The original inertial navigation systems maintained a physical platform such that it was always aligned with true north, and always level with respect to the earth's surface. Electromechanical gyros and torque motors mounted within gimbals in each of the three axes achieved these requirements. The platform is aligned with true north and levelled at the beginning of the flight; this condition is maintained throughout the flight.
A by-product of aligning and levelling the platform is that attitude information is available for use by flight instruments and other systems. Modern day commercial aircraft inertial navigation systems are equipped with strapdown devices including solid-state gyros and accelerometers. The alignment process and attitude compensation is now achieved in the computer's software, i.e. there is no physical platform.
The key principles of inertial navigation are based on accelerometers and gyro references together with a navigation processing function. These can be combined within single inertial navigation systems (INS) and dedicated crew interface. Alternatively, the accelerometers and gyros are contained within an inertial reference unit (IRU), the processing function and crew interface is then integrated within the flight management computer system (FMCS). Within the FMCS, the flight management computer (FMC) combines area navigation and performance management into a single system.
More recent technical developments include ring laser gyroscopes combined with the very latest accelerometers in state of the art Inertial Navigation Systems.