Radar System Engineering

SEC. 13.20] RESOLUTION AND CONTRAST 549
be taken as 180. On a range sweep of length R nautical miles, the
number of radar pulse lengths resolvable in principle is 12.2R/T, where 7
is the pulse length in microseconds. Thus, on a centered PPI, the
fundamental pulse-length resolution and the spot-size resolution are
equal when 12.2R/T = 180, or R = 15 when r = 1. Accordingly, on
a set with a l-psec pulse, a 15-mile centered PPI will have the full inherent
range resolution; a 100-mile PPI will reduce it approximately sevenfold,
and so on. This reduction in resolution can be overcome by the use of
expanded displays, but often this measure restricts the field of view.
Fortunately for the indicator designer, however, this is not the
entire story. For one thing the operator cannot, except on the fastest
sweeps, realize the fundamental range resolution without optical aids
even if the display were to make it available; indeed, on a 5-in. tube
the spot-size resolution for a 100-mile sweep is nearly as good as can be
comfortabley used, alt bough on a 12-in. tube it is three or four times
worse. Second, range resolution alone is of limited usefulness if it is
accompanied by poor angular resolution; here the fundamental limitation
is usually not the indicator but the azimuthal beamwidth. The angular
width of the CRT spot on any PPI is given in radians by R/180r, where R
is the radius of the tube and r is the distance from the range origin to
the spot in question. This width will be equal to the beamwidth e,
measured in degrees, when 57R/180r = e or R/r = 30. Thus, even
for a 1° beam, the display resolution exceeds the fundamental resolution
for all points farther from the origin than one-third of a tube radius.
The cathode-ray tube is usually not the limiting factor in over-all angukzr
resolution, although in certain cases of accurate range measurements or
of observations on groups of aircraft or ships at long range, better range
resolution on long-range displays would be useful. Frequently, however,
the need for high disperm”on requires expanded displays quite apart from
question of resolution.
The above discussion shows that in general the cathode-ray tube
imposes a severe restriction in range resolution on any but the fastest
sweeps. It is therefore apparent that unless fast sweeps are to be used
there is no point in going to extreme bandwidths in the receiver. In
most cases the receiver bandwidth need not be so great as to impair
seriously the signal-to-noise discrimination. It should be borne in
mind, however, that with too small a bandwidth, a signal far in excess of
the limit level has a much greater duration after limiting than has a
weak signal.
Contrast.—The contrast of the display depends upon the characteristics
of the screen and upon the way in which it is excited. Unfortunate
y present tubes of the persistent type have serious shortcomings in
contrast. The screen material has a natural color much like that of the