US Patent 7107833 Inertial reference unit with internal backup attitude heading reference system

An inertial reference unit (IRU) is described which includes a processor programmed to provide inertial data from received inertial signals, a primary sensor unit providing the inertial signals to the processor, and an input/output (I/O) unit communicatively coupled to the processor. The I/O unit provides signals, including inertial data, to an external interface of the IRU and routes signals to and from the processor. The inertial reference unit also includes a secondary sensor unit separate from the primary sensor unit which provides inertial data independent of the inertial data provided by the processor.