# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Damson
#### 3.3.5 Results and Conclusions  
Initial results measuring multipath, Doppler spread and Doppler shift with the whole system were presented in References [35; 37]. The work presented in Reference [38] justifies the necessity of using an Automatic Radio Control System (ARCS) to select the best waveform to obtain a high data rate with an acceptable bit error rate, this analysis was done illustrating the range of Doppler shifts and spreads on a adverse high latitude HF channel. The results for Doppler spread and Doppler shift during the campaign between 16 and 22 of October in 1995 is presented in Reference [39], where a disturbance takes place whereby the geomagnetic Q index reached values of 7 for several hour. This work demonstrated that the values of Doppler spread and shift are positively correlated with the level of disturbance. Multipath spread, Doppler spread and SNR were measured during the Spring, Summer and Autumn of 1995 for 4 high latitude paths in Reference [36]. Finally, these data were related to the performance of an HF data modem to estimate the availability on these paths. Doppler shift was also measured under both quiet and disturbed geomagnetic conditions in Reference [40]. In this work, longer multipath delays were observed during midday at frequencies above the predicted MUF, and higher Doppler spreads were observed during disturbed conditions, when the reflection point is located within the auroral oval. In Reference [41], the channel conditions measurements were used to evaluate the performance of two 75 bps (compliant with STANAG 4415 and STANAG 4285) and a 2400 bps waveforms, where the two 75 bps waveforms exhibited a 60-75% availability higher than the 2400 bps waveform, because the later was degraded during a geomagnetic disturbance. The work presented in Reference [42] showed how the irregularities of the ionosphere at polar cap paths latitudes produce different angles of arrival of the transmitted signal at the receiver side, and how the movement of the reflection points produces both Doppler shift and Doppler spread. An additional high latitude paths were analysed in Reference [43], extending the campaign measurement time. The results of this new work concluded that this high latitude path presents less azimuth standard deviations than in polar cap paths, which means that the roughness of the ionosphere is less abrupt. A deep analysis was presented in Reference [44], where the direction of arrival of independent signal components were related to their precise Doppler frequency and with the ionospheric irregularity drift made by the CUTLASS (geophysical instrument) radars observation. The gross deviation of the displacement of the received signal from the great circle path is due to electron density depletion and ionospheric tilts within mid latitude through sub-auroral latitudes, whereas in the polar cap they are associated to the presence of patches and arcs of electron density. A review of the state of the art in this topic, quantifying and identifying the geophysical conditions that leads to this undesirable effect, is conducted in Reference [45]. In References [46; 47], a spatial filtering has been implemented using  
Figure 6: Example DAMSON output, from Reference [36]. simple arrays of antenna to reduce the level of Doppler spread, hence there is an improvement of up to 45% in the availability. Channel simulation tools basically used the Watterson channel model, which is appropriate for mid latitudes paths. However, high latitudes paths are more complex than mid latitudes, more concretely when they have to take into account the directional effects. To tackle this issue in Reference [48], a campaign measurement took place between March 2004 and May 2005 over two paths: (a) from Svalbard to Kiruna, Sweden, and (b) from Kirkenes, Norway, to Kiruna. Finally, a channel model that incorporates this directional effects has been developed defining 7 different cases, which represents the complex range of measurements observed.