## 2 System Description  
Since June 2019, the Taiwanese American FS7/COSMIC2 program has been executing active limb sounding of the Earth's neutral atmosphere and ionosphere via GPS/GLONASS RO observations from low-Earth orbiting (LEO) satellites. Similar to the prior mission, FS3/COSMIC, the FS7/COSMIC2 is a six-LEO-satellite constellation mission but orbits at 24\({}^{\circ}\) inclination and \(\sim\)550 km altitudes (\(\sim\)720 km altitudes for parking orbits). It enhances the Global Navigation Satellite System (GNSS) receiver's capability to receive multi-channel (1.5 GHz and 1.2 GHz) GPS and GLONASS satellite signals and can provide more than 5000 RO observations per day within the region between the geographic latitudes of \(\pm\)40\({}^{\circ}\). Each RO observation can provide a set of limb-viewing measurements on GNSS signal intensity and phase from the LEO satellite altitude to the Earth's surface. Those measurements can be further retrieved into \(N_{e}\) and limb-viewing scintillation index \(S_{4}\) profiles in altitudes [13; 14]. In this study, we propose to identify a plasma bubble in the Taiwan-Philippines sector by using the FS7/COSMIC2 GPS/GLONASS RO observations. Meanwhile, the Taiwan-Philippines sector is defined as the ionospheric region located in \(20^{\circ}\pm 15^{\circ}\) N and \(120^{\circ}\pm 15^{\circ}\) E geographic coordinates, where the geomagnetic latitudes are from \(\sim-3^{\circ}\) to \(25^{\circ}\).  
The Hualien VIPIR is a modern ionospheric radar (also termed ionospheric sounder or ionosonde) that fully digitizes complex signal records and uses multiple parallel receiver channels for simultaneous signal measurements from multiple spaced receiving antennas [15]. As a usual ionospheric sounder, the Hualien VIPIR transmits pulsed waveforms in the medium- and high-frequency (MF/HF) bands and measures the envelope group delays, i.e., virtual ranges, of ionospheric echoes to produce ionograms with ionospheric trace h\({}^{\prime}\)(f) as a function of the radio carrier frequency. Details of the system can be found in [16].  
Other sources of scintillation observations are the 133 ground-based GPS receivers located in Taiwan and the surrounding islands. They are operated and maintained by the Central Weather Bureau (CWB) of Taiwan and routinely provide 1-Hz satellite navigation system data (in the RINEX format). The online processing includes determinations of scintillation index \(S_{4}\) and the two-dimensional vertical \(S_{4}\) map in the Taiwan-Philippines sector. Based on earlier investigations [17], the theoretical and experimental analyses show that scintillation index \(S_{4}\) values become underestimated when a sampling spatial scale is larger than the first Fresnel zone (FFZ). We note that the 1-Hz sampling rate from ground-based GPS receivers is high enough to determine complete \(S_{4}\) values but not to characterize the signal scintillation spectrum. Thus, we have designed and implemented a software-defined receiver in order to acquire and track GPS (and Satellite-Based Augmentation System, SBAS) L1-band C/A code signals [18; 19]. Compared with most commercial GPS receivers, software-defined GPS receivers offer added flexibility and versatility by implementing most functions in software. Another advantage of a software-defined GPS receiver is that it could have a maximum sampling rate of 1000 Hz due to the L1-band Coarse Acquisition (C/A) code duration of 1 millisecond, and the executing sampling rate can be much higher than that of a usual commercial GPS receiver. Spectrum analyses on the received signals of two software-defined GPS receivers located at Chungli (\(24.97^{\circ}\)N, \(121.19^{\circ}\)E) and Hualien (\(23.89^{\circ}\)N, \(121.55^{\circ}\)E), Taiwan will be used to characterize the targeted ionospheric irregularities.