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 comprising at least the following steps, which are to be executed iteratively, while illuminating the interior of the vehicle with radar waves by the radar sensor system, - receiving radar waves reflected from the scene, - removing signals that are representative of a static portion of the scene from range signals obtained from the received radar waves for deriving dynamic range signals assigned to a plurality of range bins, - select a plurality of predefined groups of range bins out of the plurality of range bins, wherein the groups are separated by at least one range bin, - retrieve real values of range signals assigned to each of the range bins of the selected predefined groups of range bins, - average the retrieved real values over a predefined period of time to form range signal buffers, - determine a Doppler frequency for each range bin of the plurality of predefined groups of range bins, - derive a statistic quantity from the determined Doppler frequencies representing a dominant frequency, characterized in that the method comprises the further steps - compare the dominant frequency with at least one predetermined condition concerning a predefined characteristic vital sign frequency (Th f), - if the dominant frequency fulfills all predetermined conditions, calculate a plurality of correlation coefficients, one correlation coefficient for each possible pair of range signal buffers, - group the correlation coefficients into at least two groups depending on the range bins to which the range signals of each possible pair of range signal buffers have been assigned, - compare each correlation coefficient with a group-specific threshold value (Th cc) and assign predetermined numerical values to each correlation coefficient depending on the outcome of the comparison, - for each group of correlation coefficients, calculate a numerical indicator value based on the predetermined numerical values that have been assigned to each correlation coefficient of the specific group, - compare (72, 74, 76) each numerical indicator value to a predetermined group-specific indicator value threshold (Th 1, Th 2, Ths), and - generate an output signal based on a result of the comparison for each group. - receiving radar waves reflected from the scene, - removing signals that are representative of a static portion of the scene from range signals obtained from the received radar waves for deriving dynamic range signals assigned to a plurality of range bins, - select a plurality of predefined groups of range bins out of the plurality of range bins, wherein the groups are separated by at least one range bin, - retrieve real values of range signals assigned to each of the range bins of the selected predefined groups of range bins, - average the retrieved real values over a predefined period of time to form range signal buffers, - determine a Doppler frequency for each range bin of the plurality of predefined groups of range bins, - derive a statistic quantity from the determined Doppler frequencies representing a dominant frequency, - compare the dominant frequency with at least one predetermined condition concerning a predefined characteristic vital sign frequency (Th f), - if the dominant frequency fulfills all predetermined conditions, calculate a plurality of correlation coefficients, one correlation coefficient for each possible pair of range signal buffers, - group the correlation coefficients into at least two groups depending on the range bins to which the range signals of each possible pair of range signal buffers have been assigned, - compare each correlation coefficient with a group-specific threshold value (Th cc) and assign predetermined numerical values to each correlation coefficient depending on the outcome of the comparison, - for each group of correlation coefficients, calculate a numerical indicator value based on the predetermined numerical values that have been assigned to each correlation coefficient of the specific group, - compare (72, 74, 76) each numerical indicator value to a predetermined group-specific indicator value threshold (Th 1, Th 2, Ths), and - generate an output signal based on a result of the comparison for each group.