This paper presents the system design, imaging algorithm, and first trial results of a K-band Synthetic Aperture Radar (SAR) named DemoRad, developed at the Warsaw University of Technology, Poland. The rapid growth of autonomous vehicle technology puts a demand on sensors precision, and affordability. Recent trends in avionics and navigation lead naturally towards progressing miniaturization, highly appreciated in UAV autonomous systems. Small in size, low cost, and high precision remote sensors find a wide range of applications in civilian and military domains (e.g., autonomous delivery systems, self-driving cars, driving assistance systems, military reconnaissance, and missile or fire guidance, among others). SAR imaging is a technology with high and proven potential in most of these fields. The system presented in this paper was designed to be a small and affordable radar sensor applicable for small UAV platforms. This paper introduces system architecture and a brief description of the signal processing algorithms. The preliminary results from ground-based and airborne trials are also presented.