## 2 Localization System Architecture
### Switched Beam Antenna Architecture  
The key part of the anchor system architecture is the SBA, which is based on an array of seven antennas, arranged with one hexagon at the SBA center and six perimetric pentagons. Figure 2 shows the mechanical arrangement of the SBA: it is a hemispherical structure with a maximum diameter around 146 mm, a height of 53.8 mm and with a dihedral angle of 120 degree. The dihedral angle is a design parameter depending upon the operative conditions, in particular coverage area, and noise, as discussed in [21], nevertheless its optimum value is between 100 and 130 degree for circular coverage area radius up to 2.5 m. The SBA exhibits six identical perimetric antenna, while the top antenna differentiates from the previous ones as it admits two radiation modes and corresponding input ports, [22]; the effective beams are therefore 8 in total. Each of the perimetric elements is a Circularly Polarized (CP) antenna designed by the principle explained in [23], which is basically a technique capable to generate two propagation modes with a single feeding signal, by splitting it in two orthogonal and properly phased components. This is obtained by removing the conductor in an elliptical region centered within the disk. The top antenna is instead a dual mode circularly polarized antenna capable to provide a regular cardioid pattern with the maximum of radiation in the boresight exploiting its first input pin, along with a second patch that provides a toroidal patter with the maximum radiation at 45 degree from the antenna axis obtained using its second pin. The adoption of circular polarization is motivated by two main reasons: the first consists in the need to reduce any signal fading due to cross-polarization coming from a generic target-anchor orientation. The second one consists in the inherent robustness of this propagation mode to multi-path phenomena. The SBA patch antennas are capable of operating in the 2.4-2.48 GHz band. In Figure 3 the picture of the prototype adopted in this work is reported. The pictures show the SBA assembly (Figure 3a), with the top antenna and 3 out of the 6 perimetric antennas. The top antenna implements a number of parasitic elements that improve the gain of antenna element #8, the one with a toroidal pattern, which results particularly affected by the global arrangement. The rear view visible in Figure 3b shows the control unit of the SBA and the other parts of the anchor architecture shown in Figure 1. This specific SBA is implemented in each anchor node by using a multiplexer properly synchronized with the RSSI block; this grants that the RSSI acquisition is related to the specific array element and thus to a specific space sector. The multiplexing between the antennas operates in a synchronized mode for all the anchors belonging to the same constellation to avoid signal clashes. Figure 2: The mechanical structure of the Switched Beam Antenna; (**a**): top view, (**b**): side view. The radiation pattern of the SBA is shown in Figure 4, in the azimuthal plane (\(\phi\)) and elevation (\(\theta\)). In particular, Figure 4a shows the measured radiation patterns for all the 8 antennas at elevation angle \(\theta=90\) degree. This is not the boresight direction for antennas 2-7, thus they exhibit maxima lower than their absolute maximum gain. From the graphs it is possible to see that the respective direction of maximum radiation is uniformly spread across the radial direction. In this picture, antennas 1 and 8 are observed by their horizontal angle of view, far from their maximum, and thus do not provide any pattern variation. Figure 4b reports the corresponding radiation patterns for each antenna in the elevation plane; in this case all the antennas are observed at different angle of view and thus provide gain variation. In particular, the complementarity of the antenna 1 and 8 gain patterns is clearly observable, thus providing an additional piece of information to the localization engine.