The diffusion of the UASs (Unmanned Aircraft Systems), commonly known as drones, is nowadays growing in various civil, scientific, and military applications. Thanks to their versatility, flexibility, they are used in several operation contexts, such as search and rescue, disaster assessment, urban traffic monitoring, power line inspection, agriculture crop monitoring and spraying, and in all operations that could be very dangerous or impossible for human action. Recent media events have shown a high number of UASs within urban areas, where it is necessary a high level of safety, due to presence of people, vehicles, and obstacles (fixed or mobile). Many of these aircraft are self-flying or autonomous: therefore, it is fundamental to guarantee a high safety level during their missions. In this paper a low-cost prototype of system for autonomous landing is proposed. It can be installed on-board of small UAS, in addition to the standard instrumentation, and it is able to estimate real-time landing distances, and any obstacles below it: for this performance it is similar to a radar, with high resolution level (sub-centimeter). This will also allow landing inside impervious sites or on moving platforms (e.g., installed on ships).