Full text: Technical Commission VII (B7)

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
A. Schmitt, T. Leichtle, M. Huber, A. Roth 
German Aerospace Center (DLR), Earth Observation Center (EOC), Oberpfaffenhofen, D-82234 Wessling, Germany - 
(andreas.schmitt, tobias.leichtle, martin.huber, achim.roth)(g)dlr.de 
KEY WORDS: Environment, Land Cover, Dynamic, Change Detection, SAR, Algorithms, Multitemporal, Value-Added 
Today's SAR sensors provide a variety of different image modes particularly with regard to multipolarised acquisitions. Until now, 
each polarisation mode requires a special decomposition which is a severe drawback when designing processing chains. Therefore, a 
new description for multipolarized SAR data based on the well-known Kennaugh matrix was developed that enables the uniform 
description and processing of SAR data independent of its polarisation by separating backscattering strength from polarimetric 
information. This mathematical approach subsequently is extended to the processing of multitemporal SAR data in order to stabilize 
the polarimetric information over longer periods of time and to enhance temporal changes in the polarimetric backscattering. 
Because of the high sensitivity of the Kennaugh elements a novel multilooking technique based on the Gaussian pyramid is used that 
locally adapts the look factor and thus selects the optimal balance between radiometric accuracy and geometric resolution. This 
methodology is applied to two dual-co-polarized TerraSAR-X acquisitions over the RAMSAR testsite "Upper Rhine" in order to 
generate value-added products that help to map land cover and land cover changes in consequence of water level changes. The first 
results are very promising although the interpretation of the observed polarimetric changes is not yet validated. The aim of this paper 
is to present a further application of the (Differential) Kennaugh matrix which will be the kernel of a polarimetry and change 
detection processor to be implemented in the coming years. 
This paper addresses the use of TerraSAR-X data in order to 
determine the extent and the spatial as well as the temporal 
variability of open water and flooded vegetation surfaces in 
1.1 Background 
Previous studies use either single- or quad-polarized (Brisco, 
2011) data for wetland mapping. Single-polarized data is 
always available because it is still the standard mode of the 
common space-borne SAR sensors. Open water surfaces can be 
identified in SAR images by their low backscattering like it is 
done generating the water indication mask of the TanDEM-X 
mission (Wendleder, 2011). Flooded vegetation is expected to 
show a strong double-bounce backscattering. Unfortunately, the 
backscattering mechanism cannot be identified in single- 
polarized data sets. Therefore, the extent of flooded vegetation 
areas can only be estimated by their stronger backscattering in 
each (co-pol) channel (Hess, 1990). Quad-pol data enables the 
identification of several backscattering mechanisms. But, quad- 
pol data is not easy to acquire. Most SAR satellite sensors do 
not support the polarimetric mode at any user-defined time. As 
the quad-pol mode always effects a reduction of the imaged 
area and the resolution as well, this method is not suitable yet 
for large-area applications. Hence, we focus on dual-co- 
polarized data. The combination of the complex HH and VV 
allows the identification of two scattering mechanisms: surface 
and double-bounce. The reduction of image size and resolution 
is lower than using quad-pol data. Additionally, dual-co- 
polarized data is available at any time and in almost any mode 
with TerraSAR-X and several other sensors. 
1.2 Methodology 
The two complex layers of the TerraSAR-X dual-polarized SSC 
product cannot directly be interpreted and have to be 
decomposed and geocoded. As all common polarimetric 
decompositions only concern quad-polarized data, a new 
Kennaugh matrix like decomposition has been developed 
(Schmitt, 2012). The decomposition produces four layers. The 
first comprises the total intensity of both layers, which is very 
low over open water surfaces. The second layer gives the 
intensity difference between surface and double-bounce 
scattering. Thus, high values in the second layer indicate a 
dominant double-bounce scattering while low values indicate a 
dominant surface scattering. The third layer includes the co-pol 
diattenuation which is represented by the intensity difference 
between the two measured channels. The fourth layer is 
negligible only holding complementary correlation information. 
All layers are normalized by the total intensity or the so-called 
Hyperbolic-Tangent-Normalization respectively so that their 
value range is reduced to ]-1,1[. Having a closed range the 
values can be saved in an integer format without clipping 
information at the upper or lower end. The chosen bit depth 
then only fixes the sampling which reaches its maximum near 
the centre and decreases towards higher deviations in both 
negative and positive direction. For display and interpretation 
reasons the Hyperbolic-Tangent-Scaling of Kennaugh elements 
can directly be transferred to the common unit decibel via the 
Inverse Hyperbolic Tangent function. The first layer represents 
the total intensity independent of the polarization state while the 
other layers represent the polarized contribution to the total 
intensity. Hence, intensity and polarimetric information can be 
evaluated separately. 

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