Unmanned aircraft systems (UAS) have proven to have distinct advantages compared to manned aircraft for a variety of tasks. Current airspace regulations require a capability to sense and avoid other aircraft to replace the ability of a pilot to see and avoid other traffic. A prototype phased-array radar was developed and tested to demonstrate a capability to support the sense and avoid (SAA) requirement and to validate radar performance models. Validated radar models enable evaluation of other radar systems in simulation. This paper provides an overview of the unique radar technology, and focuses on radar performance and model validation as demonstrated through a flight testing campaign. Performance results demonstrate that the prototype SAA radar system can provide sufficient accuracy to sense avoid non-cooperative aircraft.