A method of operating a radar sensor system that is configured to determine range and velocity information from radar waves reflected by a scene in an interior of a vehicle for vital sign detection. The method includes steps to decompose reflected and received signals into range and velocity information, to measure the movement over time in specified range gates and to evaluate the similarities between them. Based on the characteristics of similar behaving range bins, it can be decided whether any detected movement is related to an internal or external disturbance or by vital signs.