Since the 1990s, a number of initiatives have been launched worldwide to explore the potential applications of High Altitude Pseudo Satellites, also called High Altitude Platform Stations (HAPS). HAPS are aircraft positioned above 20 km altitude, in the stratosphere, for very-long-duration flights counted in months and even years. These unmanned aircraft may be airplanes, airships or balloons. A number of HAPS development projects have recently reached more advanced development stages, which indicates that first operational services could be expected by 2018 - 2022.