with the UB product with a reduced mean difference and RMSD. When the SMOS products are compared only against the SEM data they both agree within 1 cm with an observed mean thickness of 17 cm along the ship track of Lance.However,Lance is not an icebreaker and it s ability to navigate through the ice is limited to relatively thin ice to up to about half a meter level ice thickness. Pressure ridges appear on a very local scale and frequently resulted in an interruption of the route validation. The validation is further complicated because of the manual optimization by the helmsmen who preferably navigated through open leads (Fig. 4d). The very local navigation and the different location of the thickness transects is a likely cause for the mean difference between the (thicker) airborne ALS and HEM data in comparison to the (thinner)HEM measurements. The SEM measurements are neither correlated to HEM or ALS nor to SMOS thickness data. However, the SEM instrument provided accurate thickness measurements which were in very good agreement with visual estimates of the level ice thickness.