1. A method for detecting activity of a physical system carrying a motion sensor, the method comprising: obtaining a reference value ({circumflex over (λ)}) of a vector representing a reference axis that indicates a preferred direction; extracting a time sequence of measurements provided by the motion sensor; deducing therefrom a time observation sequence calculated using the time sequence of measurements, at least one component of the time observation sequence being calculated from the time sequence of measurements using the obtained reference value ({circumflex over (λ)}); using a statistical model of components of the time observation sequence in view of a plurality of possible predetermined states of the physical system, determining the activity of the physical system in the form of a time sequence of states corresponding to the time observation sequence; {circumflex over (λ)} determining whether a calibration is needed by calculating a likelihood value for the time observation sequence in view of said statistical model, and comparing said likelihood value with a predetermined threshold (δ); and on the basis of the comparison, performing the calibration, in response to determining that the calibration is needed, by updating the obtained reference value ({circumflex over (λ)}).