Radar is used to measure internal body motion. Radar reflections from the body are measured using a range of frequencies that includes a higher frequency band and a lower frequency band. In the higher frequency band, e.g. above 24 GHz, the radar signal hardly penetrates the skin, whereas it penetrates deeper into the chest in the lower frequency band e.g. below 10 GHz. Chest surface motion is estimated by means of the measurements using the higher frequency band. Effects of the estimated chest surface movement are subtracted from the measurements from the lower frequency band. The resulting response after removal is used to fit a model of a heart. Fitting may be performed in a series of fitting steps, including fitting parameters X of a geometric model to the measurements determining a least square solution of fit errors between the measurements and a Taylor expansion from the grid model obtained with the fitted parameters X as a function of adaptions of the grid model and subsequently determining a further adaptation of the grid model that best fits the measurements.
