Page 91 - Fundamentals of Radar Signal Processing

radius R, which is

(2.3)

Instead of isotropic radiators, real radars use directive antennas to focus the
outgoing energy. As described in Chap. 1, the antenna gain G is the ratio of
maximum power density to isotropic density. Thus, in the direction of maximum

radiation intensity, the power density at range R becomes

(2.4)

This is the power density incident upon the target if it is aligned with the
antenna’s axis of maximum gain.
When the electromagnetic wave with power density given by Eq. (2.4) is

incident upon a single discrete scattering object, or point target, at range R the
incident energy is scattered in various directions; some of it may also be
absorbed by the scatterer itself. In particular, some of the incident power is
reradiated toward the radar, or backscattered. Imagine that the target collects
all of the energy incident upon a collector of area σ square meters and reradiates
it isotropically. The reradiated power is then

(2.5)

The quantity σ is called the radar cross section (RCS) of the target. One
important fact about RCS is that σ is not equal to the physical cross-sectional

area of the target; it is an equivalent area that can be used to relate incident
power density at the target to the reflected power density that results at the
receiver. RCS will be discussed further in Sec. 2.2.3.
Because RCS is defined under the assumption that the backscattered power
is reradiated isotropically, the density of the backscattered power at a range R
is found by dividing the power of Eq. (2.5) by the surface area of a sphere of
radius R as was done in Eq. (2.3), giving the backscattered power density at the
radar receiver as

(2.6)

If the effective aperture size of the radar antenna is A square meters, the total
e