The IMU provides accurate velocity and attitude information for use in the Shuttle Orbiter guidance, navigation and control system. The inertial sensors contained in the four gimbals platform are GYROFLEX gyros and force rebalance accelerometers. Eight speed resolvers are utilized to provide precise digital gimbals angle readouts.
The Kearfott IMU has been aboard all Orbiter flights to date and has performed well within the required specifications.
The High Accuracy Inertial Navigation System (HAINS) unit contains the next generation inertial components and new technology electronics such as a microprocessor to store calibration constants. The replacement HAINS IMU provides additional enhancements in the form of higher performance, smaller volume, lower weight, more extensive Built-in Test (BIT) and lower life cycle costs than its predecessor KT-70.
The Space Shuttle IMU consists of an all-attitude stabilized platform and associated electronics to supply output data proportional to the Orbiter's attitude and velocity.
The Orbiter employs a triple redundant IMU configuration, with skewed inertial clusters, wherein both hardware and software performance and failure detection techniques are utilized. Software was developed and is operational for factory calibration/test, hangar calibration and preflight calibration and alignment. Factory-obtained calibration constants are stored and utilized for compensations within the HAINS unit. In-orbit IMU alignment updates are provided by on-board star trackers, which are mounted on a common navigation base.
The IMU interface is accomplished via a multiplexed serial data-line and it's completely compatible with the Orbiter's general purpose computers. Each IMU is completely self-contained requiring only external power and cabin cooling air.
The HAINS IMU version, because of its reduced size, can be mounted on the common navigation base in a four-IMU configuration. The triple redundancy presently employed can thus be extended to a quadruple configuration.