accelerometer is a device that measures proper acceleration ("g- force"). Proper acceleration is not the same as coordinate acceleration (rate of change of velocity). For example, an accel- erometer at rest on the surface of the Earth will measure an acceleration g= 9.81 m/s2 straight upwards. By contrast, accel- erometers in free fall orbiting and accelerating due to the gravity of Earth will measure zero. Accelerometers have multiple applications in industry and science. Highly sensitive accelerometers are components of inertial navigation systems for aircraft and missiles.