An inertial navigation system (INS) is a computerized aid used in aviation, marine, missile guidance, and space flight applications. The system utilizes a dead reckoning style of positional orientation based on the inputs of accelerometers and gyroscopes and calculated by an onboard computer. INS systems do not require any external references or inputs to maintain their navigational integrity and are accurate to within just over 600 m (650 yards). This independence from external reference makes the INS immune to most electronic warfare techniques such as jamming or decoy signaling. Unfortunately INS devices are expensive and complex which often precludes their use outside of specialized applications.
This form of navigation system supplies necessary inputs to the operators or automatic steering mechanisms of aircraft, ships, guided missiles and spacecraft. This navigational information is calculated by an onboard computer based on two primary internal inputs. The first is a manually inserted set of information regarding the departure point and all subsequent waypoints along the route. This information is generally available from charts or global positioning satellite (GPS) equipment and is typically entered into the INS computer by the crew. Once the flight or voyage is underway, motion sensors in the vehicle supply the inertial reference system with a constant stream of real time positional information which the computer uses to compare actual and predicted positions.
Should these two sets of information differ, the inertial navigation system computer will either supply updates to the crew or to an autopilot system for the necessary course corrections. The sensors which supply the positional information used as a reference by the inertial navigation system fall into two basic categories: accelerometers and gyroscopes. Accelerometers establish the proper acceleration, and thus the speed, at which the vehicle is traveling. Gyroscopes are orientation instruments which accurately measure any deviation in the axial position of the vehicle when it turns along any axis. Between them, these two instruments paint an accurate, real time picture of course and velocity for the inertial navigation system computer to use as a positional reference.
One of the strong points of an inertial reference system is that once initialized, all of the information is generated internally on a real time basis. This precludes external influences such as electronic warfare systems jamming from interfering with the vehicle's navigation. This is of particular value for INS systems in guided missiles and other military vehicles. The many benefits of inertial navigation systems are, unfortunately, largely negated by the complexity and high cost of the systems which typically restricts their use to specialized applications.