## 2 Materials and Methods
### Jamming Event Detector  
The maritime environment is not usually so harsh as land in terms of satellite visibility. It is highly uncommon for all satellites to be blocked due to a lack of visibility or obstacles. In the event of poor satellite visibility or obstacle, all signals from the same satellite would be blocked. In the case of a maritime open sea environment, a sudden drop in the Carrier-to-Noise density ratio (C/N\({}_{0}\)) of all satellites in the same frequency band is most likely some form of jamming, either intentional or unintentional. The idea here is to detect a jamming event and generate a jamming event detection flag and an alert message to the end-users based on the predefined detection threshold. The STRIKE3 project [21] proposed two standard event definitions, one of which is implemented in this manuscript. A jamming event is detected by the receiver based on the C/N\({}_{0}\) measurements. If the mean C/Nof all satellites in view drops 4 dB or more compared with the expected mean C/N0 of all satellites in a nominal situation, then a jamming event is said to be underway. The event detector incorporates one of the most commonly used KPIs (i.e., C/N0) in any GNSS receiver. As a result, when compared with traditional MFMC receivers, no considerable additional processing costs in detection and mitigation are expected. Figure 1 illustrates the detection of jamming on GPS L1 C/A and GPS L5 signals. The outcome of the jamming event detector is summarized in Table 1. Jamming events are identified on the lower and upper L-bands for about 51 s on both occasions. C/N0 is computed at the tracking stage after each correlation period (i.e., 1 millisecond for GPS L1 C/A and 4 milliseconds for Galileo E1). A jamming event is stated to be in progress, if the following condition is met. \[ \begin{gathered} Jamming_{Detection_{FBS_{signal}}}\\ =\left\{\begin{array}{ll}1,&\Delta(\text{C/N}_{0})_{f_{signal }}\geq 4\text{ dB}\\ 0,&\text{ {otherwise}}\end{array}\right.\end{gathered}  \tag{1}\]  
In Equation (1), \(\Delta(\text{C/N}_{0})_{f_{signal}}\) is the degradation in average C/N0 observed by the event detector for any specific GNSS signal can be expressed as follows:  
\[ \begin{gathered}\Delta(\text{C/N}_{0})_{f_{signal}}=\left( \overline{\text{C/N}_{0}}\right)_{f_{signal_{n}}}\\ -\left(\overline{\text{C/N}_{0}}\right)_{f_{signal_{j}}}\end{gathered}  \tag{2}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{3}{c}{**L1/E1/B1**} & \multicolumn{3}{c}{**L5/E5a/E5b**} \\ \hline \multirow{2}{*}{**Signals**} & **Event** & **Event** & \multirow{2}{*}{**Duration (s)**} & **Event** & **Event** & **Duration (s)** \\  & **Start (s)** & **End (s)** & & **Star (s)** & **End (s)** & **Duration (s)** \\ \hline \hline L1 & 97 & 148 & 51 & - & - & - \\ E1 & 97 & 148 & 51 & - & - & - \\ B1 & 97 & 148 & 51 & - & - & - \\ L5 & - & - & - & 90 & 141 & 51 \\ E5a & - & - & - & 90 & 141 & 51 \\ E5b & - & - & - & 90 & 141 & 51 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Jamming event detection on different GNSS signals. Figure 1: Jamming event detection for (**a**) GPS L1 C/A and (**b**) GPS L5. In Equation (2), \(\left(\overline{\text{C}/\text{N}_{0}}\right)_{f_{signal_{n}}}\) is the average expected C/N\({}_{0}\) in nominal scenario, and \(\left(\overline{\text{C}/\text{N}_{0}}\right)_{f_{signal_{j}}}\) is the measured average C/N\({}_{0}\) of \(f_{signal}\) at the \(j^{th}\) time instant.