## 3 Remote Sensors at the SAS
### Oblique ionosonde  
The aim of the oblique ionosonde [26] is to obtain the useful parameters to model the HF radiolink between the SAS and the EO. These parameters include link availability, the power delay profile of the channel and the frequency dispersion. The system performs soundings during the austral summer, coinciding with the period when the SAS is open. The sounder includes a transmitter, placed in the premises of the SAS and a receiver placed at the EO. A photography of the transmitter antenna is shown in Figure 1. Both transmitter and receiver are specifically designed to fulfill the requirements of this particular oblique sounder and the data transmitter explained in Section 4. The block diagram of the oblique sounder is shown in Figure 2.  
Figure 1: Antenna of the oblique ionosonde transmitter placed at the SAS.  
The most relevant interest of this oblique sounder is that the link is difficult to establish. Firstly, for the distance of the link (12,700 km), which requires at least four hops to reach the receiver. And secondly, for the fact that the path crosses the equator and four different time zones.  
The sounder analyzes the ionospheric radiolink each hour within a frequency band that ranges from 4.5 MHz to 16.5 MHz, with a frequency step of 500 kHz. Each sounding is divided into three intervals. The first one, with a length of 5 seconds, is used to tune the antenna. In the second one, a single tone (CW) at the frequency of interest is transmitted during 5 seconds in order to perform a narrow band sounding. And finally, during the third interval a pseudo-random noise sequence is transmitted to perform a wideband sounding. The narrowband sounding is mainly used to determine the link availability. The wideband sounding can be used to extract the parameters related with time dispersion (power delay profile) and with frequency dispersion (Doppler spread and Doppler shift). The results of this oblique soundings performed during the 2006/07 austral summer are presented in [26]. The main conclusions about link availability can be summarized as follows. (i) Between 21 UTC and 04 UTC availability is greater than \(95\,\%\) at frequencies in the range 8-10 MHz,(ii) between 07 UTC and 08 UTC availability is greater than \(95\,\%\) at frequencies in the range of 11-12 MHz and (iii) between 08 UTC and 09 UTC availability is greater than \(95\,\%\) at frequencies in the range of 14-15 MHz. Whereas, between 10 UTC and 19 UTC availability scarcely reaches values of \(20\,\%\) at any frequency. In addition, the data extracted from these oblique soundings has been correlated with the data recorded with the VIS, especially the maximum usable frequency for a single hop transmission at 3000 km reflected at the F2 layer (MUF(3000)) [11]. The above correlations were obtained from a chain of available VIS located nearby the radio path, resulting that the frequency with the best availability of the ionospheric channel seem to be limited by the lowest of the MUF(3000) recorded along the path. Moreover, the ionospheric conditions close to the receiver have a high 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 the received signals suffer a significant drop of power for those frequencies larger than the MUF(3000) when compared with those frequencies lower than the MUF(3000).