## 3 Sounding System
### Test Scenario  
To study the ionospheric channel, the research group installed a sounding system [29] between two points in the Catalonia region (Spain). These NVIS nodes established a link between La Salle University-URL Campus in Barcelona (41.41\({}^{\circ}\) N, 2.13\({}^{\circ}\) E) and a remote location in Cambrils, Tarragona (41.08\({}^{\circ}\) N, 1.07\({}^{\circ}\) E). Figure 7 presents a satellite picture of the terrain with the node locations highlighted in yellow. The distance between the two points without line of sight (LOS) was approximately 97 km, a value that is perfectly within the coverage area of an NVIS link. Surface wave signals did not affect our link thanks to the radiation pattern of antennas used in the experiment (their main beam is completely vertical, towards the sky) and the large distance between both points. This was verified as we transmitted different data frames throughout the whole day. During night, when  
Figure 6: Second data frame design.  
there is absence of ionospheric propagation at 5.4 MHz, we did not receive any signal in our receiver, thus confirming that surface waves do not affect our experiment. Figure 8 presents the elevation profile between the nodes.  
Because of the high interferences and electromagnetic noise in the HF band in Barcelona and its surroundings, the receiver was established in Cambrils. This configuration minimized the interferences in the receiving node, thus maximizing the robustness of the link.  
The channel study presents a sounding of 12 complete days in December 2019. A total of 8308 files of 29.7 MB were studied, resulting in more than 240 GB of collected data. In one hour, a total of 30 tests were performed. The tests are the transmissions made on our link. These tests follow the format indicated by the experiment, which indicates the data structure to be sent, the transmission power, and the order of modulation sent. These tests followed two different experiments depending on which data frame they were transmitting (Figure 5 or Figure 6). As it can be observed in Table 1, the first data frame transmitted did a transmitting power sweep for five different modulation orders. This experiment is used to evaluate the SDR's frequency instability by computing the frequency shift of the signal received.  
On the other hand, the second data frame (Table 2) did not present any modulated data (no modulation order implied) and was only transmitted at one transmitting power  
\begin{table}
\begin{tabular}{c c c} \hline
**Modulation Order** & **Transmitting Power** & **Min** \\ \hline
2, 4, 8, 16, 32 & 3 W & 05, 06, 07, 08, 09 \\
2, 4, 8, 16, 32 & 6 W & 15, 16, 17, 18, 19 \\
2, 4, 8, 16, 32 & 12 W & 25, 26, 27, 28, 29 \\
2, 4, 8, 16, 32 & 25 W & 35, 36, 37, 38, 39 \\
2, 4, 8, 16, 32 & 50 W & 45, 46, 47, 48, 49 \\
2, 4, 8, 16, 32 & 100 W & 55, 56, 57, 58, 59 \\ \hline \end{tabular}
\end{table}
Table 1: Experiment for the first data frame (see Figure 5).  
Figure 8: Elevation profile of the NVIS link.  
Figure 7: Location of the transmitter and receiver for the channel sounding.  
value. This second data frame and its experiment are used to compute the correlation coefficient between both channels (correlation of the impulse responses of both channels), the availability (data frames detected throughout a day), and the delay spread (received multipath of the signal).