There are various approaches to downscale SMOS data (or any of the passive sensors, for that matter). It depends on the resolution you want to achieve. For example, Dumedah et al (2014) showed that you could just use the SMOS L2 Tb data without much of a problem at its ~15km resolution, that should be transferable to soil moisture, too.

There are also physical and empirical downscaling algorithms, such as those of Merlin et al. and Piles et al. Both produce 1km soil moisture products using various forms of MODIS data to calculate the spatial variability within a L3 pixel (note: not L2, but that should not matter), making use of the link between thermal emissions and soil moisture.

Check out the references:

Dumedah, G., Walker, J. P., and Rüdiger, C., 2014. Can SMOS Data be Used Directly on the 15km Discrete Global Grid?, IEEE Transaction on Geoscience and Remote Sensing, 52(5): 2538-2544

O. Merlin, M. J. Escorihuela, M. A. Mayoral, O. Hagolle, A. Al Bitar, and Y. Kerr, “Self-calibrated evaporation-based disaggregation of SMOS soil moisture: An evaluation study at 3km and 100m resolution in Catalunya, Spain,” Remote Sens. Environ., vol. 130, pp. 25–38

Piles, M., Sanchez, N., Vall-llossera, M., Camps, A., Martínez-Fernández, J., Martínez, J., & González-Gambau, V. (2014). A dowscaling approach for SMOS land observations: long-term evaluation of high resolution soil moisture maps over the Iberian Peninsula. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7, 3845-3857

There are various approaches to downscale SMOS data (or any of the passive sensors, for that matter). It depends on the resolution you want to achieve. For example, Dumedah et al (2014) showed that you could just use the SMOS L2 Tb data without much of a problem at its ~15km resolution, that should be transferable to soil moisture, too.

There are also physical and empirical downscaling algorithms, such as those of Merlin et al. and Piles et al. Both produce 1km soil moisture products using various forms of MODIS data to calculate the spatial variability within a L3 pixel (note: not L2, but that should not matter), making use of the link between thermal emissions and soil moisture.

Check out the references:

Dumedah, G., Walker, J. P., and Rüdiger, C., 2014. Can SMOS Data be Used Directly on the 15km Discrete Global Grid?, IEEE Transaction on Geoscience and Remote Sensing, 52(5): 2538-2544

O. Merlin, M. J. Escorihuela, M. A. Mayoral, O. Hagolle, A. Al Bitar, and Y. Kerr, “Self-calibrated evaporation-based disaggregation of SMOS soil moisture: An evaluation study at 3km and 100m resolution in Catalunya, Spain,” Remote Sens. Environ., vol. 130, pp. 25–38

Piles, M., Sanchez, N., Vall-llossera, M., Camps, A., Martínez-Fernández, J., Martínez, J., & González-Gambau, V. (2014). A dowscaling approach for SMOS land observations: long-term evaluation of high resolution soil moisture maps over the Iberian Peninsula. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 7, 3845-3857

I forgot to add that the 1km products will become available later this year, both through the French (CATDS) and Spanish portals (CP34-BEC). No need to process the data yourself, unless you are in a rush.

Hello Christoph, now we are in August and there are no news about the 1km products at GLOBAL scale, Do you have news about it?

Thanks

Ferran