Radar Remote Sensing
Radar remote sensing is a method of remote sensing that uses radar systems to detect and analyze the Earth's surface and other objects from a distance. Microwave or radio waves are emitted from a sensor, reflected by the surface, and then received back by the sensor. This technique makes it possible to obtain information about the structure, composition, and movement of objects.
SAR technology
A satellite-based radar with synthetic aperture (Synthetic Aperture Radar, SAR) scans the Earth's surface using microwave radiation at different wavelengths. The SAR antenna transmits microwave pulses and receives the echoes scattered back from the Earth's surface. These can be used to produce high-resolution image data and data products.
SAR technology has several special features compared to other remote sensing imaging techniques. On the one hand, it is possible to observe the Earth's surface even when it is cloudy or dark. On the other hand, physical measurements such as the backscatter coefficient or distance differences in the direction of the SAR antenna can be derived. The backscatter coefficients depend on the physical properties of the observed surfaces. These are surface roughness, geometry, and dielectric properties, primarily soil moisture. Another special feature of radar remote sensing is the penetration depth or penetration of loose sediments and vegetation depending on the wavelength used (further information: EO college). For geoscientific remote sensing, the detection of surface roughness is an important parameter and is used in conjunction with optical remote sensing for geological mapping. The ability to penetrate loose sediment layers is an important parameter for the detection of tectonic structures. Similarly, the penetration of vegetation is an important characteristic for geological and tectonic mapping. Object detection based on radar data is particularly important for numerous geoscientific questions in regions with frequent cloud cover. Examples include the detection of high-resolution surface changes in the context of geohazards, monitoring of (illegal) mining activities, and associated settlement activities.
The determination of distance differences is based on the superimposition of the phase positions of two SAR images (Figure 1). This so-called “interferometric phase” allows a digital terrain model to be derived using two images from different angles or, in the case of a large number of time-shifted images, movement rates and time series of the movement to be extracted (Figure 2).
SAR interferometry is used in remote sensing to measure surface movements over large areas and thus support hazard analysis and spatial planning. The methods “persistent scatterer interferometry” and/or “small baseline subset” are used for this purpose (further information: Feretti 2014). The result for each long-term stable scatterer is the average velocity over the entire evaluation period and time series of the movement. For quality assurance purposes, the extracted surface movements are validated by terrain surveys, statistical analysis with terrestrial survey data, and visual image interpretation based on spatially very high-resolution orthorectified optical image data. Finally, the validated data products are overlaid with geodata and interpreted geoscientifically.
Advantages
Some advantages of radar remote sensing are:
- Weather independence: Unlike optical remote sensing, radar also works in cloudy conditions, fog, or at night.
- Penetration of certain materials: Depending on the wavelength, radar can penetrate vegetation, snow, or even the ground.
- Detection of movements: Using special techniques such as Interferometrischen SAR (InSAR), radar can detect tiny changes in the height of the Earth's surface, e.g., caused by earthquakes or ground subsidence.
Radar systems
Well-known radar systems include the European Sentinel-1 and the German TerraSAR-X.
Projects
Implementation of the Ground Motion Service Deutschland (BBD) (in German only)
Helmholtz Alliance: Remote Sensing and Earth System Dynamics. Semarang/Indonesien
Project contributions
Extraction of settlement areas based on data from the TerraSAR-X radar satellite