Radio makes it possible to have communication ''without wires.'' This is a highly attractive proposition for mobile personal communications; it is increasingly an economic alternative to traditional wired phone systems, and is a potential technology for highspeed Internet access. But in the terrestrial environment, radio signals are subject to scattering and multipath effects that limit the quantity of information possible to transmit in a given bandwidth, as well as the distances over which it can be communicated. In cellular and personal communication systems (PCS), radio coverage is deliberately restricted further to allow for frequency reuse. As a consequence, terrestrial wireless networks comprise numerous antenna towers, base stations, wired or microwave links, and mobile switching centers, all dispersed over wide geographical areas. Satellites can provide wireless coverage with much less terestrial infrastructure, but only by introducing considerably problems of their own. Geosynchronous satellites require expensive and bulky user terminals and introduce large signal delay because of their great distance. Nongeosynchronous satellites, because of their motion with respect to points on the ground, greatly increase system complexity. Proposed high-altitude aeronautical platforms (HAAPs) are an intriguing alternative. From a communications perspective, they would have many advantages over both their terrestrial and satellite counterparts. If HAAPs prove to be reasonably stable, reliable, and not too costly, they will offerr considerable opportunities for wireless services provision, and introduction of innovative communications concepts such as cell scanning and stratospheric radio relays.
