Schematic view of a garden-gate pendulum. θ_ns is the rotation angle, assimilable to a translation Y_ns according to the north–south sensitivity axis of the sensor. This rotation θ_ns can be generated by translations and also by rotations ψ_ew around the east–west axis and around the vertical axis. Rotation ψ_ns around the north–south axis changes the pendulum period, and the disturbances according to this degree of freedom are negligible compared to the signals of translations and rotations. The OBM pendulum moving part is strictly in the OAM plan. The displacement from B to B′ under the effect of the tilt ψ_ew, imposes a gap from M to M′. The rotation ψ_z acts in the same way as the translation .