Environmental monitoring,
earth-resource mapping, and military systems require broad-area imaging
at high resolutions. Many times the imagery must be acquired in inclement
weather or during night as well as day. Synthetic Aperture Radar (SAR)
provides such a capability. SAR systems take advantage of the long-range
propagation characteristics of radar signals and the complex information
processing capability of modern digital electronics to provide high
resolution imagery. Synthetic aperture radar complements photographic
and other optical imaging capabilities because of the minimum constraints
on time-of-day and atmospheric conditions and because of the unique
responses of terrain and cultural targets to radar frequencies.
Synthetic aperture
radar technology has provided terrain structural information to geologists
for mineral exploration, oil spill boundaries on water to environmentalists,
sea state and ice hazard maps to navigators, and reconnaissance and
targeting information to military operations. There are many other applications
or potential applications. Some of these, particularly civilian, have
not yet been adequately explored because lower cost electronics are
just beginning to make SAR technology economical for smaller scale uses.
Sandia has a long
history in the development of the components and technologies applicable
to Synthetic Aperture Radar -- 40 years in radar, antenna, and miniature
electronics development; 30 years in microelectronics; and 25 years
in precision navigation, guidance, and digital-signal processing. Over
the last decade, we have applied these technologies to imaging radars
to meet the needs of advanced weapon systems; verification and nonproliferation
programs; and environmental applications. Sandia's expertise in electromagnetics,
microwave electronics, high-speed signal processing, and high performance
computing and navigation, guidance and control have established us as
world leaders in real-time imaging, miniaturization, processing algorithms,
and innovative applications for SAR.
How does Synthetic Aperture Radar
work?
A detailed description
of the theory of operation of SAR is complex and beyond the scope of
this document. Instead, this page is intended to give the reader an
intuitive feel for how synthetic aperture radar works.
Consider an airborne
SAR imaging perpendicular to the aircraft velocity as shown in the figure
below. Typically, SARs produce a two-dimensional (2-D) image. One dimension
in the image is called range (or cross track) and is a measure of the
"line-of-sight" distance from the radar to the target. Range measurement
and resolution are achieved in synthetic aperture radar in the same
manner as most other radars: Range is determined by precisely measuring
the time from transmission of a pulse to receiving the echo from a target
and, in the simplest SAR, range resolution is determined by the transmitted
pulse width, i.e. narrow pulses yield fine range resolution.
Synthetic Aperture Radar Imaging
Concept
The other dimension is called azimuth (or along track) and is perpendicular
to range. It is the ability of SAR to produce relatively fine azimuth
resolution that differentiates it from other radars. To obtain fine
azimuth resolution, a physically large antenna is needed to focus the
transmitted and received energy into a sharp beam. The sharpness of
the beam defines the azimuth resolution. Similarly, optical systems,
such as telescopes, require large apertures (mirrors or lenses which
are analogous to the radar antenna) to obtain fine imaging resolution.
Since SARs are much lower in frequency than optical systems, even moderate
SAR resolutions require an antenna physically larger than can be practically
carried by an airborne platform: antenna lengths several hundred meters
long are often required. However, an airborne radar could collect data
while flying this distance and then process the data as if it came from
a physically long antenna. The distance the aircraft flies in synthesizing
the antenna is known as the synthetic aperture. A narrow synthetic beamwidth
results from the relatively long synthetic aperture, which yields finer
resolution than is possible from a smaller physical antenna.
Achieving fine
azimuth resolution may also be described from a doppler processing viewpoint.
A target's position along the flight path determines the doppler frequency
of its echoes: Targets ahead of the aircraft produce a positive doppler
offset; targets behind the aircraft produce a negative offset. As the
aircraft flies a distance (the synthetic aperture), echoes are resolved
into a number of doppler frequencies. The target's doppler frequency
determines its azimuth position.
While this section
attempts to provide an intuitive understanding, SARs are not as simple
as described above. Transmitting short pulses to provide range resolution
is generally not practical. Typically, longer pulses with wide-bandwidth
modulation are transmitted which complicates the range processing but
decreases the peak power requirements on the transmitter. For even moderate
azimuth resolutions, a target's range to each location on the synthetic
aperture changes along the synthetic aperture. The energy reflected
from the target must be "mathematically focused" to compensate for the
range dependence across the aperture prior to image formation. Additionally,
for fine-resolution systems, the range and azimuth processing is coupled
(dependent on each other) which also greatly increases the computational
processing.
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