Radar System Engineering

46 THE RADAR EQUATION [SEC.211
Although we have not arrived at a universal prescription for the
minimum detectable signal power, we have studied the influence of
various factors onS~i~, and weare nowin a position to predict therelutive
change in &i. which will accompany some proposed alteration in the
radar system. Onewayto establish some absolute basis isto give the
~bserved value of the ratio of minimum detectable signal power to
average noise power, for one particular system. This is done in Table
22, in which are displayed the relevant constants of the system selected.
TABLE 22.-(2HARACTERISTICS OF A SAMPLE SYSTEM
Type of indication.
PPI; P7 screen;decay time about 7 sec
I-f bandwidth. . . . . . . . . . . . . . . . . @=l.2Mc/sec
Video bandwidth.,.... ., b = 5 Me/see
Pulseduration. . . . . . . . . . . . . . . . T=l.Oxee
Pulserepetitionfrequency. 320 pps Hence about 50 pulses on one point
Scan rate..................,., 6rpm targetperscan, with negligiblescan-
Beamwidth. . . . . . . . . . . . . . . . . . . 0=6° ) to-scan storage.
Sweepspeed. . . . . . . . . . . . . . . . . . v=2X10’cm/sec Note that d/u> r, which is
Spot diameter. . . . . . . . . . . . . . . . d= O.lcm
not the best condition, but
} is a typical one.
conditions of observation. Signal occurs on one scan, at known azimuth, in
one of six rangepositions. Observermust locate
it with 90% certainty. This corresponds to a
value of rv,in Table 2.1, of the orderof 10–1,
Ratio of minimum detectabk signal
power to average nm”se
power, under above cmditiom. 1,25 (+1 db)
Working from Table 2.2, we can infer that if the system there described
had an over-all noise figure N of, for example, 15, the signal power required
at the terminals of the antenna would be S~im= 1.25NkT@ = (1.25) (15)
(1.37 X 10-23) (291) (1.2 X 10’) = 9 X 10-’4 watts. Now if for some
other system N is given, together with the new values of the parameters
listed in Table 2.2, the reader should be able to estimate Stim for that
system. The estimate will be better, of course, the less extreme the
departure from the conditions of Table 2.2. We must emphasize, however,
the limited utility of a value of Sti. obtained in this way. The
experiments upon which Table 2.2 is based were performed in the laboratory
under ideal conditions of observation; moreover, the value of w (the
parameter of Table 21) is here certainly much too large for any radar
search operation, although it might not be inappropriate for the continuous
tracking of a target already detected.
The experimental determination of the maximum range of an actual
radar set is as uncertain a matter as is the calculation of the maximum
range by the method outlined in this chapter. Some arbitrary procedure
for observation must be specified, and the limit of range must be defined