Advanced Inertial Reference Sphere
The Advanced Inertial Reference Sphere, or AIRS, is a highly accurate inertial guidance system designed for use in the LGM-118A Peacekeeper ICBM which was intended for precision nuclear strike against Soviet missile silos.
AIRS is a fluid stabilized system, as opposed to one using gimbals. It consists of a beryllium sphere floating in fluid. Jet nozzles are used to stabilize the inertial platform as commanded from the sensors. This design not only eliminates the problem of gimbal lock, but also makes it extremely accurate (drift less than 1.5×10−5°/h), accurate enough so any further improvement would give a negligible benefit to the missile's CEP.
The sensors used in AIRS are floated gas bearing gyroscopes and SFIR accelerometers which are derivatives of PIGA accelerometers. Although this type of accelerometer is most accurate it contains many precise parts which makes them very expensive to build (approximately $300,000 per unit). PIGA/SFIR accelerometers are also very susceptible to failure because of complicated design.
The AIRS was originally developed for the LGM-118A Peacekeeper. The first AIRS units were manufactured by Northrop.