A method of operating a radar sensor system for monitoring a breathing motion of a subject , the radar sensor system including a radar transmitting unit having one radar transmitting antenna and being configured for transmitting radar waves towards the subject , a radar receiving unit having one radar receiving antenna and being configured for receiving radar waves that have been transmitted by the radar transmitter unit and have been reflected by the subject , and an evaluation and control unit that is at least configured for evaluating Doppler information from the radar waves received by the radar receiving unit, the method comprising at least steps of - operate the radar transmitting unit for at least once transmitting radar waves of a predetermined radar carrier frequency ( f ) for a specified time towards a chest and an abdominal region of the subject , - operate the radar receiving unit at least once for a specified time for receiving radar waves that have been transmitted by the radar transmitting unit and that have been reflected by the chest and the abdominal region of the subject , and for generating received radar signals from the received radar waves , - calculate the autocorrelation function of the received radar signals, - calculate the Fourier transform A ( ν ) of the calculated autocorrelation function, - determine and record values of at least two peaks of the calculated Fourier transform A ( ν ), - change the number of radar wavelengths in the path between the radar transmitting antenna , the subject and the radar receiving antenna at least once by a predetermined amount, - iteratively carry out the preceding steps (34 to 44) until at least one extreme value is detected for the determined values of each one the at least two peaks, and - from the recorded determined and detected extreme values of the at least two peaks, determine at least one breathing-characteristic parameter of the breathing motion of the subject . - operate the radar transmitting unit for at least once transmitting radar waves of a predetermined radar carrier frequency ( f ) for a specified time towards a chest and an abdominal region of the subject , - operate the radar receiving unit at least once for a specified time for receiving radar waves that have been transmitted by the radar transmitting unit and that have been reflected by the chest and the abdominal region of the subject , and for generating received radar signals from the received radar waves , - calculate the autocorrelation function of the received radar signals, - calculate the Fourier transform A ( ν ) of the calculated autocorrelation function, - determine and record values of at least two peaks of the calculated Fourier transform A ( ν ), - change the number of radar wavelengths in the path between the radar transmitting antenna , the subject and the radar receiving antenna at least once by a predetermined amount, - iteratively carry out the preceding steps (34 to 44) until at least one extreme value is detected for the determined values of each one the at least two peaks, and - from the recorded determined and detected extreme values of the at least two peaks, determine at least one breathing-characteristic parameter of the breathing motion of the subject . <EOS>