Inertial Guidance

Planes flying over the oceans often rely on inertial navigation to stay on their course. Even in the early 1970s some of the first 747 jets were designed to carry several inertial-guidance systems. When more than one inertial-navigation system is in use, each can monitor the plane's position independently for improved reliability.

On long flights, as from the United States to Japan, an inertial-guidance system can control a plane automatically by providing instructions to the autopilot. At the start of the journey the intended flight path is divided into a succession of short segments, perhaps a half dozen. The pilot enters the coordinates of the end points of each of these short flights into a computer. The inertial-guidance system flies the plane to each of these waypoints in turn. The overall route is closely approximated by the series of nearly-straight line segments. The inertial-navigation system's computer knows where the plane is located and its velocity because acceleration is measured continuously. These systems are so accurate that a plane can fly non-stop from San Francisco to Japan under the control of an inertial-navigation system, arriving with a location uncertainty of about 10 ft (3m).

For longer journeys over the surface of the earth, interpreting inertial-navigation data is more complicated. The computer must project the measured acceleration onto the spherical surface of the earth to determine instantaneous position relative to the earth's coordinate system of latitude and longitude. There is an additional complication resulting from the rotation of Earth. It is not enough to know the direction of a destination when a plane takes off. As Earth rotates, the direction of the planned destination may seem to change. Navigation must continually correct for a plane's tendency to drift off course because of Earth's rotational acceleration, a consequence of the so-called Coriolis force. The inertial-navigation system's computer compensates for these challenges accurately and quickly.

With the advent of the newer Global Positioning Satellite system as an alternative to inertial navigation, inertial navigation may be less significant in the future. For the near future, navigation by INS will continue to make a valuable contribution to transportation safety and a backup to GPS-guided systems.