In this study MERIS remote sensing reflectance at sea surface for case 1 waters are validated with in-situ measurements. The in-situ data sets are collected during three ship cruises across the Atlantic Ocean, two with RV Polarstern cruises (November 2007 and April / May 2008) and one with RV Merian (July / August 2008) and taken with a system of three TriOS-RAMSES hyperspectral spectroradiometers, that measure above-water upwelling radiance, downwelling irradiance and sky radiance from which the water leaving remote sensing reflectance is calculated. Results help to evaluate the atmospheric correction applied to MERIS case-1 data and are used to interpret the comparisons of in-situ measured chla and MERIS algal-1 chl a .