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Mapping without the sun
Zhang, Jixian

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Z. Lu
U.S. Geological Survey, Center for Earth Resources Observation and Science and Cascades Volcano Observatory, Vancouver, WA
98683, USA; phone: 360-993-8911, fax: 360-993-8981, email: lu@usgs.gov
KEY WORDS: Synthetic aperture radar (SAR), Interferometric SAR (InSAR), Natural hazards
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Interferometric synthetic aperture radar (InSAR) imaging is an all-weather instrument capable of measuring ground-surface
deformation and inferring changes in land surface characteristics. The interferometric products, derived from European Remote
sensing Satellite 1 (ERS-1), European Remote-sensing Satellite 2 (ERS-2), and European Environmental Satellite (Envisat),
Japanese Earth Resources Satellite 1 (JERS-1) and Japanese Advanced Land Observing Satellite (ALOS), and Canadian Radar
Satellite 1 (Radarsat-1) synthetic aperture radar (SAR) images, enable the characterization of natural hazards induced by volcanic,
seismic, and hydrogeologic processes, landslides, wild fires, etc. Measuring how a volcano’s surface deforms before, during, and
after eruptions provides essential information about magma dynamics and a basis for mitigating volcanic hazards. Measuring spatial
and temporal patterns of surface deformation in seismically active regions is extraordinarily useful for understanding rupture
dynamics and estimating seismic risks. Measuring how landslides develop and activate is a prerequisite to minimizing associated
hazards. Mapping surface subsidence and uplift related to extraction and injection of fluids in groundwater aquifers provides
fundamental data on aquifer properties and processes and improves our ability to mitigate undesired consequences. Monitoring
dynamic water-level changes beneath wetlands improves hydrological modeling predictions and enhances the assessment of future
flood events over wetlands. In addition, InSAR imagery can provide near-real-time estimates of fire scar extents and severities for
wild fire management and control. These studies demonstrate that all-weather satellite radar imagery is critical for studying various
natural hazards and plays an increasingly important role in better understanding and eventually forecasting natural hazards.
Interferometric synthetic aperture radar (InSAR) involves the
use of two or more synthetic aperture radar (SAR) images of the
same area to extract landscape topography and its deformation
patterns. A SAR system transmits electromagnetic waves at a
wavelength that can range from a few millimeters to tens of
centimeters and therefore can operate during day and night
under all weather conditions. Using a SAR processing
technique (Curlander and McDonough, 1991), both the intensity
and phase of the reflected (or backscattered) radar signal of
each ground resolution element (a few meters to tens of meters)
can be calculated in the form of a complex-valued SAR image
that represents the reflectivity of the ground surface: the
amplitude or intensity of the SAR image is determined
primarily by terrain slope, surface roughness, and dielectric
constants, whereas the phase of the SAR image is determined
primarily by the distance between the satellite antenna and the
ground targets. InSAR imaging uses the interaction of
electromagnetic waves, referred to as interference, to measure
precise distances between the satellite antenna and ground
resolution elements to derive landscape topography and its
subtle change in elevation.
InSAR is formed by combining, or “interfering,” radar signals
from two spatially or temporally separated antennas. The spatial
separation of the two antennas is called the baseline. The two
antennas may be mounted on a single platform for simultaneous
interferometry, which is the usual implementation for aircraft
and spacebome systems such as the Topographic SAR
(TOPSAR) and the Shuttle Radar Topography Mission (SRTM)
systems, which were created for generating high-resolution,
high-precision digital elevation models (DEMs) over large
regions. Alternatively, InSAR can be created by using a single
antenna on an airborne or spacebome platform in nearly
identical repeating flight orbits for repeat-pass interferometry
(Massonnet and Feigl, 1998; Lu et al., 2007a). In the latter case,
even though the antennas do not illuminate a given area at the
same time, the two sets of signals that are recorded during the
two passes will be highly correlated to produce an
interferogram if the scattering properties of the ground surface
remain undisturbed between viewings. The topographic effects
in the interferogram can be removed with a synthetic
interferogram created from an accurate DEM and the
knowledge of InSAR imaging geometry. In this configuration,
InSAR is capable of measuring ground-surface deformation
with a precision of centimeters or sub-centimeters for C-band
sensors or a few centimeters for L-band sensors at a spatial
resolution of tens-of-meters over a large region. This is the
typical implementation for major spacebome sensors, including
European Remote-sensing Satellite 1 (ERS-1) (operated 1991—
2000, C-band at wavelength k = 5.66 cm), Japanese Earth
Resources Satellite 1 (JERS-1) (operated 1992-1998, L-band, k
= 23.5 cm), Shuttle Imaging Radar-C (SIR-C) (operated April-
October 1994, X-, C-, and L-band, k = 3.1 cm, 5.66 cm, and
24.0 cm, respectively), European Remote-sensing Satellite 2
(ERS-2) (operating 1995-present, C-band, k = 5.66 cm),
Canadian Radar Satellite 1 (Radarsat-1) (operating 1995-
present, C-band, k = 5.66 cm), European Environmental
Satellite (Envisat) (operating 2002-present, C-band, k = 5.63
cm), Japanese Advanced Land Observing Satellite (ALOS)
(operating January 2006-present, L-band, k = 23.6 cm), and
German TerraSAR-X (operating June 2007-present, X-band, k
= 3.1 cm).