Access to exact solar irradiation data is indispensable for planning and dimensioning of applications as e.g. solar power plants. The expectable amount of yearly solar irradiation has an over-proportional impact on the financing and therefore has to be known very precisely. For this reason, DLR performs investigations at PSA to improve the accuracy of irradiation data, which are measured at several locations in southern Spain and Morocco. Devices as Rotating Shadowband Pyranometers (RSP) as well as thermal pyranometers and a pyrheliometer are used. Besides well-documented and known technical aberrations, soiling of the sensors is an important source for underestimation of the measured irradiation, especially at offside stations where daily maintenance is not possible. In this paper, we present soiling characteristics of these sensors and a corresponding method for its correction.