To mitigate atmospheric errors caused by the troposphere, which is a limiting error source for spaceborne interferometric synthetic aperture radar (InSAR) imaging, a tropospheric correction method has been developed using data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the Global Positioning System (GPS).

The Original InSAR Image (left), the GPS ZTD Difference Correction Map (center), and the corrected InSAR Image (right). The black dots in the middle image correspond to the locations of the GPS stations.
The ECMWF data was interpolated using a Stretched Boundary Layer Model (SBLM), and ground-based GPS estimates of the tropospheric delay from the Southern California Integrated GPS Network were interpolated using modified Gaussian and inverse distance weighted interpolations. The resulting Zenith Total Delay (ZTD) correction maps have been evaluated, both separately and using a combination of the two data sets, for three short-interval InSAR pairs from Envisat during 2006 on an area stretching from northeast from the Los Angeles basin towards Death Valley. Results show that the root mean square (rms) in the InSAR images was greatly reduced, meaning a significant reduction in the atmospheric noise of up to 32 percent. However, for some of the images, the rms increased and large errors remained after applying the tropospheric correction. The residuals showed a constant gradient over the area, suggesting that a remaining orbit error from Envisat was present. The orbit reprocessing in ROI_pac and the plane fitting both require that the only remaining error in the InSAR image be the orbit error. If this is not fulfilled, the correction can be made anyway, but it will be done using all remaining errors assuming them to be orbit errors. By correcting for tropospheric noise, the biggest error source is removed, and the orbit error becomes apparent and can be corrected for.

After reprocessing the InSAR images using re-estimated satellite orbits, the overall rms reduction (using both tropospheric and orbit correction) spanned from 15 to 68 percent. With this tropospheric correction, low-frequency errors can be removed from InSAR images. Additionally, results show that for days with high-quality ECMWF data, the SBLM ZTD correction performs as well as the GPS ZTD correction. Finally, the tropospheric correction enabled orbit correction, and by correcting for both errors, the quality of the InSAR images increased significantly.

By correcting for the troposphere, other errors become visible. The main contributor to the remaining errors is uncertainties with determining the satellite orbit. Because the orbit error is now separated from the tropospheric error, the orbit can be corrected for more accurately.

This work was done by Frank H. Webb, Evan F. Fishbein, Angelyn W. Moore, Susan E. Owen, Eric J. Fielding, and Stephanie L. Granger of Caltech and Fredrik Björndahl and Johan Löfgren of Chalmers University of Technology for NASA’s Jet Propulsion Laboratory. NPO-46918

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Tropospheric Correction for InSAR Using Interpolated ECMWF Data and GPS Zenith Total Delay

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This article first appeared in the August, 2011 issue of NASA Tech Briefs Magazine.

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