Figure 2.3 (a) Green 8mm LEDs are used as feature points for tracking by the system. These are referred to as ‘markers’ throughout this research. (b) Markers are incorporated into beacons. These are groups of markers in fixed locations. A beacon comprising four markers is shown in this figure.
In most cases it is possible to uniquely estimate pose using only four markers . If four markers are integrated into one beacon then pose estimates can be made when a single beacon is in view instead of requiring, in the single marker per beacon case, four beacons. Figure 2.3b shows an example of a beacon using four markers placed at the edge of the structure. Beacons such as this one were used during development and were constructed
by embedding markers into a plastic housing as shown in the figure.
a number of these four-marker beacons placed around the tracking volume is preferred.
The camera modules are the eyes of the system. They calculate the positions of the markers they observe within a stream of image frames and pass this information back to the hub. From this information the hub can determine the angles to the markers. During camera motion, the relative motion of the markers’ images must be tracked as they move within each image frame.
Each module contains a powerful processing unit capable of analysing a constant stream of images from a sensor placed behind a lens and filter assembly. The processing unit is linked to a communications system implemented in hardware and software. In this research, three camera module prototypes were built. However, it is intended that more of these modules may be used in the future to provide a greater combined FOV. Due to their function to detect bright spots (markers), and the Blackfin processing unit used in the camera modules, the name ‘Black Spot’ was coined for the camera modules. The camera module firmware was written using ISO C  as described in Chapter 4.
The hub is the brains of the system. It uses the locations of the markers from the camera modules along with a model of the surroundings to construct an estimate of its pose. The