The concept of buoyancy propulsion is investigated for a lighter-than-air platform in the upper- stratosphere. This alternative to conventional thrust propulsion requires a cyclical change in net buoyancy (buoyancy minus weight). Methods have proved efficient in water. 'Underwater Gliders' undertake long missions covering vast distances using little power. High altitude, long endurance craft can perform many functions provided by satellites at potentially much lower cost. Such functions include meteorological monitoring, surveillance, and communications. The proposed craft is a...