Radar remote sensing

A satellite-based synthetic aperture radar (Synthetic Aperture Radar, SAR) scans the earth's surface by means of microwave radiation. The SAR antenna transmits microwave pulses and receives the backscattered echo from the surface. Based on this echoes high-resolution image data and data products can be produced.
The SAR technology has several special features in comparison to other imaging remote sensing methods. On the one hand, it is possible to observe the Earth's surface even on cloudy days and at night. On the other hand physical variables such as the backscattering coefficient or path length differences in the viewing direction of the SAR antenna can be derived. The backscattering coefficient depends on the physical properties of the observed surfaces. The main properties are surface roughness, geometry and dielectric properties, primarily affected by surface moisture. Another important feature of radar remote sensing is the penetration depth, respectively the penetration of loose sediments and vegetation in relation to the used wavelength (more information: EO college). For geoscientific remote sensing the measurement of surface roughness is an important parameter and therefore it is used together with optical remote sensing techniques for geologic mapping. The capability to penetrate through loose sediments is an important parameter for mapping tectonic structures. The capability to penetrate through vegetation is an important characteristic for geologic and tectonic mapping. Object recognition based on radar data is of particular significance for geoscientific applications in regions with frequent cloud coverage. E.g. the detection of surface changes with high temporal resolution, amongst others within the context of geohazards, monitoring of (illegal) mining activities attended with settlement activities.
The determination of relative path length differences is based on the superposition of the phase of two SAR images (figure 1). This so-called "interferometric phase" can be used to derive a digital elevation model or to monitor surface displacements over time (figure 2).

Fig 1: Acquisition geometry of a radar system (modified after DLR 2015) Source: https://saredu.dlr.de/

Fig 2: Relation between interferometric phase (Φ), topographic phase (Φ_topo) and differential phase (Φ_diff) (modified after DLR 2015) Source: https://saredu.dlr.de/

Within the BGR remote sensing section SAR interferometry is used to monitor surface displacements over large areas and thus assist hazard analysis and spatial planning. For this purpose, the methods "Persistent Scatterer Interferometry" and/or "Small Baseline Subset" are applied (more information: Feretti 2014). The results are the mean velocity over the entire acquisition time span and a displacement time series for each long-term coherent scatterer.
For quality assurance, the detected surface displacements are verified by field surveys and statistical analysis with independent datasets (e.g. continuous GNSS time series). Finally, the validated data products are overlaid with geoscientific datasets to interpret the observed displacement phenomenon.

PSI Source: BGR