If the vehicle's position is known at one point in time, the position can be continuously determined by advancing the known position using course, speed, time and distance travelled. This technique is known as dead reckoning. The advantage of dead reckoning is that it allows fully autonomous positioning within the vehicle. The main disadvantage of dead reckoning is its unbounded accumulation of errors. Therefore, dead reckoning is usually combined with another method for positioninig in order to eliminate errors accumulated since the last correction.
The vehicle's speed and the distance travelled can be determined using wheel odometers. Each turn of the wheel is identified and the distance travelled can be determined by the circumference of the wheels. Odometer inaccuracies result from wear and slip of the wheels. The course can be determined using magnetic or gyroscopic compasses. As the magnetic field of the Earth is very weak, the accuracy of the course determined by magnetic compasses is subject to all kinds of magnetic perturbations. Gyroscopic compasses use mechanical or optical gyroscopes to determine the course of a vehicle. A mechanical gyroscope consists of a rapidly spinning wheel set in a framework that permits it to tilt freely in any direction or to rotate about any axis. The momentum of such a wheel causes it to retain its attitude when the framework is tilted. An optical gyroscope measures the interference pattern generated by two light beams, travelling in opposite directions within a mirrored ring or fibre loop, in order to detect changes in motion.