SMOS Side Lobe Level (SLL), which is caused by the limited coverage of the measured visibility samples in the frequency domain, is responsible for non-negligible radiometric errors at pixels placed close to large brightness temperature transitions (e.g. coastline, ice-sea border, RFI, sun reflection, etc.). In order to mitigate this effect the visibility samples are tapered by means of a Blackman window before the image inversion procedure is undertaken. However, the theoretical SLL performance of such a specific taper is degraded by residual visibility calibration errors. Simulations performed in this work show that SLL performance is very sensitive to calibration errors and that the SLL performance of the rectangular window cannot be improved to a large extend, even in the case that calibration errors are constrained by very stringent requirements. Therefore, smarter inversion techniques or data handling procedures must be devised in the future to effectively deal with this issue.