Synthetic Aperture
Radar
By Robert Colburn,
IEEE History Center, Rutgers University
Synthetic
Aperture Radar, which was developed in the
1950s as a military reconnaissance tool, was
a solution to the 1940s need for an
all-weather, 24-hour aerial remote
surveillance device. In the late 1940s, the
United States Army was looking for an aerial
reconnaissance tool which could see through
clouds and would not depend on the hours of
daylight. Radar — given its ability to
penetrate clouds and fog, and its
non-dependence on the wavelengths of visible
light — seemed the logical choice. The
obstacle was that, in order to achieve a
high enough resolution to be useful, the
radar antenna would have needed to have been
about the size of a football field, far too
large for a reconnaissance aircraft to
carry.
Carl Wiley,
working at Goodyear, Arizona, (which later
became Goodyear Aerospace, and eventually
Lockheed Martin Corporation) in 1951,
suggested the principle that — because each
object in the radar beam has a slightly
different speed relative to the antenna —
each object will have its own doppler shift.
A precise frequency analysis of the radar
reflections will thus allow the construction
of a detailed image. A radar antenna a meter
or so wide can be made to acquire an image
which otherwise would have required a much
larger one. Approximately one year after
Wiley, researchers at the University of
Illinois independently developed the same
idea, as well as developing beam-sharpening
and autofocus concepts. During the summer of
1953, the University of Michigan’s “Project
Wolverine” laid the plans which would result
in the development of a practical SAR. The
processing demands stretched the limits of
the analog processors of the day, however,
Emmett Leith, one of the pioneers of
holography, believed that optical processing
of the data could satisfy the requirement.
The method worked. In 1957, airborne
synthetic aperture radar was yielding
dramatic results, and the University of
Michigan system had proven itself.
In 1974, the
National Oceanic and Atmospheric
Administration and engineers from Jet
Propulsion Laboratories began exploring the
possibilities for oceanic observations using
a satellite carrying a synthetic aperture
radar. SAR’s wavelengths make it sensitive
to small surface roughness changes, meaning
that it is ideal for monitoring surface wave
patterns and currents. SAR can measure
displacement accuracy to within several
millimeters. The June 1978 launch of
Seasat was the first civilian
application of synthetic aperture radar, and
it provided a powerful new tool to
scientists studying the earth. Prior to
Seasat, civilian image acquisition of
the earth was via Landsat cameras,
using visible light and providing
resolutions in the tens of meters. Seasat
operated until October of 1978, when it was
disabled by a massive short circuit in its
power system. Since that time, many SARs
have flown on board the Space Shuttle.
Synthetic
Aperture Radar has become one of the most
valuable tools for remote sensing of the
earth and its environment. With resolutions
down to one .3 meter or less from 55 km
away, and .11 meters from 25 km, it is used
for sea ice observation, measuring glacier
variations, wind pattern data collection,
rainfall, erosion, warning of storm surges,
vegetation structure, disaster management,
identification of potential landslide areas,
and drought prediction. The pressure of
magma building up beneath volcanoes often
causes them to rise or distort their
surfaces. Synthetic aperture radar is
capable of detecting such minute variations,
and for that reason it has become a useful
tool monitoring volcanoes and their lava
flows as a way of warning of impending
eruptions, as well as mitigating the
post-eruption hazards.
Forests play an
enormously important role in climate
interactions, and the health of forests is
related to the health of soil and to the
quality of the watershed. Forests also
typically cover vast areas, often
inaccessible ones, thus airborne or
satellite radar offers a practical means for
detailed study. Foliage reflects varying
radar returns depending on water content and
density. Forestry mapping and management,
including prediction of and monitoring of
forest fires.
Damage
assessment during floods has been greatly
improved by SAR. Because radar can penetrate
the cloud cover which accompanies severe
weather, it is often the only reliable
source for accurate information on how much
land has been inundated, the cresting and
current flows of rivers, the extent of crop
destruction, and damage to roads, bridges,
and buildings. By using SAR information,
disaster relief organizations can save lives
by choosing proper evacuation routes, and
distribute food and medical aid more
quickly.
Because of its
ability to measure minute differences in the
surface of the ground, SAR has revealed much
new information about ground subsidence
(which in itself is often a marker for water
table variations) and the role it can play
in natural disasters. Spaceborne SAR has
shown that the land under parts of New
Orleans underwent rapid subsidence in the
three years preceding the 2005 Hurricane
Katrina.
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