Radar System Engineering

SEC.7.3] AIDS TO PLOTTING AND CONTROL 223
Video Mapping—None of the methods so far described permits the
superposition of an “electronic” map on the dk,play. By this term is
meant the addition to the radar signals of signals which, by appropriate
intensity-modulation of the indicator, reproduce on the display itself a
map, a grid, or any pattern that may be desired. The position of radar
echo signals can be compared with this electronic map with complete
freedom from parallax, and, in addition, any distortions or imperfections
of the display affect the grid and the signals equally and thus produce no
reading error. A very satisfactory method of accomplishing this has
been given the name of “video mapping. ”
The signals required to produce the desired modulation are derived
from the device illustrated in Fig. 7.8. The desired map, in the form of
opaque lines on a transparent
Photomultipl)ertube
background or vice versa, is placed
7
immediately in front of an auxiliary
PPI of the highest precision
obtainable. This PPI rotates in
synchronism with the radar antenna
and executes its range sweep
m the proper time relation with
the radar transmitter, but operate?
at a constant intensity, receiving
no video signals. The cathoderay
Viewing aperture 1
tube used has a short-persist-
Transparency to~
be
ence blue screen; the intensity is
reproduced
adjusted to a medium or low level. FIG. 7.8.—Video mapping transmitter.
The aperture in front of the phototube is
As the moving spot of the auxiliary
smallenoughsothat the parallaxbetweenthe
PPI moves behind the map to be
reproduced, a photocell placed two
spot on the CRT screenandthe markson the
plotting surfaceis small.
or three feet from the map receives a signal whenever the rays from the
spot to the photocell undergo a change in absorption. Because of the
rapid motion of the spot in its range sweep, these signals are in the videofrequency
range. The fast screen avoids any appreciable “tailing” of
signals.
The signals from the photocell are amplified and mixed with the radar
echo signals with a polarity such that on the final display the lines of the
map are brighter than the background. In the absence of limiting,
gradations of light and shadow can be presented. Figure 7.9 is a repro.
duction obtained on a second PPI tube of a photographic negative used
as a subject for video-mapping transmission. Figure 6.8 \vas also produced
by video mapping, the final display being a B-scope.
This method has the very great advantage that the map is correctly
correlated with the radar display regardless of the degree of off-centering,