Introduction Though UA have traditionally been used in segregated airspace where separation from other air traffic can be assured@ administrations expect broad deployment of UA in non-segregated airspace alongside manned aircraft in the future. Current and future unmanned aircraft system (UAS) operations may include scientific research@ search and rescue operations@ hurricane and tornado tracking@ volcanic activity monitoring and measurement@ mapping@ forest fire suppression@ weather modification (e.g. cloud seeding)@ surveillance@ communications relays@ agricultural applications@ environmental monitoring@ emergency management@ and law enforcement applications. For an air vehicle to operate in non-segregated airspace there is a requirement to see and avoid other aircraft@ properly act and respond to certain weather conditions@ and remain well clear of obstacles. Two primary sensor systems are under development to allow a UAS to meet this requirement. The first class comprises sensor(s) or electronic system(s) on the air vehicle and is called aircraft-based sense and avoid (ABSA). The second class involves sensor(s) or electronic system(s) monitoring the air space from the ground and is referred to as ground-based sense and avoid (GBSA). It is anticipated that equipage of UA with an ABSA system or the use of a GBSA would be dependent on the class1 of airspace the UA operates. The goal of airspace access for appropriately equipped UA systems is to achieve a level of safety equal to that of an aircraft with a pilot in the cockpit. If UAS operate in non-segregated civil airspace@ they must be integrated safely and adhere to current operational rules that provide an acceptable level of safety similar to that of a conventional manned aircraft. Thus it is envisioned that UA will require an SA system that can maintain simultaneous tracks of nearby aircraft@ terrain@ weather@ and obstacles to replace current functionality and actions performed by the pilot on manned aircraft. Since the SA systems will be used to ensure the safety of life and property@ a radiofrequency (RF) based SA system is one of these technologies. These RF systems will need to be designated a safety service and operate in an aeronautical radionavigation service (ARNS) allocation. 1 The world's navigable airspace is divided into three-dimensional segments@ each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization (ICAO) in which classes are fundamentally defined in terms of flight rules and interactions between aircraft and Air Traffic Control (ATC). Individual States may also designate Special Use Airspace@ which places further rules on air navigation for reasons of national security or safety.