Radar System Engineering

SEC. 73] AIDS TO PLOTTING AND CONTROL 219
conveniently provided by ruling each individual set on a thin transparent
sheet. The sheets are then stacked and arranged to be edge-lighted
individual] y; thus only the chosen scale appears. Such multiple scales
are used on the indicator of the V-beam height-finding display described
in Sec. 6.12, to permit the use of range sweeps of various lengths.
The arrangement of Fig. 7.5 permits the use of only one scale, but
affords, as shown, the considerable advantage that a wax pencil can be
used for plotting the position of each radar echo of interest, with a minimum
of difficulty from parallax. As the pencil point touches the edgelighted
plotting surface, it glows brightly over the small area of contact,
and small corrections in the position of the point can be made before a
mark is actually applied to the screen. Although the operator must look
through the screen itself at its
10to20% reflection
virtual image below, the real and
virtual images can be distinguished,
providing the scales are
not too complex, by their positions
t
1
and usually by a difference in in-
A
I
tensity or appearance.
Display Projection.—An out- 1
1 t
I
standing problem in the use of
B I
radar for control is that of trans- A 1
—
ferring radar plots from the face of
*.
a small indicator tube to a large ‘A
\\
board where plotting can be done,
\\\
Nonreflect!ng surface \
other information entered, and a
\\\
display visible to many people in Wrtual image
a large room presented. Tfis FIG. 7.5.—Method of plotting with the aid of
optical superposition.
problem has been approached in
several different ways. On*that of telling grid coordinates of a target
to plotters who enter the target position on a large board—will be described
in Sees. 7”5 and 7.6. Another, which seems obvious but presents considerable
tethnical difficulty, is that of direct optical projection of an
enlarged real image of the display on an appropriate screen.
The light intensity from a persistent cathode-ray-tube screen of the
usual type is too low for satisfactory enlarged projection. Several alternative
methods of accomplishing the same result have been used or
proposed, including:
1. Splitting the cascade screen (Sec. 13.2) into two parts, the blue
component being in the tube and the persistent component on the
projection screen. Although this scheme is better than direct
projection of a cascade screen, results are far from satisfactory.