## 3 Sounding System
### System Description  
The system to perform this study relies on a software defined radio (SDR), which can be seen in Figure 2. The versatility of the SDR offers the possibility to adjust parameters for the adaptation of different scenarios. Our SDR was implemented with a field-programmable gate array (FPGA) combined with the Zynq-7010-SOC [21], which were placed in a Red Pitaya STEMlab 125-14 board [22]. This board makes all the computing operations possible, since it features analog-to-digital converters (ADC) and digital-toanalog converters (DAC) with a resolution of 14 bits, all driven by a system clock of 125 MSPS. The Red Pitaya board was connected via Ethernet to a Raspberry Pi 3 [23], which saved the received files onto a hard disk and managed the different peripherals [24]. These connections are presented below:  
Antennas: At the transmitter site, an inverted vee (V-) antenna was used, which was placed in La Salle URL in Barcelona. At the receiver side in Cambrils, two orthogonal inverted-V antennas were located. Figure 3 displays a graphical representation of the orthogonal antennas located in Cambrils. The frequency is currently set to 5.4 MHz, a value based on ionogram studies [25]. The height of the antennas is 14.5 m and the length of their legs is 14 m.  
Phasing Network: The two perpendicular inverted vee antennas worked together with a phasing network (PN; in Figure 2), which was in charge of shifting the phase of one of the two receiver antennas to make it possible to receive different and orthogonal polarizations [26]. The phasing network got a total of four wires, two from each antenna, as we duplicated the received signals using a radio frequency splitter (PDML-20A-100 from Merrimac Industries, Inc.). The route that both antennas followed was the same: one cable was lengthened with a quarter-wave phasing line to provide a 90\({}^{\circ}\) shift and connected to a radio frequency (RF) combi  
Figure 3: Diagram of the orthogonal inverted vee antennas in the receiver.  
Figure 2: System block diagram.  
Merrimac Industries, Inc.), and the other feed line was directly connected to a RF combiner. The output of the PN gave us a phase difference between the inverted vee antennas of either \(+90^{\circ}\) or \(-90^{\circ}\). A block diagram of the phasing network is displayed in Figure 4.  
* Amplifier: In order to do the sounding an A-class amplifier was used in the transmitter side. The model chosen was the Bonn BLWA 0103-250, which achieves 250 W of maximum power with an input power of 0 dBm.
* Low-noise amplifier (LNA): The model chosen was the ZFL-500 LN+, with a minimum gain of 20 dB, a frequency range between 0.05 and 500 MHz, and an operating temperature between \(-20\)\({}^{\circ}\)C and \(70\)\({}^{\circ}\)C.
* Filters: We used two band-pass filters (BPF) to avoid known interferences on both sides. On the transmitter side, we filtered the NVIS useful frequency range from 3 to 7 MHz. On the other side, we used a filter with a band pass between 4 and 6 MHz. Our system compensated for the phase delay of the BPF via software, as each data frame was corrected in both amplitude and phase before being demodulated and studied.
* GPS: A GPS was used to synchronize the transmitter and receiver in time (fundamental for the channel study performed). Time synchronization is essential to automate tests and data analysis. Our experiment had different signals sent, which depend on the minute of transmission. Thanks to the time synchronization, the transmitter knows which data file to send and the receiver tags it before saving it in order to analyze the data correctly. The transmitter and receiver were configured with extreme precision thanks to the GPS modules incorporated into the Raspberry. Furthermore, we also used PN sequences to detect the start of the received data structures and synchronize the transmitter and the receiver.