The use of UASs (Unmanned Aerial Systems) in meteorological studies has been a growing interest in the recent years but several technical and legal issues have to be overcome in order to fly safely and efficiently within the civil airspace. This paper addresses the technical aspects of the VOLTIGE (Vecteur d'Observation de la Troposphère pour l'Investigation et la Gestion de l'Environnement) project where a fleet of UAVs will fly simultaneously in and out of fog. Several improvements have been made on the communication between the payload and the autopilot in order to have an efficient sensor-based navigation. The challenges regarding the integration into civilian airspace are also presented.