# Physical Layer Definition for a Long-Haul HF Antarctica to Spain Radio Link
## 1 Introduction  
Antarctica is a remote and isolated continent of great interest for the scientific community. Many remote sensors are scattered across the continent to conduct experiments related to different disciplines such as biology, geology and physics, performing experiments that cannot be reproduced anywhere else on Earth. These sensors usually store the data during the winter until they can be downloaded the following summer or send the data periodically via a satellite link. However, in polar zones, the transmission between the sensor and the geostationary satellite is poor, thus, in this context, HF transmission is a good alternative. La Salle, together with the Observatori de l'Ebre (OE), both part of the Universitat Ramon Llull (URL), have been working in a joint research project in Livingston Island (62.7\({}^{\circ}\)S, 299.6\({}^{\circ}\)E), in the South Shetland archipelago for remote sensing and skywave digital communications modem design to be received in Cambrils, Spain (41.0\({}^{\circ}\)N, 1.0\({}^{\circ}\)E). Even though the Spanish Antarctic Station Juan Carlos I (ASJI) is only manned during the Austral summer, the recollection of scientific measurements is continuous. The information that has to beanalyzed in almost real-time is transmitted to the OE in Spain through a satellite link. To date, only a part of the recorded geomagnetic data of just one of our two variometers in operation can be sent to the OE by satellite link and made available using the facilities provided by the INTErnational Real-time MAGnetic observatory NETwork (INTERMAGNET). A link based on LEO satellite networks [1; 2] would have the capability to cover the polar zones in an efficient manner, but the aim of the project was to develop a proprietary communications system, not depending on any other commercial service or technical support. Furthermore, one of the goals of the project was to design a radiomodem, which could perform as a communications system or an oblique ionospheric sounder itself. So, a radio High Frequency (HF) transmission modem can be seen as either an alternative to satellite or a complementary backup system in case of satellite link failure, in terms of data communication. The reader is referred to [3] for more information about the origin and the history of the project, as well as the reasons to plan a backup data transmission system. The best transmission scheme for long-haul HF links with extremely low Signal-to-Noise Ratio (SNR) has to be found in order to overcome severe channel transmission conditions. Several standards [4; 5] have been studied to be applied in this case, but none of them have succeeded in making the SNR to work in the 12,760 km ionospheric link. Therefore, a new physical layer has to be defined. First, narrowband and wideband soundings were performed in [6], in order to describe the channel performance. Afterwards, the system was improved in terms of sounding frequency availability and number of soundings per day, and more complete results were evaluated in [7], giving more details on the channel parameters. Also several modulation tests using spread spectrum modulations [8] have been performed, as well as Orthogonal Frequency-Division Multiplexing (OFDM) [9] and single carrier modulations [10]. This paper has two main goals. The first one to describe in detail the final HF modem hardware, improved since [6; 11]. The second one is to continue the work of [7], explaining the narrowband and wideband soundings. In this paper, we present the study of the synchronization sequences, we analyze the channel error burst rate and we choose the best modulations and their configuration to define a physical layer proposal. In order to define a physical layer, we have identified and evaluated four different transmission situations: (i) narrowband sounding; (ii) wideband sounding; (iii) daytime data transmission and (iv) nighttime data transmission.