## 3 Setup and Measurement Campaign
### CNR Inspection and Jammer Position Determination  
The measurements were first inspected for the CNR time dependence for each of the rides and satellites. Figure 8a,b shows that the CNR acquisition is more continuous on the smartphones, while the geodetic devices typically do not report their CNR measurement when interference is detected. There are, however, some exceptions to the rule (Figure 8c,d). These exceptions are not so much in the sense of observing the opposite, but rather in the sense that the effect is less pronounced. In their paper, [PERSON] et al. [37] observed the CNR of some commercially available smartphones located beside a road, enabling them to spot the passing of a jammer. This raised the idea of obtaining the current position of the jammer. Since the jammer was inside the car, there was no effective way to directly measure its position via satellite positioning. On the other hand, the speed of the car did not allow us to measure its position using any geodetical means. However, by inspecting the CNR of various satellites and their minima, the time when the jammer was closest to the receiver could be determined. For the reasons discussed above, only the phones were used for the analysis, since the geodetic receivers did not give enough measurements in the vicinity of the strongest interference. Furthermore, it must be considered that the CNR measurements near the jammer are noisy, so using their minima is not enough for an effective time-of-approach determination. Instead of using the bare minimum of an acquired set of measurements, it would be better to fit the data to an intended function and then use the minimum of that function. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **RTKLIB** \\ \hline Constellations & GPS + GLONASS + Galileo + BeiDou \\ Observations & Carrier-phase (L1/E1 + L2/E5b + L5/E5a) \\ Ambiguity & Continuous \\ Ephemeris & Broadcast \\ Elevation angle & 10\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Processing parameters used in RTKLIB. Figure 8: _Cont._  
In order to find a sensible fitting function, the following reasoning was employed. The baseband CNR is defined as the logarithm of the ratio between the signal power and the noise power density in a 1-Hz bandwidth (in units of Hz) due to the external blackbody radiation as a contribution due to interference effects and thermal noise generated by the receiver itself. Near the jammer, the noise density increases with the contribution of the jammer's noise density. This can be expressed using the Betz equation [38],  
\[\text{CNR}=10\log\biggl{(}\frac{S}{N+J}\biggr{)} \tag{1}\]  
where \(S\) is the signal power, \(N\) is the background noise power density, and \(J\) is the jammer noise power density. If the latter is assumed to decrease quadratically with the distance (assuming isotropic radiation) and the velocity is constant, then a fitting function \(f\) can be constructed (after some basic mathematical calculation) as follows:  
\[f=-10\log\biggl{(}a+\frac{b}{(t-t_{0})^{2}+c}\biggr{)} \tag{2}\]  
where \(a,b,c\), and \(t_{0}\) are the fitting parameters, and \(t\) is the variable of this function, with \(t_{0}\) being the minimum sought for. In order to obtain some sensible results, only the satellite and receiver combinations that gave satisfactory results (for instance, as in Figure 8a) were used. This required a manual inspection of all the acquired data, after which the following selection was made. The fitting itself was performed using the Levenberg-Marquardt method with non-imposed boundaries on parameters for the least-squares calculation. After finding the proper fitting function for all the combinations in Table 10, a further manual selection of good fits was performed. This had to be done, because not all fits gave satisfactory results. While most fits went well (see, e.g., Figure 9a), in some cases the fitting failed.