Air Traffic Control Entities

     Within the United States, the Federal Aviation Administration (FAA) is responsible for controlling all air traffic within the National Airspace System (NAS).  It accomplishes this daunting task of keeping aircraft safe in ever-increasing crowded airspace through the coordinated efforts of various Control Entities. Each Control Entity is responsible for directing air traffic through, or sometimes over, their Area of Responsibility (AOR), which typically also coincides with a particular phase of an aircraft's flight from one location to another. 

    Focusing on the graphic at the top of this blog post, I'll explain the various domestic Control Entities within the United States and their responsibilities, helping to draw the similarities and differences between each. 

Ground- 

    Although not depicted above, "Ground Control" is responsible for the movement of aircraft and vehicles within an airport's taxiways, inactive runways, holding areas, and parking areas. Ground is responsible for preventing ground incursions or collisions and efficiently directing traffic to or from runways.

Tower-

    The tower is responsible for active runways and coordinates arrivals (landings) and departures (take-offs) within class B, C, and D airspace (FAA, 2016). An aircraft must have clearance from the tower to taxi onto a runway for a departure and must also be cleared to land when arriving. An aircraft that is preparing to depart has first had communications with Ground before switching over to the tower for take-off clearance. Conversely, an aircraft preparing to land will communicate with the tower after having been in communication with the Terminal Radar Approach Control (TRACON) controller.

TRACON-

    Also referred to as "Approach", TRACON is responsible for controlling air traffic between the take-off and landing phases and the en-route phase of flight (Martin, 2014). TRACON facilities are located at every major airport and every aircraft flying within an airport's airspace will communicate with the TRACOM of that area for directions into, out of, or through the airspace. When preparing to land, TRACOM will direct an aircraft to switch over to the Tower controller for clearance to land. After having departed an airport, TRACOM will direct an aircraft to contact the applicable Air Traffic Control Center (ATCC) that controls air traffic within that AOR.

Center-

    An ATCC, more commonly referred to as a "Center", is responsible for controlling traffic in the areas between airport airspace. Centers are designed to typically provide ATC services for aircraft operating under an Instrument Flight Rules (IFR) flight plan. Centers essentially "own" the airspace of their region providing route clearance, separation, and vectoring for approach on decent. It is also important to note that Centers are responsible for publishing Notice to Air Missions (NOTAM) notices that announce abnormal conditions or hazards within the National Airspace System (NAS) (Air Route, n.d.).

References

Air Route Traffic Control Center. (n.d.). CFI Notebook. https://www.cfinotebook.net/notebook/air-traffic-control/air-route-traffic-control-center

Martin, E. (2014, July 24). Airspace 201: The Air Traffic Control System. Phoenix East Aviation. https://pea.com/blog/posts/airspace-201-air-traffic-control-system/

Transportation, U.S. Department of & Federal Aviation Administration. (2016). Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B - 2016): [B/W edition]. CreateSpace Independent Publishing Platform. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak