Allen D I don't wish to delay nor to complicate your departure but perhaps you would take this query with you and respond at your convenience. An accelerometer is in essence a mass suspended between two springs and constrained to move only along the axis of its construction. Yes I'm aware that a practical device involves a great deal more than that -- amplifiers, feedback loops, damping networks, sensors, electromagnetic compensators, computer processing and probably more but that is irrelevant to the basic concept. An inertial guidance system as I understand it consists, in part, of three accelerometers -- one in each axis. That they function well is attested to by the fact that airliners once relied upon them for successful navigation as did nuclear submarines and the Apollo space vehicles including the Luna missions. In the latter service, they were touted as being sufficiently accurate as to permit unassisted return to Earth from any point in the mission should ground updates for any reason cease. However, I do not believe that the accelerometers provide any input to the navigation system if a SEPARATE and EXTERNAL force is NOT acting on the vehicle to which they are rigidly mounted ALONE. I specifically do not believe that the mass in an accelerometer will move relative to the vehicle to which it is rigidly connected if both it -- the mass -- and the vehicle are uniformly accelerated. This latter condition is satisfied when both (could hardly be otherwise!) are in free fall such as in orbit eg the Earth orbiting the Sun or in a body outside Earth's atmosphere falling toward Earth. Paul D Make the switch to the world's best email. Get the new Yahoo!7 Mail now. www.yahoo7.com.au/worldsbestemail