This article focuses on the use of airships to provide Wi-Fi broadcasting. Park an antenna high in the stratosphere and then relay signals to and from devices below. Such an airborne transceiver could blanket urban areas with wireless coverage more cheaply than satellite-based alternatives while avoiding the need to build forests of mast-mounted base stations on the ground. This spring engineers at Sanswire Networks, an Atlanta-based Wi-Fi provider, plan to test a prototype of a high-tech airship that they claim could supply mobile communications service to major metropolitan areas for as long as 18 months at a stretch. Beyond standard financial and political difficulties, these projects face formidable technical difficulties. INSET: CONTINUOUS COVERAGE.