two-axis mount therefore imperfect
in principle.
FIG. 9.36.—Deck-tilt error. The arc HB,
The computer described below
which is the bearing of the beam, is greater
than the arc HA, which is the bearing as indicated
if deck-tilt error is not compensated
provides correction for distortion
MBA represents a meridian on a sphere.
of this type. Such correction is,
however, inapplicable if the radiation is a vertically fanned beam or a
pencil beam which must oscillate rapidly in a vertical plane.
The inadequacy of two-axis ship antenna mounts for certain applications
can be corrected by the addition of a third degree of freedom, corresponding
to the cross-level axis. This axis is supported by the train
axis and is set parallel to the deck; the elevation axis is mounted on it at a
right angle. By means of this new degree of freedom a servomechanism
holds the elevation axis level in respect to the horizon. The axes of this
three-axis mount are commonly designated as the “train,” “cross-level,”
and “level” axes. A mount of this type affords freedom from the distortion
of the dkplay occurring when a two-axis mount is used. It has
the added virtue that it can follow moving targets above 45°, which with
a two-axis mount would require impracticably great servo rates. Weight
considerations have prevented the development of three-axis scanners
for airborne use.
We have mentioned a distortion in the display of an elevated target
observed from a rocking ship. A closely related distortion in bearing is
present even for surface targets: the deck-tilt error. Its origin is made
clear by an example, Fig. 9.36, which represents a ship heading north
toward the reader, listing to starboard. The circle represents a large