Special Issue on "High Altitude Platform (HAP) Systems: Technologies and Applications"

The growing exigencies for broadband mobile communications have led to the successful and rapid deployment of both terrestrial and satellite wireless networks. In parallel to these two well-established ways for providing wireless communication services, in recent years another alternative has attracted the attention of the telecommunications community. It is based on quasi-stationary aerial platforms operating in the stratosphere, often dubbed as High Altitude Platforms (HAPs). This emerging solution preserves many advantages of both terrestrial and satellite systems but also provides special advantages of its own. Mobility on demand, large coverage, payload reconfigurability, possibility for frequent takeoffs and landings for maintenance and upgrading and very favourable path-loss characteristics (with respect to terrestrial and satellite systems) are the main features which make HAPs attractive for a large class of applications and services from multicast and gap-filler broadband telecommunications, to remote sensing, environmental monitoring, disaster recovery , meteorological measurements, real-time monitoring of seismic regions, agriculture support and many more. Telecommunications is certainly one of the most promising applications because of the commercial revenues they can provide. They can support services for Global Navigation and Positioning Systems (GPS, Galileo), be integrated into GSM/UMTS systems and provide solutions for special geographical regions (islands and impervious areas) or temporary applications (Olympic Games) supporting multimedia services at satisfactory quality. Concerning the integration into the GSM or UMTS standard, different scenarios can be conceived. One scenario employs HAPs as " backup " base stations for covering areas partially served by terrestrial base stations. HAPs can also be used for providing a full-service coverage of a wide rural area, where no terrestrial network is active. A complete study of research activities in the field demands a thorough investigation of the different layers of the communications path taking into account all the relevant factors influencing the performance of the system. And a meaningful survey of applications and case studies should cover global activities and standardization efforts in the last decades and any scope for internationally accepted recommendations. Even though active and passive communications platforms date back to the early '60s, with the Echo balloon launched to reflect broadcasts from Bell Laboratories, it is only the past few years that have seen a resurgence of interest in HAP systems. Currently several projects address the usage of HAPs for future communications pointing out the renewed interest in very flexible structures. However, despite the vast literature on terrestrial and satellite systems, there exist …