# First Evidences of Ionospheric Plasma Depletions Observations Using GNSS-R Data from CYGNSS
## 2 Materials and Methods
### Data Processing  
The data processing is performed to derive the CYGNSS observables of ionospheric activity. Source data comes from the DDMs. DDMs provided by CYGNSS has a size of 11 Doppler frequency bins \(\times\) 17 delay bins, with a resolution of 200 Hz per Doppler bin, and 0.2552 chips delay bin. Since 1 C/A chip is equal to 1/1,023,000 s, it is equivalent to 293.3 m. Therefore, the resolution per delay bin is 74.8 m. The images below present two examples of CYGNSS's DDM over two different surfaces. In order to assess the conditions of sea surface during the acquisition of these measurements, sea wave height is overlaid with the specular reflection points where the DDMs were taken, and it is shown for both cases in Figure 2, at points labeled as A and B. In the case of the DDM for rough water surface (Figure 3a) the \(SNR\) (Signal-to-Noise Ratio) is 2.3 dB and the wave height is around 3 m, marked in the map as A. For the calm ocean in Figure 3b, the \(SNR\) is much higher, around 15.3 dB, and the waves are around 0.6 m, marked as B in the map. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Orbital altitude & \(\sim\)520 km \\ Inclination & \(\sim\)35\({}^{\circ}\) \\ Period & 95 min \\ Linear speed & 7.6 km s\({}^{-1}\) \\ Covered region & 40\({}^{\circ}\)N to 40\({}^{\circ}\)S \\ Number of satellites & 8 \\ Number of GNSS satellites tracked per receiver & 4 \\ DDM sample rate & 1 Hz \\ DDM size & \(11\times 17\) (Doppler, delay) \\ DDM Doppler bin resolution & 200 Hz \\ DDM Delay bin resolution & 0.2552 chips \\ Total number of DDMs per day & \(2.7\times 10^{6}\) \\ Dataset volume per day & 10.4 GB \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of CYGNSS mission. Figure 2: Sea surface roughness shown as the wave height for the selected DDMs in Figure 3 is underlaid the \(SNR\) (Signal-to-Noise Ratio) values (in dB) for some samples around the exact coordinates of the DDM, marked with labels A and B, respectively. To perform this study, the main source of information is taken from the \(SNR\) (Signal-to-Noise Ratio) measurements available in the CYGNSS database. The \(SNR\) is post-computed on the ground using the DDM values as follows:  
\[SNR=10\log_{10}\frac{DDM_{max}}{N_{avg}}, \tag{1}\]  
where \(DDM_{max}\), is the value of the \(DDM\) bin with maximum signal and \(N_{avg}\) is the average noise per bin, computed from the delay-Doppler bins before the DMM peak (i.e., delay < 0, as in Reference [8]. The value of the \(SNR\) is then given in decibels (dB). The maximum number of samples per day, considering the 8 satellites and their 4 channels, is around \(2.7\times 10^{6}\) measurements. An example of this data is plotted in a world map in Figure 4, where both the measurement over land and oceans are displayed. Colorbar indicates the \(SNR\) of reflected signal in dB. As the aim of the study is to observe the possible fluctuations in the intensity of the signal after crossing the ionosphere, the following computation is performed for every vector of data in a day. Having the 4 channels of SNR values during a whole day, it is chopped from a user-input \"start_time\" to an \"end_time\". Then, using the _quality flags_ that the CYGNSS database provides, the vector is chopped using only the points over the oceans farther away than 25 km from the coastline, and within a rectangular area that the user can define. Figure 4: SNR values on specular reflection points for the full day 21 November 2017, on all regions (sea and land) from 40\({}^{\circ}\)S to 40\({}^{\circ}\)N. Values in dB. Figure 3: CYGNSS Delay-Doppler Maps (DDMs) examples of specular reflections over two different surfaces: (**a**) rough sea surface and (**b**) calm sea surface. Plots represent the equivalent analog power in Watts received per each delay/Doppler bin, distributed around the adjusted zero delay/Doppler for the particular geometry configuration. Note the difference in the color scale. The output is a vector of data with internal discontinuities in time.