Radar System Engineering

~70 THE GATHERING AND PRESENTA TION OF RADAR DATA [SEC.6.5
Linear Deformation.—A deformed display of considerable utility,
especially in the vertical plane, can be formed by “ stretching” a polar
plot in one rectangular dimension as though it were on a sheet of rubber.
On such a display, the cartesian coordinates parallel and perpendicular
to the axis of stretch retain their original meaning and their linearity, but
they have different scale factors. Straight lines remain straight but,
except for those parallel to the coordinate axes, their directions are
changed. Circles of equal range appear as ellipses with their major axes
in the direction of the stretch.
In the horizontal plane this geometry, called the stretched PPI’ (Fig.
6.6), is useful principally in connection
with the control from a
remote point of aircraft approaching
a landing, or ships navigating
a channel. The stretching, done
in a direction perpendicular to the
desired course, aids greatly in detecting
slight deviations therefrom.
This technique finds its greatest
utility in the vertical plane
where vertical stretching is used
to enhance the accuracy of aircraft
height determination (see discussion
of RHI, Sec. 6.6).
Rectangular Presentation of
Range and A ngle.—A plane surface
is often represented in a deformed
manner by combining
range and angle in cartesian rather
than in polar coordinates. This
is accomplished by moving a range sweep laterally across the tube face in
synchronism with the antenna motion so that the origin is stretched out
into a line.
In range and azimuth these rectangular displays are of two different
sorts:
1. Displays in which no attempt is made to minimize the deformation,
either because it is unimportant in the particular circumstances,
or because certain advantages can be gained by neglecting it. Any
desired range interval and azimuthal sector may be covered,
although in practice more than 180° is rarely used. The display
is always normalized to make optimum use of the tube face.
1This has sometimesbeen called, erroneously,an “expanded azimuth” display.