3.6 Surface-Distributed Targets
49
surface element (elementary area) of which changes the phase of incident wave
by independent random way. The scattering field in this case represents a sum of
partial waves with random phases. As it is known, in this case, the power summing
of these waves is taken place. This means that incident wave at θ angle induces on
elementary area S a current, proportional to cos θ , and since BSD of this area also is
a cos θ , then area RCS of S square will be A = a 0 cos
2
θθS. For surface-distributed
targets, another parameter is introduced—back-reflection coefficient (BRC)—which
is defined as γ (θ ) = a 0 sec θ , and the most tables have been made exactly for it. The
dependence γ (θ ) from θ for different types of surface is depicted in Fig. 3.23.
Experimental measuring shows that many of statistical smooth surfaces stand the
function cos
n
θ , where n number changes from 1 up to 20 and even 50–60 as a good
approximation to normalized BSD.
Sometimes, the Rayleigh criteria can be useful by which all surfaces are divided
into smooth and rough. Examine a wave incident on rough surface at θ angle
(Fig. 3.24).
Elementary examination shows that 1 and 2 beam path difference on plane MN
will be ψ = 2kh sin ϕ. Rayleigh proposed to consider a surface as smooth at
ψ ≤ π/4. This means that if observation is carried out at ϕ angles, satisfying
the condition: sin ϕ ∼ = ϕ ≤ λ/16h or, if average height of irregularities h: h ≤
Fig. 3.23 Dependence of back-reflection coefficient from wave incident angle
Fig. 3.24 Rayleigh criteria
49
surface element (elementary area) of which changes the phase of incident wave
by independent random way. The scattering field in this case represents a sum of
partial waves with random phases. As it is known, in this case, the power summing
of these waves is taken place. This means that incident wave at θ angle induces on
elementary area S a current, proportional to cos θ , and since BSD of this area also is
a cos θ , then area RCS of S square will be A = a 0 cos
2
θθS. For surface-distributed
targets, another parameter is introduced—back-reflection coefficient (BRC)—which
is defined as γ (θ ) = a 0 sec θ , and the most tables have been made exactly for it. The
dependence γ (θ ) from θ for different types of surface is depicted in Fig. 3.23.
Experimental measuring shows that many of statistical smooth surfaces stand the
function cos
n
θ , where n number changes from 1 up to 20 and even 50–60 as a good
approximation to normalized BSD.
Sometimes, the Rayleigh criteria can be useful by which all surfaces are divided
into smooth and rough. Examine a wave incident on rough surface at θ angle
(Fig. 3.24).
Elementary examination shows that 1 and 2 beam path difference on plane MN
will be ψ = 2kh sin ϕ. Rayleigh proposed to consider a surface as smooth at
ψ ≤ π/4. This means that if observation is carried out at ϕ angles, satisfying
the condition: sin ϕ ∼ = ϕ ≤ λ/16h or, if average height of irregularities h: h ≤
Fig. 3.23 Dependence of back-reflection coefficient from wave incident angle
Fig. 3.24 Rayleigh criteria