• Traditional guidance systems used mechanical gyroscopes (rate gyros) to measure the angular velocities of the vehicle or of the gimbals on which the seeker was mounted. - A full inertial navigation system (INS) with accelerometers and rate gyros was usually too expensive for applications other than aircraft. - Integrating measured angle rates produces angle/attitude data and integrating measured accelerations twice produces position data.
• More recently, the development of solid state gyros (fibre optical gyroscopes or FOGs) and accelerometers has allowed small vehicles to be fitted with full inertial navigation systems, often supplemented with another navigation aid, such as a global positioning system (GPS).
• These allow accurate vehicle position and attitude estimates to be generated, which can significantly improve the performance of a guidance system. - However, the INS still has to be aligned to some reference and often calibrated in flight - Transfer alignment.