458 
218.4 db as the performance figure. 
For a ship with an effective height of 18 meters, 
Figure 5 gives the value of 20 log A at various dis- 
tances. With the aid of equation (5), in Chapter 5, 
the power P, returned to the radar by the ship at 
these distances can be found, using the following 
data: P; = P,= 60 kw, G = 22.5 db, and o = 7,440 
square meters. 
In Figure 6, the maximum range versus distance 
curves for a set are presented. This set has a beam- 
width of 60 degrees so that it is effective for tracking 
aircraft coming in toward the set (increasing angle 
of elevation). The near coverage then approximates 
the free-space coverage which is dependent on the 
angle of inclination of the antenna. 
In Figure 7, curves are presented for a 3-cm set. 
Estimating Ship Size 
The fact that the strength of a signal received 
from a target depends on the size of the target can 
be utilized to estimate ship size. If signal strength 
versus distance curves are obtained for surface craft 
of various sizes, then by plotting readings from an 
unknown vessel, its size may be estimated. A field 
procedure for obtaining these curves is to use a 
number of surface craft of assorted sizes and follow 
them on a radar with a calibrated A scope, recording 
signal strength versus distance. If a number of such 
runs are taken during periods judged to be standard 
and the results averaged, a fairly reliable chart 
should result. 
The curves can be obtained by calculation using 
measured power output and antenna gain and 
empirica]'y determined values of « in equation (5) 
in Chapter 5. The shape of such a curve for a “point” 
target is given in Figure 26 in Chapter 5. The radar 
curve can be obtained from this curve by changing 
the ordinate back to 20 log A [equation (8) in 
Chapter 5] and-then using equation (5) in Chapter 5. 
Since an actual surface target is generally large 
and at short distances intercepts a considerable 
part of the energy, the oscillatory part of the point 
curve of Figure 26 in Chapter 5 is smoothed out. 
The value of A for short distances is sometimes 
taken as 6Ao, or 15 db above the free-space value. 
In Figure 8, an example is given of curves com- 
puted for various ship sizes. Most of the plotted 
points fall in the 200- to 600-ton region. However, 
the precaution of a performance check is necessary 
in order to use this method most effective1y (see 
next section ). Since shape plays an important part 
in radar cross section, a ship of unusual shape may 
give a signal outside its class. ‘ 
Performance Check 
Since the performance of a set varies, before a 
chart such as has been described is used to detect 
unknown ships, it should be checked to determine 
PROPAGATION THROUGH THE STANDARD ATMOSPHERE 
tterinss Reagomiitemeters 
Figure 6. Maximum range versus height curves for 
various targets. 
tooo0o 
5000 
- 
2000 
AR 
10.0.0 | eee eer 
500 
200 
Height~ Meters 
100 
SO) 
Set 
d= 3cm, 
= hetOm GS 
? 
“So 
Ficure 7. Maximum range versus height curves for 
various targets. 
S 10 20 
Maximum Range— Kilom¥ters 
