The acceleration is a combination of the actual vehicle acceleration and the acceleration due to gravity acting on the tilting inertial platform. It can be measured by an accelerometer mounted fixed on the platform, in either the north–south or east west direction, horizontally. So this equation can be seen as a version of the equation for a simple gravity pendulum with a length equal to the radius of Earth. The inertial platform acts as if it were attached to such a pendulum.
