Corrections of stratiﬁed tropospheric delays in SAR interferometry: Validation with
global atmospheric models
, C. Lasserre
, G. Peltzer
, O. Cavalié
, C. Doubre
Laboratoire de Géologie, CNRS UMR 8538, École Normale Supérieure, Paris, France
Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph, Fourier, CNRS, BP53 38041 Grenoble Cedex 09, France
Department of Earth and Space Science, University of California Los Angeles, CA 10095, USA
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA
EOST-IPGS (UMR 7516 CNRS-UdS), 5 rue Ren Descartes, 67084, Strasbourg, Cedex, France
Received 15 May 2008
Accepted 25 March 2009
Phase propagation delay
Global climate model
The main limiting factor on the accuracy of Interferometric SAR measurements (InSAR) comes from phase
propagation delays through the troposphere. The delay can be divided into a stratiﬁed component, which
correlates with the topography and often dominates the tropospheric signal, and a turbulent component. We
use Global Atmospheric Models (GAM) to estimate the stratiﬁed phase delay and delay-elevation ratio at
epochs of SAR acquisitions, and compare them to observed phase delay derived from SAR interferograms.
Three test areas are selected with different geographic and climatic environments and with large SAR archive
available. The Lake Mead, Nevada, USA is covered by 79 ERS1/2 and ENVISAT acquisitions, the Haiyuan Fault
area, Gansu, China, by 24 ERS1/2 acquisitions, and the Afar region, Republic of Djibouti, by 91 Radarsat
acquisitions. The hydrostatic and wet stratiﬁed delays are computed from GAM as a function of atmospheric
pressure P, temperature T, and water vapor partial pressure e vertical proﬁles. The hydrostatic delay, which
depends on ratio P/T, varies signiﬁcantly at low elevation and cannot be neglected. The wet component of the
delay depends mostly on the near surface speciﬁc humidity. GAM predicted delay-elevation ratios are in good
agreement with the ratios derived from InSAR data away from deforming zones. Both estimations of the
delay-elevation ratio can thus be used to perform a ﬁrst order correction of the observed interferometric
phase to retrieve a ground motion signal of low amplitude. We also demonstrate that aliasing of daily and
seasonal variations in the stratiﬁed delay due to uneven sampling of SAR data signiﬁcantly bias InSAR data
stacks or time series produced after temporal smoothing. In all three test cases, the InSAR data stacks or
smoothed time series present a residual stratiﬁed delay of the order of the expected deformation signal. In all
cases, correcting interferograms from the stratiﬁed delay removes all these biases. We quantify the standard
error associated with the correction of the stratiﬁed atmospheric delay. It varies from one site to another
depending on the prevailing atmospheric conditions, but remains bounded by the standard deviation of the
daily ﬂuctuations of the stratiﬁed delay around the seasonal average. Finally we suggest that the phase delay
correction can potentially be improved by introducing a non-linear dependence to the elevation derived from
© 2009 Elsevier B.V. All rights reserved.
The main limitation of differential radar interferometry (DInSAR)
in measuring centimeter ground displacements, apart from coherence
loss, comes from unaccounted electromagnetic phase propagation
delays in the atmosphere (Zebker et al., 1997; Hanssen, 2001).
Atmospheric propagation delays are a geophysical signal affecting
every pixel of the radar scene and masking ground motion, and can
potentially be modelled and corrected. They limit the measurement
accuracy of post- and inter-seismic deformations in the seismic cycle,
transient creep along some fault segments, volcanic deﬂation or
inﬂation, subsidence induced by mining or ﬂuid extraction, or ground
motion due to loading or unloading of the lithosphere by the
hydrosphere. Therefore, it is particularly important to devise techni-
ques to improve our understanding of electromagnetic delays in the
atmosphere and to mitigate their effects on SAR measurements.
Propagation delays are caused by dispersive effects in the
ionosphere and by air refractivity gradients in the neutral troposphere.
Ionospheric effects in SAR interferograms have mostly been observed
in auroral zones and are more important in L-band than in C-band.
They are revealed by azimuth distortions or shifts in SAR images with
characteristic distances generally larger than 100 km, except for
kilometer scale azimuth streaks (Gray et al., 2000; Mattar and Gray,
2002; Meyer et al., 2006). Air refractivity gradients in the troposphere
Journal of Applied Geophysics 69 (2009) 35–50
⁎ Corresponding author. Laboratoire de Géologie, CNRS UMR 8538, École Normale
Supérieure, 24 rue Lhomond, 75231 Cedex 05 Paris, France.
E-mail address: email@example.com (M.-P. Doin).
0926-9851/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
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