In addition to the spectral information, the texture of the surface of the mudflats also plays an important role. Information about this is provided by radar data, which measure the backscatter of an emitted signal and in doing so provides information about the roughness of the surface.

Radar data are regularly and systematically captured by satellite-based sensors.

Radar systems actively emit signals, so they are not reliant on sunlight, which means they can also be used at night and at dark times of year.

In addition, radar beams can penetrate clouds, which allows data acquisition in bad weather conditions such as heavy cloud cover or moderate rain. In the often cloudy Wadden Sea, capturing radar data – as opposed to optical satellite data – is in principle only dependent on the tide.  

The figure on the left shows a part of the German Wadden Sea captured by the European Remote-Sensing Satellite ERS-1 radar sensor. In this image tidal mudflats not covered by the sea appear darker than the sea and the mainland.

The image was taken on 26 March 1992, at 10:25 UTC, when the sky was completely cloudy.

 

With radar systems, there are also a few factors that must be considered when analysing the data:

  • There is anisotropy of the backscattered signal (angle between the receiving direction and the alignment of a structure), which influences the image properties.
  • Precise habitat mapping based on signal strength (particularly with single-frequency data) is not possible.
  • The water cover influences the signal because the radar signal cannot penetrate water. For this reason, the tides and any residual water on the mudflats must be considered.
  • The radar signal is dependent on wind direction and wind speed.
  • Areas of flow in and around the edges of the tidal channels can influence the signal.
  • When analysing multi-frequency recordings from various sensors, the time shift between the individual recordings must be taken into account.

Data from the German TerraSAR-X and TanDEM-X as well as the Canadian Radarsat-2 are used for the analysis of satellite-based SAR images from 2012 and 2013. SAR images from 2007 to 2009, which were used for the systematic study of radar signatures of dry tidal mudflats, come from the ERS-2 and ENVISAT European satellites, the German TerraSAR-X, the Japanese ALOS, and the Canadian Radarsat-2.

A radar with a synthetic aperture (Synthetic Aperture Radar, SAR) provides two-dimensional fields of the backscattered radar output ("radar images", "SAR images"), whose high spatial resolution is achieved by a complex recording technology and processing of the raw data. The spatial resolution of SAR images depends on the satellites, their recording mode and the radar band. The highest resolutions of under one metre (spatial size of a pixel in the SAR image) are achieved with the German TerraSAR-X (X-Band) in high-resolution spotlight mode. Data with this resolution are suitable for identification of fine structures in the mudflat, whereas lower resolutions (10 – 50 m) are used for large-scale monitoring and classification of sediments.

The SAR images used were made in the L (approx. 1 GHz / 30 cm), C (approx. 5 GHz / 6 cm) and X (approx. 10 GHz / 3 cm) radar bands. The different electromagnetic wavelengths allow investigation of textured structures on the surface of the mudflats at various length scales (of the magnitude of the radar wavelengths).