Radar System Engineering

482 THE RECEI VIA’G SYSTEM-INDICATORS [SEC.132
saturate too quickly. The number of overlapping pulses (which is
often large) is given for a PM by the expression NS/ur at the center of
the arc, where N is the PRF, S is the spot size, u is the angular velocity
of the scan, and r is the distance from the origin of the display to the echo
spot. Thus for a 30-sec scan and a PRF of 400 (see Sec. 15.10) there
are about ten overlapping pulses at “ranges” corresponding to the
radius of a 7-in. tube, since S/r = ~. The property of “ building up”
in intensity because of excitation by successive members of such a group
of pulses is possessed to an adequate degree by all cascade screens, providing
the excitation is not at too high a level.
At faster scanning rates it becomes possible to provide screens having
carry-over from one scan to the next. Within the requirements set by
clarity in a changing picture, the longest obtainable persistence should
usually be used in order to average over the largest possible number of
pulses. However, the permissible decay time is so short in the case of
very rapid scans that the operator and not the screen has the longer
memory. The persistence should then be determined entirely by the
requirements of freedom from flicker on the one hand and freedom from
blurring due to motion on the other.’ If a set has several indicators
involving different scale factors, their persistence should theoretically
be graded, fast screens being used on the expanded displays where the
picture changes rapidly and slower ones on those displays covering large
areas. Fortunately, it is usually the latter on which the signal-to-noise
discernibility is of most importance , since the expanded displays are
usually confined to near-by regi ens.
Providing the scanning is not too slow (for example, if it is approximately
one scan per second) certain types of cascade screens will, if
initially unexcited, display more than twice as much intensity after two
scans as after one, and so on, e~”enthough the intensity may have decayed
manyfold in the time between scans. This property of “supernormal”
buildup was at one time believed to be very desirable from the signaldiscernibility
standpoint, on the hypothesis that it would give the
repeating signal an advantage over random noise. However, the property
is most exaggerated on an initially unexcited screen, whereas in
actual cases of successive scan integration the screen is initially excited
from previous scans. Furthermore, it is not evident a priori that the
1When extended observations are to be made cm a single target or region the
display is sometimes “frozen”; th~t is, the target motion relative to the radar is compensated
so that the picture remains stationary. It is then possible to use longer
persistence without blurring. However, since observation of the frozen display is
usually part of the “tracking” operation which controls the removal of the motion,
the persistence must not be so long that it reduces the ease of detecting small changes
in target position.