The use of satellite radar remote sensing in Catalonia

Most of the civil applications of remote sensing technology in Catalonia use optical satellite imagery. However, interest is growing in radar satellite measurements, in some cases to enhance the existing optical information but also totally new applications arise. Most of them are related to the monitoring of the part of the Mediterranean Sea that affects the Catalonian “costas”, but also some land applications are being given a chance as for example the observation of soil moisture to optimize water use in agriculture.

A major advantage of using radar systems compared to optical imagery is that the measurements are independent of the meteorological conditions, especially with the low frequencies. Besides, during the night measurements can be realized as it is an active remote sensing technique. Active microwave signals are sensitive to the dielectric properties of a medium and to its geometrical structure at the surface and in the volume. Single channel radars, operating at a given frequency and polarization, as well as polarimetric radars are in use.

Spaceborne radar systems can be separated in the following categories: altimeters, scatterometers, rain radars and imaging radars. Altimeters are used for accurate surface height measurements of for example the ocean, lakes or ice surfaces. Scatterometers measure accurately the surface backscatter across a swath of several hundred km width, but with a low spatial resolution, mostly used for the monitoring of wind velocity over the oceans. Rain radars are a type of scatterometers optimized for measuring backscatter from water drops and ice particles with a horizontal resolution of a few kilometres.

However, most of the applications use data from imaging radars with Synthetic Aperture Radar (SAR) technology. These systems provide surface images with high spatial resolution (between about 10 m and 100 m) over a swath of about 100 km to a few hundred km width. Examples are the ASAR sensor on the ENVISAT platform and the small commercial satellite TerraSAR-X. These days in Catalonia this type of imagery is used for a number of services related to the water quality of the Mediterranean Sea, as oil spill and polluter identification, detection of algal bloom and other water quality indicators.

SAR interferometry (InSAR) is another technique that is used for civil applications in Catalonia. This technique, called “differential interferometry”, is used to detect very small changes in land topography as displacements of the order of fractions of a wavelength can be distinguished. Most of the local applications can be found in the petroleum industry and the monitoring of infrastructures, especially in Barcelona. On a global scale this technique is also used for the detection of glacier motions and mapping of seismic deformation.

The use of radar polarimetry offers improvements compared to single channel SARs for the quantitative inversion of target properties and for classifications. For example, polarimetric measurements enable the separation of surface roughness and moisture effects and is therefore of interest for soil moisture monitoring. Multi-polarized imagery contains a lot of information of which only a small part is understood these days and related to the processes and state variable that lay behind. The commercial sector is scheduling more and more SAR programs for the near future. The growing amount of data available will certainly unravel more of the secrets behind multi-polarized imagery and lead soon to many new applications.

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