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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).

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