Graphics Programs Reference
In-Depth Information
SAR systems can produce maps of reflectivity versus range and Doppler
(cross range). Range resolution is accomplished through range gating. Fine
range resolution can be accomplished by using pulse compression techniques.
The azimuth resolution depends on antenna size and radar wavelength. Fine
azimuth resolution is enhanced by taking advantage of the radar motion in
order to synthesize a larger antenna aperture. Let denote the number of
range bins and let denote the number of azimuth cells. It follows that the
total number of resolution cells in the map is . SAR systems that are gen-
erally concerned with improving azimuth resolution are often referred to as
Doppler Beam-Sharpening (DBS) SARs. In this case, each range bin is pro-
cessed to resolve targets in Doppler which correspond to azimuth. This chapter
is presented in the context of DBS.
N
r
N
a
N
r
N
a
Due to the large amount of signal processing required in SAR imagery, the
early SAR designs implemented optical processing techniques. Although such
optical processors can produce high quality radar images, they have several
shortcomings. They can be very costly and are, in general, limited to making
strip maps. Motion compensation is not easy to implement for radars that uti-
lize optical processors. With the recent advances in solid state electronics and
Very Large Scale Integration (VLSI) technologies, digital signal processing in
real time has been made possible in SAR systems.
12.2. Side Looking SAR Geometry
Fig. 12.1
shows the geometry of the standard side looking SAR. We will
assume that the platform carrying the radar maintains both fixed altitude and
velocity . The antenna beamwidth is , and the elevation angle (mea-
sured from the z-axis to the antenna axis) is . The intersection of the antenna
beam with the ground defines a footprint. As the platform moves, the footprint
scans a swath on the ground.
h
v
3
dB
θ
β
The radar positi
on
with respect to the abso
l
ute origin
O
=
(
000
,,
)
, at any
time, is the vector
a
()
. The velocity vector
a
' ()
is
ó
x
v
ó
y
ó
z
a
' () 0
=
×
+
×
+
0
×
(12.1)
The Line of Sight (LOS) for the current footprint centered at
qt
()
is defined
by the vector
, where
t
c
denotes the central time of the observation inter-
Rt
()
val
T
ob
(coherent integration interval). More precisely,
T
ob
2
T
ob
2
(
t
=
t
a
+
t
c
)
;
--------
≤≤
t
--------
(12.2)
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