A significant cause of radiometric distortion in Synthetic Aperture Radar (SAR) images is the interaction of the radar signal with mountains, hills, and other sloping terrain features.
Radar signals travel in wave fronts that are reflected and scattered off the terrain. Signals from all objects illuminated within the same time interval correspond to a single pixel. If the terrain slopes toward the sensor, a large area reflects toward the sensor within one time interval, and its return energy is combined. When the terrain slopes away, less energy is returned within the interval. Also, the higher parts of the slope, which are closer to the sensor, are imaged sooner than the bases. These effects result in foreshortening, which produces short, bright, facing slopes slanted toward the sensor and dark back slopes stretching away from the sensor.
For steep slopes, the position of the peak in relation to the sensor may cause distortions called layover and shadowing. In layover the base of the slope is imaged by the sensor after the peak, which makes the peak appear to fall over toward the nadir. In shadowing all of the radar signal from the back slope is invisible to the sensor since the peak blocks the signal from reaching the back slope. Both distortions result in a loss of data in the affected areas.
The Radiometric Terrain Correction uses the digital elevation model (DEM) and the math model to identify the ground area contributing to each output pixel. It diffuses and stretches the return from the slopes facing the sensor (restores mean energy level) and concentrates and compresses the return from the back slopes. Therefore, the radiometric uniformity is restored to the image.
OrthoEngine converts the digital pixel values to radar backscatter values (sigma0 or sigma nought), and then corrects them for the radiometric terrain effect. The correction relies on the accuracy of DEM and math model. If the math model is inaccurate, the local shifts between the true and modeled pixel positions may over- or under-correct some pixels. Smoothed mountain peaks and valley bottoms, artifacts, and filtered out minor terrain features in the DEM can also adversely effect your results.
The Land Cover Correction is an experimental algorithm designed to compensate for the scattering of the radar signal due to the texture and composition of the surface, particularly for areas with uniform vegetation. The algorithm uses the three-dimensional local incident (incidence) angle to correct for the angular behavior of the scatter. The correction has no effect on slopes directly facing the sensor, but progressively brightens other slopes at increasing angles. The amount of Land Cover Correction is controlled by a coefficient (exponent) ranging from a real number greater than zero to less than or equal to two.
The Radiometric Terrain Correction supports the following sensors:
Note: Since ASAR images imported by older versions of CDASAR or OrthoEngine are not compatible with the Radiometric Terrain Correction, re-import the images using the Read CD-ROM data function.
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