DLR activities on aperture synthesis radiometry

Imaging microwave radiometer systems in Earth observation from long distances are forced to use very large antenna apertures for an adequate spatial resolution following the wavelength over diameter law. In many cases the classical imaging principle of a linescanner cannot be applied because of the required movement of the antenna beam within the desired field of view (FOV). An alternative method is given by the principle of aperture synthesis from radio astronomy having the following main advantages: i) simultaneous acquisition of all image points, i.e. quasi real-time operation is possible (snapshot), ii) a wide FOV up to a complete hemisphere can be imaged, iii) available platform structures can be used for the creation of large apertures. In the ideal case the imaging operation is carried out in the spatial Fourier space, given by the baseline components or antenna distances u and v in wavelengths, using two-by-two interferometric techniques (correlation) on a thinned array of spatially distributed antennas. The spatial brightness temperature distribution T_B(l,m) to be determined, given in direction cosines l and m, is calculated by an inverse Fourier transform (FT) of the measured complex and Hermitian visibility function V(u,v). This paper describes the CLEAN algorithm