Increasing robustness of HF communications at high latitudes - utilizing ionospheric data and space diversity

HF-radio provides long range, infrastructure-independent communications to the Norwegian Armed Forces in the Arctic. This is important in an area with increasing activity and where satellite communications is less available. The high latitude HF-channel experiences large variations in time and space depending on the space weather, and HF communications must therefore make use of robust waveform- and network designs, adaptivity and real-time channel assessments in order to achieve optimum communications. In 2017–2018, measurements of HF communications were conducted in a network of stations at high latitudes. The primary aim of the study was to investigate how much the availability of HF communications at high latitudes can be improved by exploiting space diversity. If various geographical paths exhibit very different channel characteristics, modern HF-networking radios are able to exploit this, and robustness of the HF communications can be gained. The idea was first explored in previous work conducted at FFI [4]. A secondary aim of this study was to investigate the correlation between the performance of modern HF communications and certain ionospheric parameters measured by ionospheric instruments. Real-time space situational awareness will enable the HF-operators to make optimum choices for their communications. A final aim of this report was to summarize the factors of importance to HF communications at high latitudes for educational purposes. The study has confirmed that HF-networks that utilize space diversity, increase robustness and improve availability of HF communications. In a network of 20 W radios, space diversity gains of 10–30 % were achieved by including a node positioned further south at a distance of ~400 km from the transmitter. In a network of 400 W radios, space diversity gains of 10–50 % were achieved by a southern node at a distance of ~1300 km. The gains were largest during ionospherically disturbed periods. Our comparison of the HF-measurements with ionospheric data has shown that using the latter may give useful insight in the prevailing propagation conditions, and give guidance for making favourable choices for the communication networks. Real-time ionograms in combination with riometer measurements were found particularly useful. In particular, for the short measurement paths in northern Norway with close proximity to riometer measurements in Abisko in Sweden, absorption levels above 0.2 dB (measured at 30 MHz) gave generally decreasing linking probability with hardly no linking at an absorption level of 1 dB. The long paths towards the south were also affected by the absorption measured in Abisko, but to a lesser extent. The absorption can be counteracted by the HF-operator by avoiding the area of increased absorption (utilizing space diversity) or increasing the transmit power, if that is an option. Further work should include examination of space weather data sources available on the Internet, including radio amateur tools, for relevance to high latitudes and HF communications. Real-time data from relevant sources could be included in a software application tool, tailormade for the HF-operator. The application, together with modern networking radios and smart ways of exploiting the high latitude ionosphere, would improve HF communications and thereby information exchange for the Norwegian Armed Forces. The results of this report can provide a basis for such further work.