Effects of unknown radio frequency interferences on ionospheric scintillation monitoring in urban environments

Plasma density irregularities in the ionosphere affect the propagation of Global Navigation Satellite System (GNSS) signals, causing rapid and random fluctuations in amplitude and phase, a phenomenon known as ionospheric scintillation. Due to the fact that ionospheric scintillation severely degrades the GNSS receiver performance, accurate scintillation monitoring is required. In recent years, more Ionospheric Scintillation Monitoring Receivers (ISMRs) are deployed to monitor scintillation. However, scintillation monitoring can be affected by a wide range of factors, such as multipath effects, Non-Line-of-Sight (NLOS) signals and Radio Frequency Interference (RFI), which are more likely to occur in urban environments. These factors can affect the accuracy of scintillation monitoring to different extents. In this paper, based on the GNSS data collected at the ionospheric monitoring reference station in urban area in Beijing, China, we investigated the effects of unknown RFI on scintillation monitoring. It is shown that the interference mainly affects the amplitude scintillation index S4 and has no significant effect on the phase scintillation index. During the monitoring period, three kinds of anomalies, namely, overall rise, a stair-step increase and spikes, are found to occur in the S4 index. The possible causes of these S4 anomalies are analyzed separately by exploiting the carrier-to-noise density ratio (C/N₀), estimated by the Variance Summing Method (VSM) method, and the detrended signal intensity. The spectrum of the intermediate frequency (IF) data in the presence of S4 anomalies is also analyzed utilizing the Short-Time Fourier Transform (STFT) method for frequency domain analysis. The proposed study is of great significance for better understanding the impact of RFIs on ionospheric scintillation monitoring. It can also contribute to explore the methods to mitigate the interference of RFIs in accurate scintillation monitoring.