## 3 Remote Sensors at the SAS
### Ionosonde: Vertical incidence soundings of the ionosphere  
In order to have a sensor providing ionospheric monitoring in this remote region, a vertical incidence ionospheric sounder (VIS) was installed at the SAS during 2004-2005 Antarctic survey. This ionosonde is nowadays also being used to provide information for the HF radio link employed for data transmission from the SAS to Spain. A brief description of the HF radio link and of the results obtained from it will follow after the current discussion. The VIS operating at the SAS is the Advanced Ionospheric Sounder (AIS) developed by the _Istituto Nazionale di Geofisica e Vulcanologia_ (INGV) of Rome, Italy. Details of the sounder are available in [9]. Data provided by the VIS serve for conducting ionospheric research at that remote region, mainly to characterize the climatology of the ionospheric characteristics and to investigate the ionospheric effects caused under geomagnetically disturbed periods (e.g., [10; 11]). The typical pattern of the summer behavior of the ionospheric F2 characteristics and of their variability was evaluated for different Spanish Antarctic surveys, and the differences of the above patterns from survey to survey was related to the different solar activity. Such a pattern at mid-high latitudes of the SAS was also compared with the summer behavior at other mid-latitude stations and it was noticed a largerpost-sunset electron density enhancement at the SAS than that recorded in other regions. The significant decrease of the critical frequency of the F2 region, foF2, and their strong uplift as observed by the significant increase in the virtual height, h'F2, for the geomagnetically disturbed period of 18 February 2005 was also investigated and explained in terms of response of the ionosphere to the geomagnetic storms [12, 14]. In addition, the data recorded with the VIS, especially the maximum usable frequency for a single hop transmission at 3,000 km reflected at the F2 layer MUF(3000), has been correlated with data extracted from the HF radio link established between the SAS and Spain [11]. The above correlations were obtained for a chain of available VIS located nearby the radio path, resulting that the frequency with the largest availability of the ionospheric channel seem to be limited by the lowest of the MUF(3000) recorded along the path and that the ionospheric conditions close to the receiver have the larger influence on the daily behaviour of the channel availability for most of the time under the investigated seasonal conditions. In [11] it is also stated that frequencies of HF radio link delivered at the receiver suffer a significant drop of power for those frequencies larger than the MUF(3000) as recorded at the receiver compared to those frequencies lower than the MUF(3000). The above results obtained with the ionospheric sensors installed at the remote region of the SAS fit with current ionospheric research carried out by other teams for regions nearby the Antarctic latitudes and other latitudes of the globe. As a brief summary and just focusing in the more recent works, we would like to highlight the following examples which are organized according to climatological studies, meteorological events and radio HF investigations. [13, 15] discussed about the spatial correlation of the ionospheric measurements and of their implications on global ionospheric models. [16] stressed the need for long-term climatological observations of solar-terrestrial phenomena to benefit the science and applications. [17] highlighted the role of the geomagnetic activity in the secular change of the ionosphere and confirms the latitudinal dependence of the trends.