Validation of solar altitude-dependent measured values of several measurement channels. Measured values of the measurement channels that take the solar altitude into consideration are stored. Stored ratios of the stored measured values for all solar altitudes are calculated and stored. An expected ratio is determined by calculation of a frequency distribution over all stored ratios A current ratio of a currently measured measured value to the current measured values of the other measurement channels are calculated The currently measured measured values are validated. If the expected ratio corresponds to the stored ratio and the stored ratio corresponds to the currently ratio, the current measured value is valid. If the expected ratio corresponds to the stored ratio and the stored ratio deviates from the current ratio, a disturbance is present. If the expected ratio deviates from the stored ratio, the validity for this measured value is not evaluated.