By tomography of GNSS slant delay data the refractivity fields of the atmosphere can be reconstructed. The resolution of the tomography field is about 40 km in horizontal and several 100 m in vertical direction. In 2009 about 270 GNSS stations were available in Germany. The tomographically reconstructed humidity fields have to be validated with other observation methods which are able to detect the refractivity profiles of the atmosphere. The data used to compare is from radiosonde soundings, by which the refractivity can be calculated. The first results of the comparison of GNSS data with radiosonde data from two radiosonde stations in Germany are shown.