Tracking and Orientation

The biggest challenge facing developers of augmented reality is the need to know where the user is located in reference to his or her surroundings. There's also the additional problem of tracking the movement of users' eyes and heads. A tracking system has to recognize these movements and project the graphics related to the real-world environment the user is seeing at any given moment. Currently, both video see-through and optical see-through displays typically have lag in the overlaid material due to the tracking technologies currently available.

For augmented reality to reach its full potential, it must be usable both outdoors and indoors. Currently, the best tracking technology available for large open areas is the Global Positioning System. However, GPS receivers have an accuracy of about 10 to 30 meters, which is not bad in the grand scheme of things, but isn't good enough for augmented reality, which needs accuracy measured in millimeters or smaller. An augmented-reality system would be worthless if the graphics projected were of something 10 to 30 meters away from what you were actually looking at.

There are ways to increase tracking accuracy. For instance, the military uses multiple GPS signals. There is also differential GPS, which involves using an area that has already been surveyed. Then the system would use a GPS receiver with an antenna that's location is known very precisely to track your location within that area. This will allow users to know exactly how inaccurate their GPS receivers are, and can adjust an augmented-reality system accordingly. Differential GPS allows for submeter accuracy. A more accurate system being developed, known as real-time kinematic GPS, can achieve centimeter-level accuracy.

Tracking is easier in small spaces than in large spaces. Researchers at the University of North Carolina-Chapel Hill have developed a very precise system that works within 500 square feet. The HiBall Tracking System is an optoelectronic tracking system made of two parts:

• six user-mounted, optical sensors
• infrared-light-emitting diodes (LEDs) embedded in special ceiling panels

Photo courtesy Tracking Project at UNC-Chapel Hill The Hiball Tracking System uses an optical sensing device and LED-embedded ceiling tiles to track movements over a short range.

The system uses the known location of the LEDs, the known geometry of the user-mounted optical sensors and a special algorithm to compute and report the user's position and orientation. The system resolves linear motion of less than .2 millimeters, and angular motions less than .03 degrees. It has an update rate of more than 1500 Hz, and latency is kept at about one millisecond.