CONCLUSION - The impact of the fringe wash effects on the radiometric sensitivity as well as on the spatialresolution of the SMOS instrument has been estimated using two regularized reconstruction methods. Thesetwo approaches lead to similar results: provided the modelling of the fringe washing function is well-known andthe reconstruction method is regularized, the influence of the spatial decorrelation effects are mitigated and thepropagation of input noise is still under control. This remains true even with the digital correction proposed byFishman et al. Finally, a hardware modification of the SMOS instrument is not necessary with regards to thefringe wash effects.

REFERENCESM. A. FISHMAN et al., “How digital correlation affects thefringe washing function in L-band aperture synthesisradiometry’’, IEEE TGRS, 40(3), pp.671-679, 2002.E. ANTERRIEU, “Stabilized image reconstruction algorithm forsynthetic 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 atlow microwave frequencies. Within this context, the European Space Agency (ESA) iscurrently 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 inverseFourier transform reconstruction have shown a large degradation of the spatialresolution as well as of the signal to noise ratio (SNR). In order to reduce the fringewashing 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 dimensionsof the instrument lead to a weaker influence of the fringe wash phenomenon on theimage reconstruction. Furthermore, regularized reconstruction methods involving themodelling G-matrix allow to improve the quality of the reconstruction even when thesignal is blurred by a radiometric noise. It is concluded that a hardware modification ofthe SMOS instrument is not necessary with regards to the fringe wash effects.

ACKNOWLEDGMENTSThis work is supported by ESA, CNES, CNRSand the Région Midi-Pyrénées.

ON THE SPATIAL RESOLUTION AND ON THE RADIOMETRIC SENSITIVITYOF THE SMOS INSTRUMENT

Bruno PICARD(1), Eric ANTERRIEU(1), Gérard CAUDAL(2) and Philippe WALDTEUFEL(3)

(1)CERFACS42 avenue Gaspard Coriolis31057 Toulouse – FRANCE

(3)IPSL-SAB.P. 3

91371 Verrières le Buisson – France

(2)IPSL-CETP10-12 avenue de l’Europe

78140 Vélizy – France

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IMPACT OF THE FRINGE WASHING FUNCTION

SimulationsThe mean values computed here for arealistic instrument with the regularizedreconstruction methods are identical tothose obtained for an ideal instrument witha simple Fourier transform.

The fringe washing function does notdegrade the performances of the instrumentif the appropriate modelling is used, evenin the case of the digital correction.

Results

for the baselineleading to the worthdecorrelation effectsfor SMOS

time lag t [ns]

INSTRUMENT MODELLING REGULARIZED RECONSTRUCTIONS

physicalregularization

band-limited solution:

mathematicalregularization

minimum norm solution:

is the modelling operator is the experimental frequency coverage is the FOURIER 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 devicesthe objective is to restore: