On the other hand, they do not suffer from distortion effects in the presence of metals and can have high update rates because of the speed of light. Ultrasonic trackers have a more limited range because of the loss of energy with the distance traveled. Also they are sensitive to ultrasonic ambient noise and have a low update rate. But the main advantage is that they do not need line of sight. Systems using radio waves such as the Global navigation satellite system do not suffer ambient light, but still need line of sight. Spatial scan A spatial scan system uses (optical) beacons and sensors. Two categories can be distinguished: Inside out systems where the beacon is placed at a fixed position in the environment and the sensor is on the object Outside in systems where the beacons are on the target and the sensors are at a fixed position in the environment By aiming the sensor at the beacon the angle between them can be measured. With triangulation the position of the object can be determined. Inertial sensing The main advantage of an inertial sensing is that it does not require an external reference. Instead it measures rotation with a gyroscope or position with an accelerometer with respect to a known starting position and orientation. Because these systems measure relative positions instead of absolute positions they can suffer from accumulated errors and therefore are subject to drift. A periodic re-calibration of the system will provide more accuracy. Mechanical linkage This type of tracking system uses mechanical linkages between the reference and the target. Two types of linkages have been used. One is an assembly of mechanical parts that can each rotate, providing the user with multiple rotation capabilities. The orientation of the linkages is computed from the various linkage angles measured with incremental encoders or potentiometers. Other types of mechanical linkages are wires that are rolled in coils. A spring system ensures that the wires are tensed in order to measure the distance accurately. The degrees of freedom sensed by mechanical linkage trackers are dependent upon the constitution of the tracker's mechanical structure. While six degrees of freedom are most often provided, typically only a limited range of motions is possible because of the kinematics of the joints and the length of each link. Also, the weight and the deformation of the structure increase with the distance of the