## 4 Data Transmission  
The sensors placed at the SAS premises are only manned during austral summer but they are active during summer and winter. Hence, in order to process the data in close to real time, they have to be send to the research laboratories in Spain. Since 1998, the data from the geomagnetic sensor have been transmitted by means of a geostationary satellite link from the SAS to the INTERMAGNET GIN in Edinburgh. During 2007-2008 Antarctic campaign, the new geomagnetic sensor transmitted the data to a GIN placed in Ottawa, with high reliability. The aim of this research project is to establish a low-rate low-power skywave link between Antarctica and Spain that serves as a backup or even an alternative to the satellite link. The skywave radio link between the Antarctica and Spain has a set of characteristics and impairments that have to be taken into account to properly design the analog front-end and the digital signal processing (DSP) modules:* The Antarctic Treaty System regulates international relations with respect to the Antarctica. Among others, it regulates the environmental laws, including aesthetic factors, and restricts the deployment of big structures because of the environmental impact. Hence, high gain antennas that would need reinforce concrete underpinning are discouraged. * Frequency availability of this HF link is highly time dependent [26]. Therefore, the transceiver must be able to reconfigure the oscillators frequency and the antennas must cover a wide HF frequency range. * Low transmission power is required not only to fulfill the International Telecommunication Union (ITU) recommendations but also because during austral winter, the power consumption at the SAS is restricted as it is only served by batteries charged by wind and solar power. * The ground path between the SAS and EO is approximately 12,700 km, which corresponds to a minimum of 4 hops. A one-hop link is considered to be less than 4000 km [27]. * The attenuation of a transmitted signal at 10 MHz is of 135 dB only due to free space path loss. * The noise and interference levels at the receiver are very high. e.g. the signal to interference level at the antenna output at frequencies between 4.5 MHz and 9 MHz can be as low as \(-75\) dB [28]. * The ionospheric channel is a multipath channel since reflections occur at different layers of the ionosphere. Also, since the ionization level of each layer varies in time, the channel is time variant. * A simplex ionospheric link is considered. Thus no channel state information exists at the transmitter site and no automatic repeat-request (ARQ) error control method can be implemented. The HF band serves a wide range of applications and purposes: broadcasting, surveillance, monitoring, _etc._, and might have a wide range of coverage. Potentially, global coverage may be reached by taking advantage of the two modes of propagation: surface wave and sky-wave propagation. So, a global organism must take care and coordinate the access to the ionospheric medium. ITU is the leading United Nations Agency for information and communication technology issues. Since mid nineteenth century, the ITU Radiocommunication Sector (ITU-R) has coordinated the shared global use of the radio-frequency spectrum. The primary objective of the ITU-R is to ensure interference free operations of radiocommunication systems. Consequently, the world has been divided into three different regions. Region 1 comprises Europe and Africa, region 2 comprises Asia and Oceania and America forms region 3. In each region, the ITU-R suggests the attribution of a frequency band to one or several services, such as fixed, maritime mobile, aeronautic mobile, broadcasting, _etc_. When more than one service merge into the same frequency band then they are categorized into primary and secondary services.