Graphics Programs Reference
In-Depth Information
1.72
h
rms
≈
0.025
+
0.046
S
state
(6.6)
where is the sea state, which is tabulated in several cited references. The
sea state is characterized by the wave height, period, length, particle velocity,
and wind velocity. For example, refers to a moderate sea state,
where in this case the wave height is approximately between
, the wave period 6.5
to
4.5 seconds, wave length
, wave velocity
S
state
S
state
=
3
0.9144
to
1.2192
m
1.9812
to
33.528
m
20.372
to
25.928
Km
⁄
hr
, and wind
velocity
22.224
to
29.632
Km
⁄
hr
.
Clutter at low grazing angles is often referred to as diffuse clutter, where
there are a large number of clutter returns in the radar beam (non-coherent
reflections). In the flat region the dependency of on the grazing angle is
minimal. Clutter in the high grazing angle region is more specular (coherent
reflections) and the diffuse clutter components disappear. In this region the
smooth surfaces have larger
σ
0
σ
0
than rough surfaces, opposite of the low graz-
ing angle region.
6.2.1. Radar Equation for Area Clutter - Airborne Radar
intersection of the antenna beam with the ground defines an elliptically shaped
footprint. The size of the footprint is a function of the grazing angle and the
antenna 3dB beamwidth , as illustrated in
Fig. 6.5.
The footprint is
divided into many ground range bins each of size
θ
3
dB
(
c
τ 2
⁄
)
g
sec
, where
τ
is
the pulsewidth.
From Fig. 6.5, the clutter area
A
c
is
c
τ
2
-----
A
c
≈
R
θ
3
dB
sec
ψ
g
(6.7)
θ
3
dB
R
ψ
g
footprint
A
c
Figure 6.4. Airborne radar in the look-down mode.
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