Sense–n-Avoid
Separation and avoidance of unmanned aerial vehicles (UAVs) among other UAVs and manned aircraft in the NAS can be accomplished, but careful consideration must be given to the design and implementation of those UAVs that require the technology, so that it functions as designed. Since not all UAVs are the same size and have different types of airframes to perform the dull, dirty or dangerous (DDD), their payload configurations may surely not be able to accommodate the current technology available and necessary to operate in unrestricted airspace. Those UAVs that are very small in nature, such as the Micro UAV or MAV, which are designed specifically for operations in urban environments, such as within or near buildings, would not require such technology as it operates well below the service operations of both manned and unmanned aircraft.
Some UAVs, such as the Global Hawk, already have a forward-looking video camera dedicated to an operator looking ahead for other aircraft (Austin, 2010). In uncontrolled airspace it is very difficult for light aircraft pilots to spot another small aircraft approaching if it is on, or nearly on, a collision course (Austin, 2010). The view that a pilot may have of a UAV ‘head-on’ is likely to be even smaller (Austin, 2010). It would be an obvious outcome than to equip a UAV with a sensing system that enables it to detect an object encroaching on its predetermined flight plan and it could be programmed to autonomously avoid that object by whatever means capable of the UAV. A study by the European Defense Agency (EDA) has concluded that a sense and avoid system for long endurance UAV is feasible and that certification of a system is expected by 2015 (Austin, 2010).

Manned Technology
Current manned technology can be incorporated into those UAVs that are allowed to operate in unrestricted airspace (Austin, 2010). There are a number of ways that current manned aircraft communicate and a number of reasons why they communicate or receive signals (Clot, n.d.). Aircraft surveillance, navigation and data communications are all functions that require some form of communications as the following list covers some examples:
Surveillance/Collision avoidance
• Automate Dependent surveillance (ADS)
• Tactical Collision Avoidance System (TCAS)
ADS: Most of the globe is not covered by radar (Clot, n.d.). Using Automated Dependent Surveillance (ADS), however, an Air Traffic Control Centre (ATCC) can see the current position of an aircraft almost anywhere in the world (Clot, n.d.). A controller can also examine the aircraft’s intended flight path and other information held in their onboard navigation systems (Clot, n.d.). This data can be downloaded even in airspace not covered by radar, such as the oceans or sparsely populated areas (Clot, n.d.). An aircraft reports its position via an orbiting satellite (Clot, n.d.). The message is routed to the current ATCC for that aircraft (Clot, n.d.). If the ATCC needs to send instructions to the pilot, it can do this using other datalink systems to send data messages, or satellite voice services to speak to the crew directly (Clot, n.d.). There are already aircraft testing the system and the concept will revolutionize the management of aircraft in remote regions (Clot, n.d.).
TCAS: Aircraft that are TCAS equipped emit a signal (Mode S) which is received by participating aircraft and advisory de-confliction messages are provided to the pilot (Clot, n.d.). This allows the pilot to take avoiding action when necessary (Clot, n.d.). However, if another aircraft is not TCAS equipped it will not be identified.

References
Austin, R. (2010). Unmanned aircraft systems: UAVS design, development, and deployment. Chichester, West Sussex, U.K: Wiley.
Clot, A.J. (n.d.). Communication, command, and control: The crowded spectrum. Middlesex, UK: Remote Services Limited. Retrieved from http://ftp.rta.nato.int/public//PubFulltext/RTO/EN/RTO-EN-009///EN-009-02B.pdf