# Effects of Equatorial Plasma Bubbles on Multi-GNSS Signals: A Case Study over South China
## 2 Data and Methods  
In this study, data from an all-sky airglow imager and a GNSS receiver at Zhuhai (22.35\({}^{\circ}\)N, 113.58\({}^{\circ}\)E) in southern China are used. The all-sky airglow imager is KEO Sentry 4 monochromatic imager. The imager is equipped with five filters, including a 630 nm filter for imaging the ionospheric density in the F2 region. The 630 nm airglow intensity image is obtained with an exposure time of 60 s and an acquisition interval of 5.8 min. Assuming an airglow height of 300 km, each data grid point is projected onto geographic latitude and longitude coordinates based on the distance and azimuth from the zenith. The radius range of the valid airglow image is approximately 400 km (approximately 120\({}^{\circ}\) field of view). To remove interference from background light sources, the average of the previous seven images is subtracted from each image. A negative value in the final airglow image means that the ionospheric density is depleted. Figure 1 shows an example of a post-processed airglow image at 15:24 UT on 3 April 2022. The unit of the post-processed airglow image is the same with the intensity value of the original airglow image (from 0 to 65,536, 16 bit), and we focus on the relative magnitude of airglow value. The characteristics of the EPBs are clear, and the airglow value can be extracted at a fixed location and compared with GNSS observations at the ionospheric pierce point (IPP) location at an effective height of 300 km.  
The GNSS receiver is a Septentrio PolaRx5S ionospheric scintillation monitoring receiver (ISMR), which records signal messages from multi-frequency and multi-GNSS constellations; generates carrier phase, pseudorange, and carrier-to-noise ratio measurements with a temporal resolution of 1 s in mixer observation data file; and outputs the S\({}_{4}\) scintillation index with a temporal resolution of 60 s. The STEC is obtained from dual-frequency carrier-phase observations and converted to vertical TEC (VTEC) [36]. The differential code bias (DCB) is assumed to be constant daily. The observed TEC is assumed to be equal to the TEC from the IGS ionospheric TEC map, after which the DCB is obtained [37]. Observations with elevation angles less than 20 degrees are discarded to mitigate observation errors. The trend of the daily VTEC variation (VTEC baseline) is obtained by the Savitzky-Golay filter [38], which is a least-squares smoothing filter to filter high-frequency signal components, and the deviation VTEC (dVTEC) is subsequently extracted after removing the trend.  
In addition, variations in the loss of phase observation and phase cycle slip (CS) are used to assess the GNSS signal quality. Loss of phase observation is identified as the absence of a recorded carrier phase due to the low signal strength [39]. Moreover, the G-nut Anubis v3.6 tool ([[https://gnutsoftware.com/](https://gnutsoftware.com/)]([https://gnutsoftware.com/](https://gnutsoftware.com/)), accessed on 1 March 2024) has been used to detect cycle slips [40]. The principles of the cycle slip detection algorithm in G-nut Anubis  
Figure 1: An example of a post-processed airglow image at 15:24 UT on 3 April 2022. (**a**) Geographical distribution of relative airglow values with pseudocolor. The black star represents the location of the ground station, and the black triangle represents the ionospheric pierce point (IPP) location of BDS GEO satellite C03 at 300 km. The dashed and dash-dot lines mark the longitude and latitude of the IPP, respectively. (**b**) Airglow values at a fixed latitude of 21.2\({}^{\circ}\) extracted from panel (**a**). The black dashed line indicates the airglow value extracted at the fixed IPP.  
tool are based on independent linear combinations of triple-frequency undifferenced GNSS observations [41]. Losses of phase and cycle slips are identified as error epochs in this study.  
The EPB events on 3 April 2022 are selected, with a mean Kp of 1.75 and F10.7 of 140.2, suggesting quiet geomagnetic conditions.