Browsing by Author "Cao, Wenxi"

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Open Access
Change detection using SAR data
(2013) Cao, Wenxi
The objective of this thesis is to find changes caused by natural disaster from two co-registered calibrated TerraSAR-X images. Three methods are used in this thesis. The first method, histogram thresholding, uses the histogram of the SAR intensity ratio image to classify the ratio image into three classes. This technique was originally proposed by Kittler et al. (1986) and modified by Bazi et al. (2005) and Moser et al. (2006) based on the Bayesian formula. In this thesis their methods are combined together to detect three classes. The relative difference of the cost function is used to detect the number of the classes instead of the determinant of the Hessian matrix suggested by Bazi et al. (2005). The second method formulates the classification problem as a hypothesis testing problem. This idea was originally used by Touzi et al. (1988) and Oliver et al. (1996). In this thesis the analytical method by Touzi et al. (1988) is replaced by using the properties of the Gamma distribution. The third method, graph-cut algorithm, is a post-processing method, which improves classification results from the first and second methods. The provement is equivalent to the global optimization of an energy function in a Markov random field (MRF). A modern method proposed by Kolmogorov et al. (2004) and Boykov et al. (2004) is used in this thesis. This method transforms the energy function of a MRF into an equivalent graph and solves the global optimization problem using a max-flow/min-cut algorithm. These three methods are applied to the test data on Queensland, Australia, and Leipzig, Germany. Most SAR ratio images can be classified into three classes successfully. The remaining problem is that the interpretation of the changed classes is still ambiguous. Other data sources should be combined to assist or improve the interpretation of the detected change.
Open Access
Literature analysis of SWOT mission from geodetic perspective
(2012) Cao, Wenxi
Satellite radar altimeter has been used for nearly twenty years to observe the variety of the global ocean surface topography. It has advanced our understanding of global ocean circulation and sea level change. However the conventional radar altimeter can not resolve the submesoscale features in the oceans because of its large spacing between satellite ground tracks and coarse ground resolution. On the other hand altimetry technique is expected to be able to observe large rivers, lakes and monitor the storage of freshwater on land. These new challenges require a new technique and a new mission. In 2016 a satellite mission called Surface Water and Ocean Topography (SWOT) will be launched by NASA and CNES according to plan. This term paper will summarize the general measurement principle, orbit design issues and applications of SWOT in the literature.