## 4 Data Transmission
### System description  
In this section, the transceiver used in the Antarctica to Spain HF link is described. HF receivers are typically implemented by means of several super-heterodyne conversions. Multiple intermediate frequency stages are used because of the complexity to achieve high selectivity and gain to any single frequency. However, high quality synthesizers are required and local oscillators should have to be phased locked to a reference frequency. Moreover, high quality automatic gain control systems are required in each stage to preserve the receiver performance. The aim of the transceiver architecture used in this research project is to place the ADC as close to the receiver antenna as possible, digitize the whole HF band and then, in the digital domain, select and downsample the channel of interest to baseband. This is only possible due to the development of high performance ADCs and digital signal processing (DSP) technology. The HF transceiver developed and used in this HF link is based on a digital platform with a Field Programmable Gate Array (FPGA) from Xilinx plus a high speed digital to analog converter (DAC) and an ADC [26]. The signal is fully processed in the digital domain and therefore only amplification and some filtering are performed in the analog domain. This architecture benefits from the advantages of Software Defined Radio technology [29] and thus it allows us to implement any communication system by just introducing changes to the software. The transmitter platform, see Figure 2.a, upsamples the sampling frequency by means of a digital upconverter (DUC), filters and upconverts the baseband signal to the proper frequency that maximizes link availability. The upsampled analog signal feeds a low pass filter (LPF), then it is amplified up to 250 watts and finally delivered to a 7.5 meter monopole. In the receiver site, see Figure 2.b, the incoming signal is received with a 7.5 meter monopole, amplified with a low noise amplifier (LNA), filtered by a tunable bank of band pass filters (BPF), amplified with a variable gain amplifier (VGA) that adjusts the received power level 6 dB below the full scale range of the ADC and finally it is low pass filtered to reduce aliasing components. In the digital domain, the same FPGA digital platform used in the transmitter site mixes the received signal to baseband, filters it and decimates the sampling frequency by means of a digital down converter (DDC). Eventually, data demodulation are obtained in non real time mode after DSP. Prior to transmit the data from the sensors, a preamble, depicted in Figure 3, must be transmitted for tuning and synchronization. Since the wide range of frequencies that maximize availability of the ionospheric link between the SAS and EO, transmit and receive antennas must have a wide frequency range of acceptable performance. Furthermore, they must be tuned to match its impedance with the transmitting and receiving elements at each different frequency. Approximately, 5 seconds are needed for antenna tuning. Clock frequency mismatch between transmitter and receiver oscillators due to time and  
Figure 3: Transmitted frame. Figure 2: Transmitter (a) and receiver (b) system architecture based on Software Defined Radio Technology. temperature drifts must be fixed, otherwise they may have severe effects on data demodulation. During 5 seconds, a non modulated tone is transmitted and used at the receiver site to estimate the frequency deviation between transmitter and receiver clocks. This non modulated tone is also used to determine frequency availability and signal to noise ratio (SNR) statistics. The Global Positioning System (GPS) provides coarse timing synchronization between transmitter and receiver. Fine timing synchronization is acquired by means of a number of Maximal Length pseudo noise sequences of 255 chips each, transmitted at chip rate of \(\frac{1}{T_{c}}=5.4\) kcps, approximately during 200 ms. The good autocorrelation properties of these sequences assure an accurate timing synchronization.