Synthetic Aperture Radar
2.1.2 Synthetic aperture radar (SAR)
As shown by Equation 2.18, the azimuth resolution for a real-aperture SLAR is given by
.
For a typical airborne SLAR, (e.g.
= 8 km,
= 24 cm,
= 4 m, and
= 30°), the azimuth resolution
would be about 550 m. A synthetic-aperture technique may be used to improve
the azimuth resolution, using the fact that a given target may be illuminated
by many pulses as the imaging swath passes over it. Consider the situation
depicted in Figure 2.4 of an imaging swath passing over a target at some
incidence angle 0° <
< 90°. It can be shown that an array of antennas lying along a single
line is equivalent to a single antenna moving along that line as long as the
signals received by that single antenna are recorded coherently and added
together in the same way as the signals from the array would be combined in a
waveguide network.
Figure 2.4 Formation of a synthetic antenna arraythis fig from fig. 6-30 Elachi.
is the horizontal beam width on the groundFrom Equation 2.18, the angular horizontal beam width of a real-aperture radar is[1]
,
(2.21)
where
is the antenna length and
is the wavelength [Ulaby, Moore, and Fung 1986b]
If the radar is a distance
from the target, the resulting horizontal beam width on the ground
is given by
(2.22)
Figure 2.4 illustrates that as the radar moves along a straight line, signals
are received at
,
,
,
...,
from a target as the width of the illuminated area
passes over it. If these signals are recorded coherently (both phase and
amplitude are recorded), they can be used to synthesize a linear array of
antennas, or effectively one long antenna [Elachi 1987]. The
synthesized array's effective beam width corresponds to the length of the
synthetic array rather than of the physical antenna. The synthetic aperture
has a maximum length of
because that is the maximum distance the antenna can travel while still keeping
the target in view. The corresponding azimuth resolution is simply the product
of the effective horizontal beam width and the slant-range distance to the
target:
.
(2.23)
Here,
is the horizontal beam width of the synthetic aperture, and is half that of a
real aperture [Ulaby, Moore, and Fung 1986b]. If the maximum
aperture length is used,
is replaced by
in Equation 2.23. The corresponding azimuth resolution is the smallest that
may be achieved with the given antenna,
.
(2.24)
The final form is independent of both the wavelength
and the distance from the radar to the target
With the azimuth resolution directly proportional to the length of the actual antenna, very small resolution cells are possible. However, most SAR data are not processed at the minimum possible resolution because of the need to average the returns over several neighboring resolution cells, (i.e. collect many independent samples) to reduce noise [Ulaby, Moore, and Fung 1986b]
Last Modified: Wed Apr 14, 1999
CSR/TSGC Team Web