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The launch of ERS-1, the first operational radar satellite, in 1993, has led to considerable interest in images acquired at radar wavelengths {1-50 cm) as means of gathering information about agricultural crops and their condition. The main advantage of radar systems with respect to optical satellites is that they can acquire images of the earth's surface irrespective of cloud cover, since clouds are transparent at these wavelengths. This is especially important for agricultural applications, where reliably timed images are needed throughout the growing season in order to follow continously the status of the crops as they develop.Multi-frequency polarimetric radar systems represent a convincing evolution of radar systems and their capabilities. The X-SAR/SIR-C radar-instrument, which was flown twice on the space shuttle in April and October 1994, allowed many researchers access to high quality radar images aquired from a space-borne system for the first time. Thus far, the full potential of radar has not been realized, mostly because existing radar satellites such as ERS-1 only aquire information at a single-wavelength and polarization. By restricting the collection of information to a single channel means such images often lack the extra information needed to characterize crops. However, recent studies using multifrequency-radar have shown that vegetation type, structure and condition can be better characterized when data is aquired at several wavelength and polarizations.In order to obtain significant differences in growth stages for identical crop types during the first 10-day long shuttle mission, winter barley fields were chosen along a gradient from southern Italy to southern Germany. These test fields were surveyed coincidentally with shuttle passes over them. One of the main results was that the spread in fresh biomass measured in (g m"^) between winter barley and maize fields as a function of their different growth stages, could be detected with great precision from the backscatter data. The best parameter for the measurement of winter barley and maize biomass was the ratio HH/VV at L-band. For the extraction of dry biomass in winter barley fields, the cross-polar polarization HV at C-bard showed the best results.

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