A self-contained navigation system. It consists of gyroscopes and accelerometers to provide attitude, heading, position, attitude, body/inertial velocity, and acceleration information. A primary navigation data source. INS loses accuracy with time due to drift of gyroscopes. INS-DNS is moderately accurate over land, not good over water.
Inertial Navigation System - a navigation system that guides an aircraft based upon its position relative to a pre-determined point, usually its home base. The INS is aligned just before launch at a point on the taxiway (called the 'hammerhead') of which the coordinates are known.