Abstract
Multipath interference can occur in ground-based radar data acquired with systems with a large antenna beam width in elevation in an upward looking geometry, where the observation area and the radar are separated by a reflective surface. Radiation reflected at this surface forms a coherent overlay with the direct image of the observation area and appears as a fringe-like pattern in the data. This deteriorates the phase and intensity data and therefore can pose a considerable disadvantage to many ground-based radar measurement campaigns. This poses a problem for physical parameter retrieval from backscatter intensity and polarimetric data, absolute and relative calibration on corner reflectors, the generation of digital elevation models from interferograms and in the case of a variable reflective surface, differential interferometry. The main parameters controlling the interference pattern are the vertical distance between the radar antennas and the reflective surface, and the reflectivity of this surface. We used datasets acquired in two different locations under changing conditions as well as a model to constrain and fully understand the phenomenon. To avoid data deterioration in test sites prone to multipath interference, we tested a shielding of the antennas preventing the radar waves from illuminating the reflective surface. In our experiment, this strongly reduced but did not completely prevent the interference. We therefore recommend avoiding measurement geometries prone to multipath interferences.
Highlights
Ground-based radars increasingly gain importance in various geoscientific fields
We showed that multipath interferences in radar images can be caused by the superposition of direct waves and indirect waves that have been reflected at a surface between radar and observation area
The resulting interference pattern appear in all orders of statistical signal evaluation: in intensity images, coherence and phase of all interferometric and polarimetric covariances and in interferometry derived digital elevation models (DEMs) differences
Summary
Ground-based radars increasingly gain importance in various geoscientific fields. With their ability to measure with a high temporal resolution in the order of minutes, they have a crucial advantage to satellite-borne radars, which have return times of several days. High measurement frequencies make ground-based radars ideal for the monitoring of fast moving and decorrelating surfaces. Many ground-based radar campaigns focus on the monitoring of regions prone to landslides and rockfall, e.g., [1,2,3,4,5,6,7,8,9]. In [20,21], reviews of recent ground-based radar systems and their applications are provided. Many of the aforementioned applications could be affected by multipath interferences (MPI) presented here if the measurements are performed in an unfortunate geometry
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