Radar System Engineering

80 PROPERTIES OF RADAR TARGETS [SEC. 3.9
of the definition of cross section can be made in a natural and simple
fashion if the target is not too near to the radar horizon. Assuming a
flat earth with reflection coefficient –1, the power per unit area at the
target is modified by the factor
where hl and h2 are the heights of the antenna and the target, respectively,
k is the radar wavelength, and R the range (see Sec. 2.12). The same
factor has to be applied again in computing the return signal to take
account of the interference along the return path: The assumption of a
flat earth is a good approximation’ up to ranges somewhat greater than
4h,hz
R, = ~. The cross section is thus redefined by the equation
s=g.4:R2
–“”14 ‘in2(2+a12”
An attempt has been made in RL Report No. 401 to extend the
validity of this definition to include ship targets by determining experimentally
an “effective value” for R,. This is done by observing the
range at which the attenuation of the return signal becomes greater than
that predicted by the inverse fourth-power law. In this way the cross
sections in Table 3.3 were obtained. The procedure amounts to a replace-
TABLE 3.3,—RADAR CROSSSECTION OF SHIPS
Type of ship [
1 >=1 Ocm
I
Tanker. 24 X 103
Cruiser. ... ,, ~ 150
Small freighter . . . . . . . . 1.5
?Jedium freighter ., 80
Large freighter . . . . . . . . . . . . . . . . 160
Small submarine (surfaced). 0.4
I
Crosssection, ft’
I A=3 cm
24 x 10’
150
1,5
80
160
1.5
ment of the real target, which extends over an elevation range from zero
to its actual height and whose illumination varies with elevation, by a
target at an “effective height” determined experimentally.
A different attempt has been made by M, Katzin,’ who computes the
10. J. Baltzer, V. A. Counter, W. M. Fairbank, tf”. 0, Gordy, E. L. Hudspeth,
“ OverwaterObservationsat .Y and ,’5’Frequencies,” RL Report ATO.401, June 26.
1943,
2Navy Report RA3A213.\.