*3.3. GNSS TEC Observations and Methods*

The TEC from GPS stations operating within the seismogenic zone of the earthquakes was estimated by measuring the phase and amplitude (at a rate of 50 Hz) and code/carrier divergence (at 1 Hz) for each satellite, and the observation interval was 30 s. This calculates the slant TEC (STEC) from the combined frequencies by pseudo-range and carrier-phase measurements using the following equations [22,23]:

$$\begin{aligned} \text{STEC} &= \frac{f\_1^2 f\_2^2}{40.28 \left(f\_1^2 - f\_2^2\right)} \left(L\_1 - L\_2 + \lambda\_1 (N\_1 + b\_1) - \lambda\_2 (N\_2 + b\_2) + \varepsilon\right) \\ \text{STEC} &= \frac{f\_1^2 f\_2^2}{40.28 \left(f\_1^2 - f\_2^2\right)} \left(P\_1 - P\_2 - (d\_1 - d\_2) + \varepsilon\right) \end{aligned} \tag{4}$$

where *f*<sup>1</sup> and *f*<sup>2</sup> are carrier phase frequencies of GPS signals at the two ends, *P* and *L* represent the pseudo-range and carrier phase observation of the delay path of the GPS signal. *λ*, *N*, and *ε* are the wavelength of GPS signals, the ambiguity of the ray path, and random residual of the GNSS signal along ray path, while *b* and *d* are the instrumental biases of the carrier phase and pseudo-range of the derived signal. Moreover, the STEC was converted into VTEC using the following equation [24]:

$$\text{VTEC} = \text{STEC} \ast \cos\left(\arcsin\left(\frac{R\sin z}{R+H}\right)\right) \tag{5}$$

where *R* and *H* are the radius of the Earth and the height of the top ionospheric layer in atmospheric altitude, respectively. *Z* is the elevation angle of the satellite for the ionosphere pierce point.

The ionosphere ranges from 60 km to 1000 km above the Earth's surface; to simplify the ionospheric model, the single-layer ionosphere assumption model is usually introduced. So, in this work, the height of the top layer of the atmospheric altitude was taken as 350 km [25].

#### **4. Analysis and Results**

#### *4.1. Geomagnetic Activity Background*

The spatial ionospheric variation is closely related to the solar and geomagnetic activity indices [26,27]. Thus, ionospheric anomalies caused by geomagnetism and solar activity should be avoided, and abnormal ionospheric disturbances related to earthquakes were obtained. Figures 3 and 4 show the time series for the F10.7, Dst, and Kp indices from 15 to 29 November 2019. The Dst index does not exceed −30 nT and the Kp index is less than 4, indicating that the geomagnetic field is calm and in a quiet ionospheric period. Therefore, the ionospheric anomalies caused by solar activity and geomagnetic activity can be excluded, and other geophysical signals (e.g., solar and geomagnetic activities) can be distinguished. The data were obtained from NASA's website at: https://omniweb.gsfc. nasa.gov/ (accessed on 15 July 2021).

**Figure 3.** Changes in F10.7, Dst, and Kp indices from 15 November to 29 November 2019.

**Figure 4.** TEC time series diagram of ionospheric anomalies. The red line represents the lower bound (LB), the green line represents the upper bound (UB), the blue line represents the total electron content (TEC), the vertical dashed lines represent the mainshock, dTECu represents the PEIA. (**a**) TEC time series of the Albanian earthquake center; (**b**) TEC time series of earthquake centers in Bosnia and Herzegovina; (**c**) TEC time series of the central grid of the Greek earthquake; (**d**) TEC time series of grid points at the confluence of the Greek and Albanian seismogenic regions.
