## 5 Results
### Data Processing  
The presented setup allows pre-programming the motion of the receiver unit and capturing RSS values on a fine grid that is spread in the test tank. This is useful to study the EM field in the workspace. It allows analyzing tank specific field characteristics like reflections and noise. The measured signal strengths at the four different frequencies (433.90 MHz, 434.05 MHz, 434.20 MHz, 434.35 MHz) are  
Figure 14: Experimental setup to validate the embedded EM localization system.  
illustrated as contour plots in Figure 15. The measurements were taken on a grid with a \(50\,\mathrm{mm}\) by \(50\,\mathrm{mm}\) resolution, where each of the 1952 grid points initiated a 5-s motion-free measurement sequence. The contour plots show the mean values of these measurement sequences. In each of the subfigures, the RSS is largest at the position of the respective beacon and decreases with distance, as expected. For the case of an ideal omnidirectional antenna, the potential lines of equal RSS values form circles. All beacons demonstrate this characteristic. However, with increasing distance from the beacons and in the vicinity of the tank walls, the circular contours ravel out. This is due to the noise floor level, which the RSS measurements reach, which begins for our system at approximately \(-85\,\mathrm{dBm}\). The disturbances close to the tank walls are due to reflections from the tank walls, which are made of steel beams, wood and glass. The contour lines in Figure 15b are elliptically shaped around Beacon 2. This is likely due to an antenna mounting error, which results in a slight tilt of the antenna. Thus, the emission plane of the antenna is not aligned with the horizontal measurement plane of the receiver node.  
An important aspect in the context of RSS data processing is the dynamic range of the receiver unit. The dynamic range is a measure for how well the weak signals can be detected in the presence of stronger signals on neighboring frequency channels. Hence, high dynamic range is desirable as it allows one to detect signals from far away beacons in the vicinity of other beacons. The NooElecTM NESDR Mini DVB-T dongle has a dynamic range of approximately \(60\,\mathrm{dBm}\).  
The RSS data can be used to validate the URSM (2) introduced by [3] first and summarized in Section 2.2. The measured data presented in Figure 16 are used to fit \(\alpha_{i}\) and \(\Gamma_{i}\) of (2). Figure 16 illustrates the measured RSS values as a function of distance for a single frequency and the fitted URSM. The data correspond to the analytical model, whereby the scattering increases with distance.  
Figure 15: Contour plots of RSS values measured over 5 s on each grid point at the beacons’ frequencies. Black markers depict the beacons.  
Moreover, the measurements deviate from the URSM in direct vicinity of the emitting beacon. This effect is due to a vertical offset between the horizontal transmission plane of the beacon and the receiver node. For distances of more than 1.8 m the RSS measurements deviate significantly from the URSM, as the RSS reaches the noise floor level of the receiver unit. There are two options to resolve the effect with modifications of the beacons: first, increasing the power of the emitter and, second, using antennas with higher directivity. Both will increase average RSS in the test tank and, thus, push the noise floor to further regions. Another option is to add more beacons to the test tank, i.e., a total of six beacons arranged in a three by two pattern. In this case, the localization system does not need to rely on the RSS from far away beacons, as there are closer beacons whose EM signals allow more accurate RSS measurements. However, this approach would require a distance-dependent weighting of the RSS measurements in the filter algorithm to reduce the influence of inaccurate RSS measurements on the localization.