EASA learning objective 022.05.01.01.08 states: 'State that a strap-down system is fixed to the structure of the aircraft and normally consists of three laser ring gyros and three accelerometers.'

Unlike first generation Inertial Navigation Systems (INS) , Inertial Reference Systems (IRS) fixes its acceleration sensors ( 3 accelerometers) to the airframe to measure linear acceleration in all 3 planes (hence the name ‘strap down’). It replaces the stabilised platform and mechanical rate gyros used in INS with 3 Ring Laser Gyro's (RLG's) to measure rate of angular rotation around the aircraft pitch, roll and yaw axes. IRS sensors are ideally placed as close to aircrafts C of G as possible for maximum accuracy in sensing aircraft motion and simplification of the mathematical modelling.

RLG's work on the principle that if two beams of high frequency laser light are made to travel in opposite directions around a triangular path then at the point where they meet a stable interference pattern will be produced. Angular rotation of this triangular path will cause a phase difference in the light beams which will change the interference pattern, this can be used to measure the rate of rotation around the axis perpendicular to the optical path of the lasers. Using this principle the 3 RLG's are mounted at 90° to each aircaft axis which enable the IRS to keep track of the aircraft attitude.