Analysis on Effects of L-Band Solar Radio Bursts on GNSS

Abstract

The strong L-band solar radio bursts will affect the navigation signals captured and tracked by the Global Navigation Satellite System (GNSS) receivers located in the sunlit hemisphere (sunny side), thus affecting the stability and accuracy of GNSS services. In this paper, an L-band solar radio burst erupted on August 28, 2022 was detected and analyzed based on GNSS Monitor System. Similar to December 13, 2006, the incident affected the GNSS tracking stations in the sunny side to varying degrees. Based on IGS data and low orbit satellite data, the characteristics and influence of the two events are compared. The analysis results show that: (1) solar L-band radio bursts will have different degrees of impact on the sun side navigation users; (2) the influence degree is positively correlated with the solar altitude angle; (3) the influence mode is similar to space-based suppression interference.

1 Introduction

Solar activity often affects satellite navigation system services. During the active periods of the solar system, it often causes severe disturbances in the ionosphere and changes in the gradients of electron density. In some severe cases, it may cause the decrease of the carrier-to-noise ratio of the signal, the increase of measurement error, even the decrease of the accuracy of ionospheric correction, etc. Solar radio bursts are direct interference from the sun. When its frequency band covers the frequency of navigation signals, it will cause varying degrees of interference to GNSS receivers in the form of strong noise. The eruption mechanism of solar radio bursts is very complicated, generally accompanied by solar flares, with a wide burst frequency band and a variety of burst types. However, the occurrences of L-band strong solar radio bursts are rare. Since the wide use of GNSS services, the most representative ones are the four consecutive solar flare events in December 2006, all of which were accompanied by L-band solar radio bursts. The radio burst on December 13, 2006 happened to be on the sunny side of China [1]. On August 28, 2022, based on the GNSS monitoring system, this type of event was detected again. Through the analysis of IGS tracking station and low-orbit satellite data, the characteristics, principles and impacts of the two events were compared.

2 Background

2.1 Surveillance Background

From 17:45 to 18:40 on August 28, 2022 (UTC time), Beidou, GPS, GLONASS, Galileo and other satellite navigation systems have an abnormal drop in carrier-to-noise ratio at all frequency points in nearly a hundred of IGS tracking stations located in Americas, western Europe, Atlantic Ocean, and the eastern Pacific Ocean [2]. After a joint analysis with the space environment monitoring departments, it was concluded that the solar activity interfered with the GNSS service signal on the sun-facing side of the earth during this period. According to the data analysis of IGS tracking stations, the tracking stations with abnormal carrier-to-noise ratio basically cover half of the earth, and they are all within the twilight line (i.e. the sun irradiated surface, at 18:00 on August 28, 2022); during the abnormal period (around 18:00), the subsolar point is near 9.7°N, 97.2°W, located in Central America. Similar to the radio burst during 2:00–4:30 on December 13, 2006, the parts of Asia and Oceania was just on the sunny side, and the subsolar point was near 23°S, 126°E, which was located in Australia.

2.2 Fundamentals of Data

Through the observation files and ephemeris files output by the IGS tracking station receivers distributed around the world, the downlink signal situation and service performance of the satellite navigation system can be calculated and analyzed. In addition, in the past 10 years, low-orbit satellites with an orbital altitude below 1000 km have developed rapidly, and GPS satellites can be tracked by on-board GPS receivers for orbit determination. Among them, the Swarm satellite is a type of low-orbit satellites launched by the European Space Agency (ESA) in 2013 for geomagnetic observation. It consists of 3 small satellites A, B, and C, with an orbital height of about 680km and a period of 2.2 h. On the same orbital plane, there are more than 6 satellites tracking GPS throughout the day [3]. The GPS receiver on the Swarm satellites can receive GPS satellite signals and generate observation files. The following analysis is based on the IGS station data of the two events in 2022 and 2006 and the Swarm low-orbit satellite data in 2022. Since in 2006, only the GPS system was officially providing services among the four major satellite navigation systems, and the Swarm satellite receiver only supports GPS, so we will take the L1 and L2 frequency of the GPS system as an example, from the carrier-to-noise ratio of signal, positioning accuracy, etc. to analyze its impact.

3 Effects on Receiving Signals

The carrier-to-noise ratio is used to measure the quality of the signal and indirectly reflect the measurement accuracy of the carrier phase. Carrier-to-noise ratio can also be used as a measure of the magnitude of interference a signal endures.

3.1 Analysis of Carrier-To-Noise Ratio Interference of GPS Signal in IGS Tracking Stations

As illustrated in Fig. 1, the twilight line of 18:00 on August 28, 2022 is drawn, and we select the data of four IGS tracking stations located on the sunny side for analysis. Figures 2, 3, 4 and 5 are the time sequence diagrams of the GPS signal carrier-to-noise ratio of the four selected stations. Among them, the upper picture is the L1 frequency point, the lower picture is the L2 frequency point, the left side of the vertical axis is the CNR value, the horizontal axis is the time axis, and the colored curves are the CNR timing curves of different satellites. It can be seen from the figures that the carrier-to-noise ratios of the four tracking stations all decreased in varying degrees between 17:45 and 18:40, with a magnitude of 5-17dB.

Fig. 1.

Subsolar point, twilight line and site distribution map (2022.08.28 18:00)

Fig. 2.

Time sequence diagram of carrier-to-noise ratio of GLPS station

Fig. 3.

Time sequence diagram of carrier-to-noise ratio of SGPO station

Fig. 4.

Time sequence diagram of carrier-to-noise ratio of POVE station

Fig. 5.

Time sequence diagram of carrier-to-noise ratio of ASCG station

In addition, the G05 satellite enforced a power boost operation during this period, which has a certain compensation effect on the decrease of the carrier-to-noise ratio caused by the L radio burst. Taking the TLSE station in France as an example (shown in Fig. 6), the right side of the vertical axis is the value of the solar altitude angle, and the black curve is the solar altitude angle curve. At 17:47, the sun elevation angle at TLSE station was about 5 degrees, and at the L2 frequency point, while the signal carrier-to-noise ratio of other GPS satellites decreased by about 5dB, after the power enhancement operation of G05 satellite, the carrier-to-noise ratio still has an increase about 7dB.

Fig. 6.

Time sequence diagram of carrier-to-noise ratio and solar altitude of TLSE station

For the BOAV station in Brazil (Fig. 7), the solar altitude angle is about 60 degrees, and the power enhancement operation of the G05 satellite does not significantly improve the signal carrier-to-noise ratio of the station.

Fig. 7.

Time sequence diagram of carrier-to-noise ratio and solar altitude of BOAV station

The IGS tracking station ALIC data on December 13, 2006, which was located in the sun side, was selected for analysi. The timing sequence diagram of its GPS signal carrier-to-noise ratio was shown in Fig. 8. It can be concluded from the figure that the carrier-to-noise ratio of the ALIC station drops more or less between 2:00 and 4:30, and the carrier-to-noise ratio drops by about 20dB from 3:31 to 3:36.

Fig. 8.

Time sequence diagram of signal-noise-ratio and solar altitude of ALIC station

3.2 Interference Analysis of Carrier-To-Noise Ratio of Swarm Satellites GPS Signal

By analyzing the onboard GPS observation files and ephemeris files of the three Swarm satellites [4], the sub-satellite point trajectories of the three satellites from 17:45 to 18:40 are given, as shown in Fig. 9, where the trajectory of Swarm A and C satellites are almost the same. From the timing sequence chart of carrier-to-noise ratio of receiving GPS signals (from Figs. 10, 11 and 12), it can be seen that at about 17:50 on August 28, the altitude angle of the sun at the sub-satellite point of Swarm A and C satellites was about 60 degrees, and the carrier-to-noise ratio decreased about 5-15dB; the altitude angle of the sun at the sub-satellite point of B is about 28 degrees, so the carrier-to-noise ratio decreased slightly.

Fig. 9.

Sub-satellite point trajectory of Swarm (17:45–18:40) and twilight line (18:00)

Fig. 10.

Time sequence diagram of GPS carrier-to-noise ratio of Swarm-A

Fig. 11.

Time sequence diagram of GPS carrier-to-noise ratio of Swarm-B

Fig. 12.

Time sequence diagram of GPS carrier-to-noise ratio of Swarm-C

3.3 The Relationship Between Altitude Angle of the Receiver to the Sun and the Change of Its Carrier-To-Noise Ratio

By drawing the twilight lines and the position of the subsolar point at the time of radio bursts, we illustrate the distribution of IGS tracking stations (Fig. 1) and the position of the sub-satellite point of the Swarm satellites at the same time (Fig. 10). Except for a few tracking stations, the GNSS service signals received by almost all the tracking stations located on the sunny side of the earth are affected. We select and calculate the sun altitude angle and GPS carrier-to-noise ratio decrease of four tracking stations and three Swarm satellites at 18:00. The result is shown in Table 1. Similarly, we analyze the sun altitude angle and GPS carrier-to-noise ratio decrease at 3:30 of the three tracking stations on December 13, 2006, and the result is shown in Table 2. As it shows, except for a few tracking stations/receivers far away from the subsolar point, almost all other tracking stations/receivers on the sunny side are affected, and the degree of the influence is proportional to the altitude angle of the sun, i.e. the receivers near the subsolar point are greatly affected, while those far away from the subsolar point are less affected.

Table 1. Corresponding table of position, solar altitude, carrier-to-noise ratio decreasing at the peak time of SRB event (2022.08.28)

Table 2. Corresponding table of position, solar altitude, carrier-to-noise ratio decreasing at the peak time of SRB event (2006.12.13)

3.4 Characteristic of Interference

The decrease of the carrier-to-noise ratio of the GNSS receiver is caused by the increase of the background noise. Observing the time sequence diagram of the carrier-to-noise ratio at the time when each receiver is interfered by two solar radio burst events, it can be found that the curve trend is basically the same, only the amplitude is different, and the disturbed pattern is similar to suppressive interference. It can be concluded that they come from the same interference source according to the distribution of total electron content (TEC) of the global fast ionospheric product released by the European Orbit Determination Center (CODE) in Fig. 13, it can be found that there is no abnormality in the ionosphere. Therefore, the GNSS cycle slip and loss of signal caused by radio bursts are essentially different from those caused by changes in the ionosphere. Such phenomena can be regarded as space-based suppressive interference initiated by the sun.

L-band solar radio bursts and ionospheric scintillations have similar effects, both of which can give rise to a variation of GNSS signals in C/N0. The difference between scintillations and SRBs is that SRB events cause an increase in the spectral noise density of the GNSS receiver, whereas scintillations cause variations in the GNSS signal power [5].

Fig. 13.

TEC Map of CODG (2008.08.28 18:00)

4 Effects on Positioning

The strong electromagnetic interference environment will affect the strength of the received signal, causing the satellite signal to be too weak or even loss the signal, or the increase of the positioning error, even the interruption of the service. On August 28, 2022, taking the GLPS station which experienced the most affected signal carrier-to-noise ratio as an example (Fig. 2), it can be seen from the DOP value and single-point positioning accuracy that the positioning service is not affected (Fig. 14).

Fig. 14.

Time sequence diagram of number of visible GPS satellites, DOP and positional accuracy of GLPS station (2022.08.28)

The influence of the solar radio bursts on precision positioning was analysed with the observation-minus-computation (OMC) residuals and ambiguities variation of carrier phase. As can be seen from Fig. 15, although the phase ambiguity has changed during solar radio bursts of 28 August 2022, it does not have an impact on precise positioning results which can be seen from the subplot of phase OMC residuals.

Fig. 15.

Time sequence diagram of GPS phase OMC residuals, and phase ambiguities for GLPS station (2022.08.28)

On December 13, 2006, taking the ALIC station which experienced the most affected signal carrier-to-noise ratio as an example (Fig. 8), the number of visible satellites dropped significantly (less than 4) from 3:31, and the positioning was interrupted for 6 min, and it occurred several times that the DOP value was unstable between 2:00 and 4:30 (Fig. 16).

Fig. 16.

Time sequence diagram of GPS carrier-to-noise, Number of visible GPS satellites, DOP and Positional accuracy of ALIC station (2006.12.13)

The influence of the solar radio bursts on precision positioning was analysed with the observation-minus-computation (OMC) residuals and ambiguities variation of carrier phase. As can be seen from Fig. 17, unlike the solar radio burst on August 28, 2022, the solar radio burst on December 13, 2006 has a significant impact on the carrier phase observations. During this solar radio burst, the large variations of phase observations can be seen, and the continuity of the phase ambiguities is destroyed, and this have a significant impact on precise positioning results which can be seen from the subplot of phase OMC residuals.

Fig. 17.

Time sequence diagram of GPS phase OMC residuals, and phase ambiguities for ALIC station (2006.12.13)

5 Conclusion

In this paper, an L-band solar radio burst erupted on August 28, 2022 was detected and analyzed based on GNSS Monitor System. Similar to December 13, 2006, the incident affected the GNSS tracking stations in the sunny side to varying degrees. Based on IGS data and low orbit satellite data, the characteristics and influence of the two events are compared. The analysis results show that:

• (1) Solar L radio bursts will have varying degrees of impact on the signal carrier-to-noise ratio and positioning services of GNSS on the sunny side;

• (2) The impact on the carrier-to-noise ratio is due to the increase of the noise, which is irrelevant to the ionosphere;

• (3) The disturbed pattern is similar to suppressive interference, and the source of the interference is the sun, i.e. we can regard the sun as a space-based interference source;

• (4) Regardless of whether it is a ground tracking station or a low-orbit satellite onboard GPS receiver, the degree of the impact is positively correlated with the sun altitude angle, and the tracking stations near the subsolar point is most affected;

• (5) The current GPS II-RM/IIF satellite can adjust its power ranges from 5 to 7dB, but this capability has limited compensation for receivers that are closer to the subsolar point, and only for receivers that are farther away from the subsolar point this technology has obvious advantage;

• (6) The impact on the positioning accuracy depends on whether it causes the satellite to lose signal and the number of lost locks. On August 28, 2022, the positioning was not affected, but on December 13, 2006, signal interruption and carrier phase cycle slip has been found, resulting in severe impacts such as decreased positioning accuracy and even the interruption of the positioning service;

• (7) It can be found from the carrier-to-noise ratio time sequence diagram of each monitoring station that the carrier-to-noise ratio of the GPS L2 frequency point has a significantly higher variation than the L1 frequency point, and the detailed reasons of this phenomenon are under our investigation;

• (8) Based on the data analysis of globally distributed tracking stations such as IGS, it can provide timely and accurate surveillance information for space environment, and mitigate the issues such as insufficient overseas monitoring capabilities and the lack of real-time information.