This paper discusses the flight test setup and preliminary results of a precise relative navigation and separation assurance system to support safe simultaneous operation of general aviation aircraft and UAS at low altitudes in the vicinity of an uncontrolled airfield. With an increased use of UAS for a wide variety of applications, such as law enforcement, search and rescue, agriculture, infrastructure inspection, maintenance, mapping, filming and journalism, it is expected that UAS will be taking off, landing or otherwise operating at airfields at the same time as manned aircraft. In that case, it will be required that the UAS not only has precise knowledge of its location and velocity to enable an automatic flight at lower altitudes with sufficient accuracy, integrity, availability and continuity to support its operation (e.g. takeoff , landing, flight patterns), but also to support broadcasting its position and velocity to the other aircraft in the vicinity, for improved awareness of the UAS location and to achieve separation assurance