1. CONCLUSION - The impact of the fringe wash effects on the radiometric sensitivity as well as on the spatial resolution of the SMOS instrument has been estimated using two regularized reconstruction methods. These two approaches lead to similar results: provided the modelling of the fringe washing function is well-known and the reconstruction method is regularized, the influence of the spatial decorrelation effects are mitigated and the propagation of input noise is still under control. This remains true even with the digital correction proposed by Fishman et al. Finally, a hardware modification of the SMOS instrument is not necessary with regards to the fringe wash effects. REFERENCES M. A. F ISHMAN et al., “ How digital correlation affects the fringe washing function in L-band aperture synthesis radiometry’’, IEEE TGRS, 40(3), pp.671-679, 2002. E. A NTERRIEU, “Stabilized image reconstruction algorithm for synthetic aperture imaging radiometers”, Proc. IGARSS’02, Toronto (Canada), pp. 1642-1644, 2002. ABSTRACT - It is now well established that Synthetic Aperture Imaging Radiometers (SAIR) promise to be powerful sensors for high-resolution observations of the Earth at low microwave frequencies. Within this context, the European Space Agency (ESA) is currently developing the SMOS space mission. A recent study has simulated fringe washing effects on a particular SAIR configuration, including the impact of coarse correlation. The results obtained with a simple inverse Fourier transform reconstruction have shown a large degradation of the spatial resolution as well as of the signal to noise ratio (SNR). In order to reduce the fringe washing effects, the authors have suggested to split the received signals into sub- signals with narrower bandwidths. In the particular frame of the SMOS space mission, the geometry and the dimensions of the instrument lead to a weaker influence of the fringe wash phenomenon on the image reconstruction. Furthermore, regularized reconstruction methods involving the modelling G-matrix allow to improve the quality of the reconstruction even when the signal is blurred by a radiometric noise. It is concluded that a hardware modification of the SMOS instrument is not necessary with regards to the fringe wash effects. ACKNOWLEDGMENTS This work is supported by ESA, CNES, CNRS and the Région Midi-Pyrénées. IMPACT OF THE FRINGE WASHING FUNCTION ON THE SPATIAL RESOLUTION AND ON THE RADIOMETRIC SENSITIVITY OF THE SMOS INSTRUMENT Bruno PICARD (1), Eric ANTERRIEU (1), Gérard CAUDAL (2) and Philippe WALDTEUFEL (3) (1) CERFACS 42 avenue Gaspard Coriolis 31057 Toulouse – FRANCE (3) IPSL-SA B.P. 3 91371 Verrières le Buisson – France (2) IPSL-CETP 10-12 avenue de l’Europe 78140 Vélizy – France distance from the center of the FOV [km] normalized brightness temperature distance from the center of the FOV [km] spatial resolution [km] instantaneous field of view (FOV) distance from the center of the FOV [km] normalized brightness temperature IMPACT OF THE FRINGE WASHING FUNCTION Simulations The mean values computed here for a realistic instrument with the regularized reconstruction methods are identical to those obtained for an ideal instrument with a simple Fourier transform. The fringe washing function does not degrade the performances of the instrument if the appropriate modelling is used, even in the case of the digital correction. Results for the baseline leading to the worth decorrelation effects for SMOS time lag t [ns] INSTRUMENT MODELLINGREGULARIZED RECONSTRUCTIONS physical regularization band-limited solution: mathematical regularization minimum norm solution: is the modelling operator is the experimental frequency coverage is the F OURIER transform operator is the projector onto the space of -band limited functions is the zero-padding operator beyond is the apodization window digital correlatorsanalog correlators The fringe washing function - accounts for spatial decorrelation effects - degrades SNR by reducing the amplitude of - maximum degradation for: - large spacing antennae (small details in FOV) - scene areas away from the center of the FOV is an ILL-POSED problem should be REGULARIZED to provide a UNIQUE and STABLE solution SAIR are band-limited imaging devices the objective is to restore: