## 3 System Description
### Transmission System  
From the beginning, we discarded the use of commercial transceivers, as we wanted to analyze the channel in bandwidths greater than the 3 KHz standard bandwidth. Moreover, the system was conceived as a Software Defined Radio platform, so the carrier frequency, the bandwidth and the modulation can be changed by software. Figure 3: The triaxial fluxgate variometer at LIV. The transmission system hardware of the channel sounder and the radio modem has been updated several times since the 2003/2004 survey. Three main Software Defined Radio (SDR) platforms have been installed in both the transmitter and the receiver. The first one, Sounding System for Antarctic Digital Communications (SANDICOM) was based on a HF radio controlled by a PC. The baseband signal was injected to the audio input of the transceiver, so the bandwidth was limited to 3 KHz and affected by the audio filter response [30]. During the 2005/2006 survey, a new software radio platform was developed using a Virtex FPGA and an external Digital Up/Down Converter responsible for mixing the RF signal to baseband, attenuating the adjacent channel interference and decimating the sampling frequency. This platform allowed us to test any bandwidth up to 15 KHz, and to cope with Signal-To-Interference Ratio (SIR) up to 80 dB [31]. However, the system had some relevant limitations. First, the maintenance was manual, which means that the daily configuration had to be changed connecting a laptop to the transmitter. The maintenance operations were done from 12:00 to 17:00 UTC, so neither sounding nor data transmissions were available during that period. As a consequence, the frequency range above 16 MHz, which propagate optimally during daytime, could not be observed. Moreover, both the GPS and the reference clock were medium quality devices, so the time and frequency accuracy was not very high. For these reasons, the absolute propagation time and the Doppler frequency shift could not be obtained. Finally, in the 2009/2010 survey, a third generation software radio platform was installed in both the ASJI and OE (see Figures 5 and 6). It is a weather resistant compact design installed near the antenna at the top of a hill close to ASJI (see Figure 7). The core of the system is a Virtex-4 XtremeDSP development platform from Xilinx with three FPGA that perform all the signal processing operations, included the up and down conversion. A new GPS unit with the Pulse per Second (PPS) signal increases the time synchronization accuracy (1 \(\upmu\)s), making the measurement of the absolute propagation time of the wave possible. In order to improve the frequency synchronization accuracy, a 100 MHz Oven Controlled Crystal Oscillator (OCXO) was installed in both the transmitter and the  
Figure 4: The tower with the antennas of the VIS system. Behind them (on the bottom left) one can distinguish four white huts, which constitute the geomagnetic observatory. receiver side. Moreover, the transmitter power amplifier can be switched off in case of severe impedance mismatch by means of a wattmeter that controls the forward and reverse transmitted power. Figure 5: Block diagram of the current transmitter hardware. Figure 6: Block diagram of the current receiver hardware. An embedded PC configures the transmitter and is remotely accessed by using a Wireless Local Area Network (WLAN) from the laboratory. Hence, the configuration is much faster and the system is able to work 24 h a day with an extended frequency range from 2 MHz to 30 MHz [20]. The transmitting antenna was a simple 7.5 m monopole with an antenna tuner able to operate in the whole HF band (see Figure 8). However, both the impedance matching and the radiation pattern were highly dependent on the goodness of the ground plane, thus, we installed a set of 32 radials that improved significantly the behavior in almost all the frequencies and the effective radiated power.