A method of operating a radar sensor system (10) for monitoring a breathing motion of a subject (24). The radar sensor system (10) includes one radar transmitting antenna (12) and one radar receiving antenna (16) and an evaluation and control unit (22) for evaluating Doppler information from the received radar waves (18). The method comprises transmitting radar waves (14) towards a chest (26) and an abdominal region (28) of the subject (24), receiving radar waves (18) reflected by the subject (24), calculating (38) the autocorrelation function of the received radar signals, calculating (40) the Fourier transform A(v) of the calculated autocorrelation function, determining and recording (42) values of at least two peaks of the calculated Fourier transform A(v), and, from the recorded determined values of the at least two peaks, determine (46, 48) at least one breathing-characteristic parameter of the breathing motion of the subject (24). (Fig. 3)
