We describe the development of a BVLOS (Beyond Visual Line-Of-Sight) model aircraft (UAV). The broad design requirements included: 1. BVLOS Legal authorization by the Swiss Federal Aviation authority 2. Suitable for land and water take-offs/landing 3. More than 60 minutes flight autonomy 4. Modularity of the imaging package 5. Real-time IR/RGB imagery Requirements 1 to 3 are driven by the goal of improving estimates of lake surface energy flux, since such fluxes have a major impact on long-term lake temperatures and hence ecological status. Requirement 4 allows the UAV to be readily adapted to other applications. The need for real-time imagery permits modifications of on-going missions to map areas of specific interest as they are detected