The Base Station

Figure 7: Prototype base station consisting of two lighthouses and the resulting 2D coordinate system.

Figure 7 shows a picture of the prototype base station. It consists of two mutually perpendicular lighthouses. The main lighthouse platform takes about $t_\mathrm{turn} = 60$s for one rotation. The platform is driven by a geared electro motor manufactured by FTB [34], which has a low flutter of about 0.1% of the rotation speed. Using an LM317 [36] adjustable voltage regulator, the voltage supply of the motor and thus the rotation speed of the platform can be adjusted. The two bars that extend from under the platform are used to move the center of gravity of the platform to the rotation axis, such that the platform rotates at a constant speed.

The power supply for the rotating platform is implemented by a stereo jack and associated plug. While the plug is fixed to the axle of the rotating platform, the jack is affixed to the chassis using a thin steel wire. This way, the round plug can rotate in the jack.

Beam generation is based on rotating mirrors as described in Section 4.2. Both rotating mirrors are driven by a single Graupner SPEED 280 electro-motor. In order to reduce vibrations, we did not use a rigid axle to connect the mirrors to the motor. Instead, we used small steel springs as axles. The rotating mirrors are supported by two ball bearings each. Two 1mW 650nm semiconductor laser modules with adjustable focus point their beam at the rotating mirrors.

The supply voltage of the motor and thus its rotation speed can be adjusted using an LM317 voltage regulator. The mirror rotation speeds of the two lighthouses are slightly different ( $t_\mathrm{mirror} = 4$ms and $t_\mathrm{mirror} = 5$ms for one rotation, respectively), such that the observer can distinguish the two lighthouses based on the time interval between successive light flashes, which will be explained in more detail in Section 4.3.2. Hence, in order to detect a beam, the observer's photo detector must at least be hit twice by the rotating laser beam. Note that due to the fast rotation of the laser beams, the average light intensity is low enough to be eye-safe.

There is a slight chance that the photo detector is hit by the beams of both lighthouses at the same time. We will explain in Section 4.3.2 how an observer can detect and handle this situation. However, since the diameter of the laser beams is rather small, the likelihood of this event is small. By selecting slightly different platform rotation speeds for the two lighthouses, we can ensure that for each observer this happens only once in a while. In our experiments this happened about every 100 lighthouse rotations at a single fixed observer.