the autopilot in a modern large aircraft typically reads its position and the aircraft's attitude from an inertial guidance system. inertial guidance systems accumulate errors over time. they will incorporate error reduction systems such as the carousel system that rotates once a minute so that any errors are dissipated in different directions and have an overall nulling effect. error in gyroscopes is known as drift. this is due to physical properties within the system, be it mechanical or laser guided, that corrupt positional data. the disagreements between the two are resolved with digital signal processing, most often a six-dimensional kalman filter. the six dimensions are usually roll, pitch, yaw, altitude, latitude, and longitude. aircraft may fly routes that have a required performance factor, therefore the amount of error or actual performance factor must be monitored in order to fly those particular routes. the longer the flight, the more error accumulates within the system. radio aids such as dme, dme updates, and gps may be used to correct the aircraft position.