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One important application field of radar remote sensing is the interpretation of SAR data in agricultural areas. The backscatter coefficient from land surfaces is primarily determined by structural and dielectric attributes of the target. Diurnal variations of the backscattering coefficient mainly results from changes in soil moisture, plant water content or the occurrence of free vegetation water on the plant surface due to dew and intercepted rainfall. Besides this, geometric variations must be considered, e.g. wind effects. For orbit planning issues, such as for the German TerraSAR Project, the effect of surface wetness on crop recognition is a crucial issue and initiated the study. Due to the scheduled orbit parameter the overflight time of TerraSAR-X over Germany will take place in the early morning hours. Consequently, the acquired SAR images will be often affected by the presence of dew or intercepted rainfall. Until now only a few studies have been published regarding the effect of plant surface wetness on the radar signal, whereby most investigations referred to forest or the crop type wheat. In summary it could be stated, that most examinations documented an increase in the received radar signal under wet conditions by 1 – 4 dB. The influence varies with frequency, polarization and vegetation structure. The underlying mechanisms causing the increased backscatter signal are still poorly understood. Regarding the thematic interpretation of SAR data acquired under wet conditions inconsistencies in the literature exist. Due to missing experimental data, it was impossible until now to draw a general conclusion of the effect of dew and interception on the radar backscatter as well as on the thematic analysis of affected SAR data in regard to different frequencies, polarizations and crop types. To contribute towards a better understanding of this problem, the TerraDew project was initiated. It is funded by the German Ministry of Education and Research (BMBF, FKZ 50EE0035) and focuses on the quantitative and systematic evaluation and modelling of the effects of free vegetation water on multifrequency and polarimetric radar data under special consideration of different crop types. |
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