Applications of small unmanned aerial vehicles (UAVs) have increased steadily over the past years. Applications are found in Earth observation, reconnaissance, monitoring, and surveillance missions using imaging systems. The big advantage of radar sensors is their all-weather capability. Especially imaging radars possess several benefits over optical systems, as they are not affected by rain, snow, fog, or the time of day. The advantages using small unmanned platforms come at the cost of having to deal with restrictions on weight and volume. Hence, these parameters are the predominant design drivers for UAV sensor developments. This paper presents the concept, the technical realization and the status of a 3D imaging radar system suitable for small UAVs. ARTINO (Airborne Radar for Three dimensional Imaging and Nadir Observation) combines a real aperture, realized by a linear array of nadir pointing antennas, and a synthetic aperture, which is spanned by the moving airplane.