# RF Exposure Assessment by Drone-Based Technology
## 3 Results and Discussion  
The PEM generates a large amount of information, as it provides data in 20 frequency bands every four seconds. However, in most of them, the detected values do not exceed the equipment's detection threshold (see Table 1). For this reason, we have focused solely on the telephony bands in which the mobile base station located at the Polytechnic School operates. These are the bands referred to in the personal exposimeter: LTE 800 DL, GSM + UMTS 900 DL, GSM 1800 DL, and UMTS 2100 DL. In both the walking measurements and those performed with the drone, we obtained all data above the detection threshold (0.005 V/m), which is logical given the proximity of the antenna to the measurement points. Tables 2 and 3 present statistical summaries of the electric-field levels recorded during the samplings conducted at ground level and with the drone, respectively, across the four mobile telephony bands. From these data, we note that the central parameters (mean and median) of the levels detected at 20 m height are between 2.9 and 4.4 times higher than those detected at ground level when comparing mean values, and between 3.1 and 4.8 times higher when using medians. Figure 4: Theoretical spherical variogram. Regarding the maximum values, they were 0.968 V/m at ground level in the GSM + UMTS 900 DL band and 3.504 V/m at 20 m height in the UMTS 2100 DL band. To understand the significance of these values, we consider the reference levels from the ICNIRP 2020 standard, namely 41.25 and 58.34 V/m for the frequencies of 900 and 1800 MHz, respectively. Although our data cannot be directly compared to the ICNIRP 2020 standard (as it establishes reference levels for exposures averaged over 30 min and for whole-body exposure), we can conclude that they are clearly below the recommended limits. Another noteworthy aspect of these tables is that in all cases (the four frequencies and the samples taken at ground level and at 20 m height), the standard deviation represents a high percentage (in some cases exceeding 100%) of the mean value. The same is true for the interquartile range and the median. This indicates that the data dispersion is high. Additionally, the mean values were higher than the medians in all cases, which is typical of non-Gaussian frequency distributions with positive skewness. Such distributions are very common in environmental data and are characterized by a large number of small values and few high values. As an example, Figure 5 shows the frequency distributions of the electric-field values in the GSM + UMTS 900 DL band in the samples at 20 m height. In the rest of the cases, the histograms showed a similar appearance. Based on the discrete data obtained experimentally, we constructed continuous distributions to visualize the electric field of the different frequency bands using heat maps. To achieve this, we used the kriging geostatistical method. This technique requires normal or quasi-normal frequency distributions, as abnormally high values increase the variance of the dataset. The variogram is very sensitive to these high values, and the lack of normality in the data can affect the accuracy of the kriging process [17]. \begin{table}
\begin{tabular}{c c c c c} \hline
**E (V/m)** & **LTE 800 DL** & **GSM + UMTS 900 DL** & **GSM 1800 DL** & **UMTS 2100 DL** \\ \hline Min & 0.007 & 0.014 & 0.01 & 0.015 \\ P25 & 0.022 & 0.069 & 0.043 & 0.047 \\ P50 & 0.036 & 0.11 & 0.061 & 0.077 \\ P75 & 0.065 & 0.211 & 0.094 & 0.139 \\ P95 & 0.163 & 0.413 & 0.235 & 0.295 \\ Max & 0.517 & 0.968 & 0.516 & 0.942 \\ Average & 0.06 & 0.16 & 0.08 & 0.11 \\ SD & 0.06 & 0.15 & 0.08 & 0.12 \\ IQ & 0.043 & 0.142 & 0.051 & 0.093 \\ \hline \end{tabular}
\end{table} 
Table 2: Statistical summary of the electric field measured at ground level in the four mobile-telephone bands. P25-P95 are the percentiles. SD: standard deviation. IQ: interquartile range. Sample size: 331.