# Low-Cost Sensor Based on SDR Platforms for TETRA Signals Monitoring
## 3 Results and Discussion
### Results of the Field Measurements (Real Case)  
Figure 14 shows the Downlink spectrum of the TETRA signals following the acquisition made with the portable sensor in the field (SDR ADALM Pluto platform). Several active channels of different amplitudes can be seen. Additionally, 10 dB higher levels than the floor noise are observed, even for the channels with the lowest levels.  
For testing the sensor and the GNU Radio field application in motion, the sensor was placed inside a vehicle. Three measurements were made successively in which the vehicle moved with constant but different speeds: 60, 90, and 120 km/h, respectively. Figure 15 comparatively presents the power levels measured on TETRA channel no. 14 (center frequency 390.3625 MHz) and no. 131 (center frequency 393.2875 MHz). It can be visually observed that, on the route, the displayed power levels for the two TETRA channels are comparable, regardless of the value of the vehicle speed.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**FFT Number of** & **Adj. Ch. Level 1** & **Adj. Ch. Level 1** & **Adj. Ch. Level 2** & **Adj. Ch. Level 2** \\
**Points** & **Lower [dB]** & **Upper [dB]** & **Lower [dB]** & **Upper [dB]** \\ \hline
2048 & \(-30.6\) & \(-27.8\) & \(-36.8\) & \(-35.5\) \\ \hline
4096 & \(-44.1\) & \(-42.5\) & \(-49.9\) & \(-49.2\) \\ \hline
8192 & \(-59.2\) & \(-58.7\) & \(-59.7\) & \(-59.4\) \\ \hline \end{tabular}
\end{table}
Table 4: Influence of FFT resolution on the attenuation of the adjacent channels, for the ADALM Pluto SDR platform (TETRA channel no.131, input signal level \(-42\) dBm).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Equipment** & \begin{tabular}{c} **Adj. Ch.** \\ **Level 1 Lower** \\ **[dB]** \\ \end{tabular} & \begin{tabular}{c} **Adj. Ch.** \\ **Level 1 Upper** \\ **[dB]** \\ \end{tabular} & \
\begin{tabular}{c} **Adj. Ch.** \\ **Level 2 Lower** \\ **[dB]** \\ \end{tabular} &
\begin{tabular}{c} **Adj. Ch.** \\ **Level 2 Upper** \\ **[dB]** \\ \end{tabular} \\ \hline R\&S FSV13 & \(-44.85\) & \(-48.83\) & \(-61.18\) & \(-61.85\) \\ \hline NI USRP 2930 & \(-42.2\) & \(-46.3\) & \(-60.5\) & \(-60.28\) \\ \hline ADALM Pluto & \(-29.9\) & \(-27.6\) & \(-35.63\) & \(-34.2\) \\ \hline Hack RF One & \(-25.3\) & \(-26.3\) & \(-27.3\) & \(-28.2\) \\ \hline \end{tabular}
\end{table}
Table 3: Attenuation of adjacent channels by the three types of SDR platforms and for the R&S FSV13 spectrum analyzer (TETRA channel no. 131, input level \(-62\) dBm).  
Figure 14: Downlink spectrum of TETRA channels measured in a field location by the ADALM Pluto SDR platform.  
Figure 15: Representation of the measured TETRA channel powers (channel no. 14 and no. 131) when the receiving sensor was moving constantly on a route at different speeds: (**a**) channel 14, speed—60 km/h; (**b**) channel 131, speed—60 km/h; (**c**) channel 14, speed—90 km/h; (**d**) channel 131, speed—90 km/h; (**e**) channel 14, speed—120 km/h; (**f**) channel 131, speed—120 km/h.  
In terms of the accuracy of the positioning of the measuring points by the GNSS receiver, in Figure 16 we can observe the distance between two consecutive points measured with the Google Earth application. In the case of 120 km/h speed, compared to the theoretical distance of 33.33 m between two consecutive points, it was observed that the measured distance is 33 m. The difference of 0.33 m is insignificant and falls within the positioning accuracy of the GNSS receiver.  
Figure 17 indicates the measured power levels of a TETRA channel, when the sensor is moving with constant speeds of 60 and 120 km/h, respectively, along a short section extracted from the monitoring itinerary. We observed that the influence of the sensor's speed on the measurement results is insignificant, even when doubling the speed. This illustrates that placing a sensor for monitoring TETRA signals on a mobile tracker, even if it will have a variable speed, does not lead to misinterpretations of the characteristics of the acquired signals (position, amplitude, etc.).