# LoRa Mobile-To-Base-Station Channel Characterization in the Antarctic
## 3 Measurement Results
### Received Signals  
#### 3.2.1 Line-of-Sight Propagation  
The LoS or NLoS conditions are represented in Figure 9 by the top trace, plotting the altitude above sea level, where asterisks indicate LoS operation and dots NLoS conditions with respect to the base station. The bottom part of the graph displays the received power of the transmission as a function of distance. The profile of the terrain is the dominating factor influencing the received signal level. Lower signal powers correspond to locations with no direct LoS, occurring due to a quickly decreasing altitude with respect to the base station. LoS is lost at approximately 7.5 km, 10 km, 14 km, and between 16.5 km and 21 km. Clearly, the smaller decrease in height around 10 km results in a much smaller drop in power compared to the drop around 7.5 km and 14 km produced by a steeper declination. Signals are still received via diffraction around the curvature of the terrain--a phenomenon that is well known to occur in the very high frequency (VHF) and ultra high frequency (UHF) bands [18]. After the dip in elevation, around 21 km from the base station, the altitude of the mobile vehicle increases again, restoring LoS conditions at a distance of 22 km. Note the total absence of other obstacles between the end points of the link, which leaves terrain elevation the only factor directly influencing the propagation. Figure 8: Route from the station Princess Elisabeth Antarctica (PEA) to Vesthaughen followed during the measurements. Due to the varying terrain elevation, the path loss exponent is very difficult to determine. The measured signal drop is about 2 or 3 dB when going from a distance of 20 to 27 km, suggesting a path loss exponent in the range of 1.5 to 2.3 for the log distance path loss model. For comparison, Ref. [19] reports that indoor environments exhibit a path loss exponent of 1.6 to 1.8 in LoS. In free space, the path loss exponent equals 2. For urban areas, the exponent is 2.7 or higher. The snow surface is not actually flat. Therefore, the ground reflection is probably diffuse instead of specular. The path loss extracted from the measurement also indicates this. More measurements are necessary to confirm this assumption. At a distance of 28.7 km, large SNR fluctuations are present in the measurement. These fluctuations are caused by the vehicle driving back and forth between two science setups at nearly equal distances from the base station receiver, as shown in Figure 10. Point A is at 1144 m elevation whereas point B is at 1179 m elevation, hence the distance to the receiver changes by only 50 m. In Figure 11, we observe large differences in recorded SNR, but when the vehicle returns to approximately the same spot (A or B), the values are quite similar again. Figure 12 demonstrates that the corresponding packet loss (expressed as the number of packets lost per 100 packets) remains in the same order of magnitude when returning to the same location. These observations confirm the dominating influence of terrain elevation on the radiowave propagation. Figure 9: SNR as a function of distance. LoS: Line-of-Sight; NLoS: Non-Line-of-Sight. The packet loss observed during the LoS measurements is displayed in Table 2. To obtain enough packets for an accurate packet loss calculation, we consider the distance range described in the first two columns. The number of received packets is indicated, together with the percentage of packet loss. Up to a distance of 25 km, not a single packet was lost. Only in the furthest part of the measurement range--between 20 and 30 km--was a limited packet loss of 16.42% recorded. This means that communication is perfect within a 25 km range for a mobile user, and also highly reliable up to 30 km for a fixed user in locations providing clear LoS, such as location A in Figure 11.  
Figure 11: SNR measured at two positions separated by 500 m.  
Figure 10: Locations A and B, where the vehicle stopped for a longer period of time.