Page 92 - Fundamentals of Radar Signal Processing

backscattered power collected by the receiving antenna will be

(2.7)

It was shown in Chap. 1 that the effective aperture of an antenna is related to its
2
gain and operating wavelength according to A = λ G/4π. Thus
e

(2.8)

Equation (2.8) describes the power that would be received if an ideal radar
operated in free space and used no signal processing techniques to improve

sensitivity. Various additional loss and gain factors are customarily added to the
formula to account for a variety of additional considerations. For example,
losses incurred in various components such as the duplexers, power dividers,
waveguide, and radome (a protective covering over the antenna), and
propagation effects not found in free space propagation, can be lumped into a
system loss factor L that reduces the received power. System losses are
s
typically in the range of 3 to 10 dB but can vary widely. One of the most

important loss factors, particularly at X band and higher frequencies, is
atmospheric attenuation L (R). Unlike system losses, atmospheric losses are a
a
function of range. If the one-way loss in decibels per kilometer of Fig. 1.3 is
denoted by α, the loss in decibels for a target at range R meters (not kilometers)
is

(2.9)

In linear units, the loss is therefore

(2.10)

Atmospheric loss can be inconsequential at 10 GHz and moderate ranges, or
tens of decibels at 60 GHz and a range of a few kilometers. (This is the reason

why 60 GHz is not a popular radar frequency.) This example also shows that,
like system losses, atmospheric loss is a strong function of radar frequency.
Incorporating atmospheric and system losses in Eq. (2.8) finally gives