Session B1: Atmospheric Effects, GNSS Remote Sensing, and Scientific Applications

Effects of Unknown Radio Frequency Interference on Ionospheric Scintillation Monitoring in Urban Environments
Weilin Gong, Kai Guo, Zhipeng Wang, Yiming Wang, Beihang University
Location: Beacon B

Plasma density irregularities in the ionosphere affect the propagation of Global Navigation Satellite System (GNSS) signals, causing rapid 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. In this paper, based on the GNSS data collected at the ionospheric monitoring reference station in Beijing, we investigated the effects of unknown RF interference 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 the three S4 anomalies are investigated separately.
The scintillation data analyzed in this study are collected by a Septentrio PolaRxS Pro receiver installed at the National key laboratory of CNS/ATM in Beijing (Latitude 39°58?N, Longitude 116°20?E). The receiver is dedicated for ionospheric monitoring and capable of generating amplitude and carrier phase measurements at 50 Hz. It can output amplitude and phase scintillation indices at a rate of 1 min. The data recorded on the B1I signals of the Beidou Satellite Navigation System (BDS) are chosen for the analysis. The antenna for receiving GNSS signals is installed on the roof of the laboratory building. A RF cable with a length of around 150 meters is used to connect the antenna to the receiver in the lab. In this paper, BDS data from May 1 to 31, 2024 are selected to analyze the effect of interference on scintillation. The satellite elevation mask angle is set to 30 degrees to eliminate the effect of multipath.
The S4 index analyzed is calculated from the signal intensity and corrected by using the carrier-to-noise density ratio C/N_0. By analyzing the amplitude scintillation index S4 of all visible satellites output by the receiver, three types of anomalies are detected, i.e., (1) an overall rise of the S4 ranged from 0.06 to 0.12; (2) a stair-step increase in S4 between 0.18 and 0.22 and lasts for 5-6 hours and (3) spikes with the S4 index above 0.5 for all satellite signals, while with a very short duration.
All these phenomena are not possible at mid-latitudes as scintillation merely occurs in this region. To rule out the effect of receiver configuration in scintillation monitoring, the BDS signals emulated by a Spirent GNSS signal simulator GSS9000 are fed into the same receiver in the laboratory. The measured S4 index did not appear any abnormal, which means that the receiver configuration is correct.
To further investigate the factors that induced the S4 anomalies, the detrended signal intensity and C/N_0 of the open-sky signals are further analyzed. A sixth-order Butterworth low-pass filter with a cutoff frequency of 0.1 Hz is used to detrend the intensity measurements. The analysis shows that during the overall S4 rise period, the detrended signal intensity undergoes decay with a period of 10 seconds and depth of around -4 dB. A short time Fourier transform (STFT) is conducted on the detrended signal intensity. By analyzing the normalized STFT magnitude, it is found that there is a peak of Power Spectral Density (PSD) at 0.1 Hz. Therefore, the presence of periodic pulse interference of lower intensity may contribute to the overall increase of S4. In addition, it is shown that the C?N_0 is maintained at around 40 to 42 dB-Hz, which is generally lower than the normal C?N_0 power and may due to the long cable between the receiver and the antenna. The low C?N_0 may cause the S4 exponential correction term to be unreliable. This needs to be further validated. The results will be the shown the in the paper.
Next, the factors for the stair-step increase in S4are analyzed. When an increase occurs, the detrended signal intensity presents a periodic decay with a period of 10 seconds and the depth up to -10 dB. Then another periodic decay occurs with the same period about 5 seconds apart from the first decay. The second decay depth is less than -10 dB but also exceeds -5 dB. By performing the STFT on the detrended signal intensity, it is found that the normalized STFT magnitude shows a larger peak at 0.1 Hz than during the overall rise of S4, while a second peak occurs at 0.2 Hz. This indicates that there is a new periodic interference jointly causing the step increase in S4. In addition, there is a step decrease in the C?N_0 to about 38 dB-Hz. the C?N_0 returns to normal after the end of the step increase.
Finally, the factors contributing to the pulsed spikes in the S4 index are analyzed. During the spikes, the detrended signal intensity fluctuates irregularly, with a maximum decay of up to -20 dB. The C?N_0 showed a similar decreasing spike, falling below 35 dB-Hz. This indicates that the spikes in S4 are also due to the presence of impulsive interference with high intensity. The BDS signals are received through the RF front-end universal software radio peripheral (USRP) X310. The analysis is currently in progress and will be shown in the paper.
This study analyzes the amplitude scintillation index anomalies observed in May 2024 during the daily monitoring of the ionosphere at the Beijing reference station. The causes of the S4 anomalies are investigated. The C?N_0 and signal intensity are analyzed. The anomalous signal in the frequency domain is studied. Based on these analyses, the following conclusions can be drawn.
There are three types of anomalies identified in the S4 index, namely, an overall rise, a stair-step increases and spikes. The overall rise raises the S4 to between 0.06 and 0.12. A stair-step raises the rise to between 0.18 and 0.22. A spike raises the S4 above 0.5, but for a very short period of time.
Based on the analysis of the signal intensity in time and frequency domain, it is found that the overall rise of S4 is caused by a weak periodic pulse interference with a period of about 10 seconds. The stair-step increase occurs due to the mixing of a new periodic pulse interference that is stronger. The spikes occur as an unknown interference with a very short duration. The spectral distribution of the IF data will be analyzed later to determine the frequency range where the interference is located.
The S4 index is corrected by using the carrier-to-noise density ratio C/N_0. When C/N_0 is relatively low, and interference may be present, the correction term becomes unreliable. The S4 correction formula needs to be validated.
These results are important for better understanding the impact of unknown RF interference on ionospheric scintillation monitoring in urban environments. It could also be useful for modeling GNSS signals under RF interference for distinguishing between RF interference and scintillation.