Image Fusion of High Resolution TerraSAR-X and Multispectral Electro-Optical Data for Improved Spatial Resolution

Image fusion is a technique that is used to combine the spatial structure of a high resolution panchromatic image with the spectral information of a low resolution multispectral image to produce a high resolution multispectral image. In this study, first results of the attempt to fuse image data of TerraSAR-X with optical multispectral data will be published. TerraSAR-X is a new German Radar satellite which was launched in June 2007. The satellite is equipped with an advanced high-resolution X-Band Synthetic Aperture Radar. It combines the ability to acquire high resolution images for detailed analysis as well as wide swath images for overview applications. In the spotlight mode, the satellite is able to record data with one meter spatial resolution. First test data are already available for scientific investigators. Using TerraSAR-X data as a panchromatic input for pansharpening is a difficult task, due to speckle noise inherent in the Radar images.
To generate fused images, use was made of the Ehlers fusion, a fusion technique that was developed for preserving maximum spectral information. It has already proven its superiority over standard pansharpening techniques such as IHS, PC, Brovey and multiplicative fusion methods. The Ehlers fusion is based on an IHS Transformation combined with filtering in the Fourier domain and was developed for fusing panchromatic and multispectral electro-optical data. The IHS transform is used for optimal color separation. This technique is extended to include more than 3 bands by using multiple IHS transforms until the number of bands is exhausted. As the spectral characteristics of the multispectral image bands are preserved during the fusion process, there is no dependency on the selection or order of bands for the IHS transform.
A subsequent Fourier transform of the intensity component and the panchromatic image allows an adaptive filter design in the frequency domain. Using fast Fourier transform (FFT) techniques, the frequency (spatial) components to be enhanced or suppressed can be selected directly. The intensity spectrum is filtered with a low pass filter (LP) whereas the panchromatic spectrum is filtered with an inverse high pass filter (HP). After filtering, the images are transformed back into the spatial domain using an inverse FFT and added together to form a fused intensity component. This fused intensity exhibits the low-frequency information from the low resolution multispectral image and the high-frequency information from the high resolution panchromatic image. The new intensity component and the original hue and saturation components of the multispectral image form a new IHS image. As the last step, an inverse IHS transformation produces a fused RGB image that contains the spatial resolution of the panchromatic image and the spectral characteristics of the multispectral image. The order of bands and the inclusion of spectral bands for more than one IHS transform are not critical because spectral information is preserved during this process.
So far filtering within the Ehlers fusion has been done with isotropic high pass filters. Due to the high frequency noise level in Radar data, however, these filters are not sufficient. Therefore the filter design procedure in the frequency domain has to be adapted to be applicable to the TerraSAR-X data. A precedent spatial domain filtering with noise suppression filters such as Frost, Lee, or median filter before the frequency domain filtering has shown better results. Other test series used different band pass filters to cope with the high frequency speckle noise. The Ehlers Fusion was then modified to integrate radar data with optical data. The fused images were first visually compared. Each band of the fused image was compared to the appropriate original multispectral band for preservation of the spectral characteristics. Then, the identical band combinations of the fused and original images were compared, such as true color or false color infrared combination. Especially those areas that show different reflections and structures in the original multispectral image were selected. For the spatial resolution evaluation each band of the fused images was compared to the high resolution TerraSAR-X image. Particular attention was paid to the changes of contrast and gray values near edges like streets and buildings. Due to the fact that visual comparison is very subjective; the fused images were also evaluated using quantitative and statistical methods. Initial results will be presented at the EARSeL Workshop.