As unmanned aircraft systems (UAS) become more important to the US military and other users, the pressure to allow them to fly in the national airspace increases. The greatest impediment to this is the lack of an alternative means of compliance with federal “see and avoid” regulations to provide the capability to avoid airborne conflicts between the UAS and manned aircraft. To provide this alternative means of compliance, the US Army is leading the development of a Ground-Based Sense and Avoid System (GBSAA). The system uses ground-based radars, threat detection and alerting logic, and decision support display aids to provide an air picture of the UAS’s operating environment and follows the DO-254 and DO-178C standards for safety critical avionics hardware and software, respectively. This system will allow greater airspace access and lower cost operations by replacing ground observers in the field with a centralized system, thus consolidating the observer function. The first GBSAA deployment site is expected to go live in 2016 at Fort Hood Army Air Field, Fort Hood, TX, operating under the FAA’s Certificate of Authorization process. This paper provides an overview of the system and of a human-in-the-loop simulation-based test exercise that is a key component of the certification of the system. During this test exercise, 19 self-separation violations, in which intruders came within 1 nmi horizontally and 100 ft vertically of the ownship, and no near-mid-air collisions (NMACs) occurred during 195 hours of simulation, including many stressing multi-intruder scenarios, during which reported workload was low and situation awareness was high throughout. All participants ultimately stated that the GBSAA system was appropriate for UAS operations within the National Airspace System (NAS).