## 4 Impact of Strong Scintillations on GNSS Receivers  
One of the consequences of scintillation is LoL of the tracking stages of the receiver, affecting the quality of navigation solution and also making impossible the calculation of the scintillation activity through the classical indices. In case of LoL, the computation of indices using tracking loop outputs is not reliable, thus leading to false alarms and missed detections of the monitoring unit.  
An example of LoL due to strong amplitude scintillation in closed loops is reported in Figure 3. From top to bottom, the values of the I and Q prompt correlators, the estimate of the \(C/N_{0}\), the values of the Phase Lock Loop (PLL) discriminator output, the estimate of the Doppler frequency \(f_{D}\) and the estimated \(S_{4}\) value are reported. Figure 3a reports the tracking results obtained running a classical second order tracking loop. After about five minutes, the fading induced by amplitude scintillation is so strong that the PLL loses the lock on the signal. As a consequence, the \(S_{4}\) values provided by the scintillation monitoring engine, and computed upon the values of the I and Q correlators, cannot be trusted. As a benchmark, the results obtained running an experimental third order loop, especially configured in terms of bandwidth and loop parameters to be able to track such extreme fading situations, are reported in Figure 3b. The third order loop is able to keep the lock on the signal, even though the quality of the measurements is low. In this case, the estimate of the \(S_{4}\) can then be considered valid. It is worth noting that, for the first five minutes, the two architectures provide the same value of the \(S_{4}\) index, while, after the loss of lock, the values provided by the 2 nd order PLL are false, and, in particular, they underestimate the intensity of the scintillation event.  
In general, under significant scintillation events, cycle slips, degradation of \(C/N_{0}\) and corruption of data bits affects the quality of the estimated position (since some satellites might have to be excluded from the solution). In addition, the estimations of \(S_{4}\) values for the affected channels are wrong, as shown in Figure 3a.  
Various techniques are proposed in the literature to reduce the probability of a LoL in the presence of scintillation. The straightforward solution is indeed to exploit a third order PLL [26; 27], as shown by the previous example, but also advanced receiver architectures have been proposed, [12]. However, the only solution able to completely overcome the problem of LoLs is to abandon the closed loop implementation in favour of an open loop architecture [18; 28]. It is, of course, well known that  
Figure 2: Generic architecture of a Software Defined Radio (SDR) satellite navigation receiver.  
closed loop solutions are optimal in terms of limiting the jitter on the code and phase estimations, but they show limitations for the specific use in scintillation monitoring receivers in the presence of strong events.  
In the following sections, an open loop architecture will be exploited to extract from the incoming signal the information related to the amplitude scintillation intensity, defining a metric alternative to the \(S_{4}\) parameter. This architecture has been firstly presented in a previous paper of the authors [18], but has been refined and extended in the present work. A similar approach has been proposed in other works (such as [29; 30]); however, the architecture proposed in this study overcomes several limitations, among which the need of a fine time assistance.