sensors Review Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review Marcos Hervás 1 , Pau Bergadà 1,2 and Rosa Ma Alsina-Pagès 1,* 1 GTM—Grup de recerca en Tecnologies Mèdia, La Salle—Universitat Ramon Llull, c/Quatre Camins, 30, 08022 Barcelona, Spain;
[email protected] (M.H.);
[email protected] (P.B.) 2 Wavecontrol, c/Pallars, 65-71, 08018 Barcelona, Spain * Correspondence:
[email protected]; Tel.: +34-932-902-445 Received: 03 April 2020; Accepted: 23 April 2020; Published: 28 April 2020 Abstract: High Frequency (HF) communications through ionospheric reflection is a widely used technique specifically for maritime, aeronautical, and emergency services communication with remote areas due to economic and management reasons, and also as backup system. Although long distance radio links can be established beyond line-of-sight, the availability, the usable frequencies and the capacity of the channel depends on the state of the ionosphere. The main factors that affect the ionosphere are day-night, season, sunspot number, polar aurora and earth magnetic field. These effects impair the transmitted wave, which suffers attenuation, time and frequency dispersion. In order to increase the knowledge of this channel, the ionosphere has been sounded by means of narrowband and wideband waveforms by the research community all over the world in several research initiatives. This work intends to be a review of remarkable projects for vertical sounding with a world wide network and for oblique sounding for high latitude, mid latitude, and trans-equatorial latitude. Keywords: HF; vertical sounding; oblique sounding; skywave; ionosphere; communication; Doppler spread; delay spread; SNR 1.