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g 2008 
REDUCTION OF ATMOSPHERIC WATER VAPOUR EFFECTS ON ENVISAT ASAR 
INTERFEROGRAMS USING MERIS NEAR IR MEASUREMENTS 
Zhenhong Li 
Department of Civil, Environmental and Geomatic Engineering, University College London, London WC1E 6BT, UK 
Now at Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK 
Zhenhong.Li@ges.gla.ac.uk 
Commission VII, WG VII/2 
KEY WORDS: SAR, Change Detection, Radar, Integration, Monitoring, Mapping, Surface, Geodesy 
ABSTRACT: 
Atmospheric water vapour effects represent a major limitation of repeat-pass Interferometric SAR (InSAR) techniques including 
InSAR time series approaches (e.g. permanent scatterers (PS) and small baseline subset (SBAS)). In this paper, it is demonstrated 
that atmospheric water vapour effects greater than 4 cm can be observed even in desert regions (e.g. Southern California) and the use 
of MERIS correction model can improve the accuracy of InSAR derived deformation signals from 9.9 mm to 4.1 mm. It is also 
shown that, using an advanced integration technique titled InSAR Time Series with Water Vapour correction model (InSAR TS + 
PWV) for reduction of water vapour effects with coincident MERIS near IR water vapour data, a time series of postseismic motion 
after the 2003 Bam (Iran) earthquake is achievable with about 50% reduction in RMS model misfit. It is believed that this study not 
only contributes directly to the ENVISAT mission, but also will benefit space agencies’ plans to design and launch InSAR missions 
because it aids in the identification of necessary characteristics of their future InSAR missions. 
1. INTRODUCTION 
Land surface deformation is a major worldwide hazard which 
can result from natural processes such as earthquakes, 
volcanoes and landslides, or from anthropogenic processes 
including extraction of groundwater, oil and coal. Surface 
displacement causes many problems including economic loss 
and human suffering. One estimate placed the direct annual cost 
of subsidence damage and mitigation within the USA alone at 
over $100 million (Panel on Land Subsidence, 1991). Indirect 
costs of subsidence are even greater, such as the indication that 
subsidence was a contributing factor to the levee failures in 
New Orleans when hurricane Katrina hit in 2005 (Dixon et al., 
2006). On 8 October 2005 one of the deadliest earthquakes in 
modem times struck the region of Kashmir in South Asia and 
claimed 80,000+ lives. Therefore, land deformation mapping is 
of crucial importance not only for sustainable economic 
development but also for the safety of the general public. 
Interferometric SAR (InSAR) has been already revolutionizing 
our ability to image surface deformation at spatial resolutions of 
a few tens of metres in the last two decades, which in turn has 
led to many new insights into geophysical processes, such as 
earthquakes, volcanoes and landslides (Massonnet and Feigl, 
1998). It is well-known that InSAR techniques are limited by 
atmospheric effects: any difference in radar signal propagation 
delays between SAR acquisitions leads to additional shifts in 
phase measurements, which largely result from changes in 
water vapour content in the troposphere (Hanssen, 2001). 
ESA’s ENVISAT provides a unique opportunity to investigate 
atmospheric water vapour effects on SAR interferograms due to 
its two main payloads, Advanced Synthetic Aperture Radar 
(ASAR) and MEdium Resolution Imaging Spectrometer 
(MERIS): (1) There is no time difference between acquisitions 
of water vapour and SAR data; (2) MERIS full-resolution mode 
has a high spatial resolution of up to 300 m; (3) MERIS and 
ASAR have a virtually identical propagation path. 
2. ATMOSPHERIC EFFECTS ON INTERFEROGRAMS 
2.1 Atmospheric Effects on Interferograms - Theory 
After removing the flat earth and local topography, the 
unwrapped differential interferometric phase at pixel (x,r) 
computed from the SAR acquisitions at times t M , the start time, 
and t s , the end time, can be written as follows (Berardino et al., 
2002): 
i>, uh (x,r) = 8<t>\°^ (x,r) + ôtfil {x,r)+S<f>“' u l (x,r) + (x,r) 
,Sft T ( V ).fE^V, AZ +'-> 
usX ’ 1 r sin 6> 
4 n 
S K<s (■ x ’ r ) = ~r[ d - ¿(/ M ,x,r)] 
CD 
5 Kl ( x ’ r ) = X[ d *m (t s >x>r) - d a , n {t M ,x,r)~\ 
where  is the transmitted signal central wavelength (in mm, 
e.g. c. 56.3 mm for ASAR), d(t s ,x,r) and d(t M ,x,r) 
represent the cumulative deformation in the line of sight at 
times t s and t M respectively, with respect to the reference 
instant t 0 , i.e., implying d(t 0 ,x,r) = 0,V(x,r) ; AZ(x,r) is 
topographic error present in the DEM used for interferogram 
generation, and its impact on deformation maps is also a 
function of the perpendicular baseline component B lt t , the 
sensor-target distance r , and the look angle 0 . The terms 
datm{ t M’ x ’ r ) an d d aM (t s ,x,r) account for temporal