The Next Generation Air Transportation System (NextGen) is an initiative from the FAA to modernize all components of the current Air Transportation System.  Its main goal is to overhaul the existing infrastructure to better track aircraft, enhance communications, navigation, and air traffic control (Federal Aviation Administration, 2016).  One of the main enablers of NextGen is the transition from relying on radars for tracking and aircraft separation to the use of the Automatic Dependent Surveillance Broadcast (ADS-B).  This system relies on Global Positioning System (GPS) data for aircraft location; it relays positioning information to Air Traffic Control (ATC) and to nearby aircraft at the same time (Federal Aviation Administration, 2016). 
Positioning information is utilized by upgraded software part of the En Route Automation Modernization (ERAM) system, which increases the quantity of aircraft controllers can track at a time.  “En Route controllers are able to track 1,900 aircraft at a time instead of the previous 1,100 flight capability. Additionally, now coverage extends beyond facility boundaries, enabling controllers to handle traffic more efficiently. This extended coverage is possible because ERAM can process data from 64 radars versus the 24 radar processing with the legacy Host system” (Federal Aviation Administration, 2015).  More precise positioning information and tracking enables Performance Based Navigation (PBN); this allows for safer operations while reducing the spacing between aircraft.  Having reliable positioning and a higher degree of ATC increases the overall safety of operations in the NAS.
Unmanned Air Systems (UAS) provide a significant challenge to the NextGen architecture.  It will challenge the technologies incorporated into the new architecture by supporting aircraft that are of an extreme variety of sizes and form factors.  The size of the UAS is an issue at the crux of their incorporation in the NAS; the FAA differentiates between them by weight, but this should not be the only differentiation factor.  Integrated separation strategies must be developed for manned and unmanned aircraft to chare the skies; guidance and control also become problematic when having both types of aircraft sharing an airspace.  An air traffic controller can easily transmit routing instructions to a pilot verbally; the same might not be possible with an UAS, where the pilot (assuming a low level of automation) could be located on the other side of the world.  This by itself is a significant human factors challenge; how can an operator safely receive instructions when geographically separated from the controller?  Perhaps the solution would be to eliminate the pilot and enforce a high level of automation.  The UAS would be integrated in the NextGen ERAM system, and a ADS-B equivalent could be installed on the aircraft.
Automation is inevitable.  Lack of autonomous UAS operations would interfere with the evolution of the role of the pilot.  It is recognized that manned aviation pilots need to evolve into flight managers of technically advanced aircraft (Robertson, 2010).  Development of unmanned UAS operations will provide the necessary level of research and development that will ultimately benefit manned aviation as well.  Automation would address lost link situations, and is the one way of addressing Sense and Aviodance (SAA) issues.  The incorporation of UAS in the NAS should start gradually, starting with flights over sparse populated or non-populated areas.  Once technology develops and reliability is shown, public perception will develop and the incorporation of UAS in the NAS would be able to move forward.

References

Federal Aviation Administration. (2015, April 29). En Route Automation Modernization (ERAM). Retrieved from FAA.gov: https://www.faa.gov/air_traffic/technology/eram/
Federal Aviation Administration. (2016, June 6). Automatic Dependent Surveillance-Broadcast (ADS-B). Retrieved from Federal Aviation Administration: http://www.faa.gov/nextgen/ programs/adsb/
Federal Aviation Administration. (2016, February 13). NextGen Works. Retrieved from FAA.gov: http://www.faa.gov/nextgen/works/
Robertson, C. L. (2010). Determining Appropriate Levels of Automation. University of North Dakota, FITS SRM Automation Management Research.