A Realistic System

Figure 4: A rotating lighthouse with a ``virtual'' parallel beam whose outline is defined by two parallel laser beams. Rotating (a) or deflectable mirrors (b) are used make the laser beams scan the northern hemisphere of the lighthouse.

During the first experiments it turned out that actually building a lighthouse with a sufficiently exact parallel beam is very difficult, at least given the limited technical capabilities that were available to us. This has the unfortunate consequence, that the model described in Section 4.1 cannot directly be used due to the resulting high inaccuracies. To understand the reason of these inaccuracies, consider the following example, where we assume a beam width of 10cm. Even if the angle of beam spread is only $1^\circ$ (instead of $0^\circ$ for an ideal parallel beam), the width of the beam at a distance of 5m would be about $18.7$cm, resulting in an error of almost 90%. The relative error could be reduced somewhat by increasing the width of the beam. However, a large beam width also results in a large and clumsy base station device.

Therefore, instead of building a system perfectly matching the requirements of Section 4.1, we have to adapt our model to a system which can actually be built. In order to develop such a model, we first have to examine ways of generating near-parallel beams.