In 2015, NASA began a four-year project to provide guidance for low-altitude (under 500 ft/150 m) air traffic management for unmanned aircraft systems (UAS). This UAS Traffic Management (UTM) project set out to define all aspects of lowaltitude traffic management including airspace management and geo-fencing, weather and wind, predict and manage congestion, terrain and man-made objects and safe separation. The program provided problem sets with increasing complexity, from unpopulated land (UTM-1) to autonomous, beyond visual line-ofsight, urban operations (UTM-4).