Introduction and Background
compensate for drift in measurements from accelerometers and gyroscopes.
Electromagnetic trackers, such as the Polhemus Fastrak , use magnetic fields to deter-
mine pose. These systems use source and sensor units. DC magnetic fields that are picked up by the sensor unit
The source units create
Either the source unit
magnetic fields in the three orthogonal directions.
s noted, these systems suffer from
distortions due to ferromagnetic objects in the vicinity of the object being tracked. Unlike optical tracking systems, and to a lesser degree radio frequency tracking systems, magnetic trackers do not require the source and sensor to have a line of sight.
Radio frequency (and microwave) trackers use electromagnetic waves to enable tracking.
Usually these systems use time of flight, however, this is more difficult to implement com- pared with acoustic trackers as the speed of light is many times faster than the speed of sound and precision electronics are required. Perhaps the most well known radio fre- quency tracking system is the Global Positioning System (GPS) run by the United States
Department of Defence . This system uses a constellation of between 24 and 32 Medium Earth Orbit satellites. Using signals from a number of satellites, approximate positional tracking (in comparison to other systems) can be performed. Radio frequency tracking sys-
tems suffer from multipath issues. This occurs when waves bounce off objects and cause multiple signals to be received. This adds to the difficulties of designing a radio frequency tracker. The author is not aware of any precision radio frequency trackers available on the market.
The last type of tracking system and the type chosen for this research is optical tracking. These systems use light sources (optical markers) and optical sensors. The position of the
markers with respect to the sensors enables these systems to calculate pose. some of the available optical systems is given in Section 1.2.
Existing optical systems
large amount of work has been performed in the area of optical tracking.
tical tracking systems exist and these systems can be broken into two broad categories: inside-looking-out systems and outside-looking-in systems. n inside-looking-out system uses markers outside of the tracking volume. These are observed by the optical system mounted on the object whose pose is being estimated. n outside-looking-in system has the observation component of the system on the outside of the tracking volume and uses optical markers mounted on the object within the tracking volume. n example of an op- tical tracker that uses the inside-looking-out model is the 3rdTech HiBall tracker  and
an example of an outside-looking-in tracker is the optical tracking system from