Concepts and main applications of high-altitude-platform radio relays

Novel radio systems using high-altitude platforms (HAPs) at altitudes around 20 km above the ground are proposed as a way of overcoming the drawbacks of conventional satellite and terrestrial radio systems. This paper provides an introduction to the field, acting as a precursor to several extended papers on HAPs that will be featured in a future Radio Science Bulletin special issue. The concept, genesis, development, and recent status are detailed, and the proposed applications and advantages of different categories of HAPs are discussed. The possibilities of microwave-energy transmission from ground stations to receiving rectifier antennas (rectennas) on HAPs are described as a way of overcoming the long periods of darkness, when solar cells are not usable. Finally, the paper describes the new HAPS projects being realized, followed by a short forecast of future trends in the area. A thorough comparison of radio communication and remote sensing using GEO and LEO satellites, terrestrial radio systems, and HAPS is presented in tabular form.