A processing apparatus (6) for determining a respiration signal (34) of a subject (100) is presented. The processing apparatus (6) is configured to perform the steps of obtaining a movement signal (22) descriptive of a respiratory movement, determining a first quantity (24) descriptive of a rotation axis and/or rotation angle based on the obtained movement signal (22), and estimating a rotation axis (25) and/or rotation angle (26) based on the first quantity (24) and a rotation model, wherein the rotation model models the respiratory movement as a rotation around a single rotation axis. This model of the rotation can be further used as a feature for an instantaneous classifier descriptive of a movement artifact descriptive of a non-respiratory movement. Furthermore, a processing method, a respiration monitor (1, 91), a computer-readable non-transitory storage medium and a computer program are presented.