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Team: SAR Topography

Introduction:

Since 1991 various kinds of operational radar systems have been available for earth observation. In contrast to optical systems, which passively record the sunlight reflected from the surface of the earth, radar is an active system which emits signals and then recaptures them when they are reflected back by some object. This makes it possible for radar to function independently of the time of day and even to penetrate clouds, so that images of an area can be secured for whatever time period is desired.
In order to obtain images with high ground resolution, so-called SAR systems (SAR= Synthetic Aperture Radar) are employed. These instruments illuminate the required location on the ground several times while flying over it. These separate sets of signals are then relayed to ground stations where the data is processed to yield a single image.

The character of the returning radar signal is determined by what reception system is employed and by the nature of the ground surface which was illuminated. The parameters of radar systems are the carrier frequency (f) or the wavelength (λ), the polarization, and the viewing angle used to observe the earth. By polarization the direction of oscillation of the emitted and received radar signals is meant. The reflection from the earth’s surface is influenced by the electrical material properties, the surface roughness, and the moisture of whatever is illuminated. In satellite remote sensing X band (λ = 2.4 – 4.5 cm; f = 7 - 12 GHz), C band (λ = 4.5 – 7.5 cm; f = 4 - 7 GHz) and L band (λ = 15 – 30 cm; f = 1 -2 GHz) are mainly used.

Processing SAR satellite data to obtain geoinformation products:

A number of processing steps are required to derive geoinformation products from SAR data, and the SAR Topography team is involved in their development, implementation and to some extent the operational consequences. These include

• Preprocessing to standardized SAR images (SAR processing)
• Orthorectification to generate image maps without terrain distortions
• Radiometric correction (calibration)
• Texture analysis and image segmentation to improve image classification
• Polarimetric analysis of the raw data to improve recording of the backscatter
• Analysis of changes which took place in the interval between different measurements (change detection)
• Implementing procedures for operational systems (optimizing throughput, following programming standards, integration in an efficient IT infrastructure, linking to the DLR-DFD data and information management system).

Besides technical realization, the development of geoinformation products requires close interdisciplinary cooperation to guide and support the SAR Topography team with expertise in specialized areas.

Since terrain relief plays a decisive role in the imaging process as well as when analyzing recorded radar data, the team manages an operational databank to store, analyze and process digital elevation models.

Using radar interferometry and radargrammetric stereo analysis methodologies it is possible to derive elevation models from SAR data. As part of the TanDEM-X mission, elevation models generated interferometrically from individual images are calibrated and combined to wide coverage data sets.