Radar System Engineering

SEC, 152] NEED FOR SYSTEM TESTING 591
(Sees. 210 and 211), such test signals are generally not equal to Smirs but
differ from it by a constant ratio, The ratio between pulse power and the
power of such a test signal, P/ST, expressed in decibels, is called the
“radar performance figure” and is a suitable measure of the ability of
thesystem to see radar targets.
Figure 15.1 shows the range performance on various types of radar
targets as a function of the radar performance figure. As explained in
Chap. 2, radar range performance does not always follow the fourth-power
law of Eq. (1), but the performance
figure remains of vital importance in
determining what fraction of the
maximum radar range can be realized
by a given system against a given
type of target, regardless of the
existing propagation conditions.
The Inadequacy of Guessing Performance.—It
has often been wrongly
assumed that over-all radar performance
can be adequately judged without
using test equipment by means oo-
of one of the following “rule-ofthumb”
Dechis below rated performance
criteria: (1) the general
appearance of the picture seen on
FIG. 15. 1.—Relation between radar
performance deficit and available radar
range for various types of target.
the radar indicator, (2) the maxi-
mum range at which target signals can be seen, or (3) the signal strength
above noise of the echo from a “ standard” target.
Both (1) and (2) are strongly affected by changes in propagation
conditions’ as well as by interference effects resulting from composite
targets,’ tidal changes,3 etc. Although the effect of anomalous propagation
can be practically eliminated by choosing a near-by weak target
signal as a standard, such a favorable choice of’ ‘standard target” does not
necessarily eliminate interference effects. Targets that do not show such
effects in some degree are exceedingly rare.
These rule-of-thumb criteria are inadequate for the consistent judgment
of radar performance, as has been shown by surveys of radar
performance made on radar equipment in the field during the war.
Unfortunately during most of the war adequate test equipment was
commonly lacklng; when it was present, it was often in charge of inadequately
trained maintenance personnel who did not appreciate the need
for using it regularly. Figure 15.2 shows the combined results of a
1SeeSec. 2.14.
2SeeSew. 3.8 and 3.9.
sSec. 2.12.