1. An apparatus for providing spatial orientation data signals, comprising: an inertial platform accelerometer cluster having three orthogonal axes and a plurality of accelerometers arranged to provide signals indicative of linear acceleration of said platform along each of the three axes and rotational acceleration of said platform about each of the three axes, said accelerometers all having their centers of measurement coplanar; processing means for receiving said accelerometer signals and providing output signals representative of linear acceleration along each of the three axes and rotational acceleration about each of the three axes.