A system and method for determining CPR induced chest compression depth using two sensors while accounting for different orientations of the two sensors. The system may include a first motion sensor operable to generate motion signals corresponding to motion in a first coordinate frame defined by a first set of axes and a second motion sensor operable to generate motion signals corresponding to motion in a second coordinate frame defined by a second set of axes and a control system operable to receive the motion signals from the first motion sensor and the second motion sensor, rotate the motion signals from the first motion sensor into the second coordinate frame to obtain rotated motion signals corresponding to the motion signals from the first motion sensor, and combine the rotated motion signals with the motion signals from the second motion sensor to generate an output indicative of said displacement.