A portable instrument provided with a device for controlling or managing a sports or well-being activity of a person wearing the portable instrument. The control device includes at least a motion sensor and a pressure sensor connected to a calculation unit, and a GNSS receiver module connected to the calculation unit. The calculation unit of the control device is arranged to activate the GNSS receiver module for a time of activation defined subsequent to variations in movement detected by the motion sensor differing from known movement data or walking profiles in order to determine at least one reference speed of the person, and to deactivate the GNSS receiver module after the activation time for a deactivation time greater than the activation time.