On the impact of rockfall catch fences on ground-based radar interferometry

Rockfalls are major natural hazards in mountainous regions and as such monitored if they pose a high risk to people or infrastructure. Ground-based radar interferometry is a relatively new technique suitable for such monitoring. It offers the potential for determining sub-mm- to mm-level displacements by remote measurements under various weather conditions. To avoid damage from smaller rocks and debris, critical surfaces are often protected by rockfall catch fences. We present an experimental investigation proving that the radar measurements are indeed significantly affected by a catch fence made of steel even if its mesh size is larger compared to that of the wavelength of the radar. A stable rock wall in a quarry was monitored by means of a ground-based synthetic aperture radar for 2 days. Different fences varying in shape, size, and density of mesh and in various geometrical configurations were erected at different locations for short periods of time during the experiment. We show that for surfaces observed through the fence, the reflected power can be reduced by 20 dB and thus the signal-to-noise ratio is significantly deteriorated. We also observed spurious interferometric phase shifts. Even parts of the rock wall not covered by the fences are affected. Side lobes and mixed pixels result, e.g., in severe loss of coherence and thus potentially mask actual displacements.