Part two of my series on air traffic control. I’m not an ATC professional; this is what I learned from a lot of reading. Sources are at the bottom, and corrections are welcome.
The first thing that surprised me when I started digging in: a single airline flight gets passed between a lot of controllers. A routine trip from New York to Chicago might talk to half a dozen different people, in several different buildings, before it reaches the gate. Nobody “flies” the whole trip from the ground. It’s a relay race, and the handoffs are where much of the skill lives.
Following a Flight#
The tower. It starts at the airport. At a busy airport, one controller issues the clearance (the route the flight has been approved to fly), a ground controller handles taxiing, and a local controller in the tower cab clears the aircraft onto the runway and into the air. Larger airports also use surface surveillance systems like ASDE-X to track aircraft and vehicles on the ground, which helps a lot at night and in bad weather.
The TRACON. A few miles after takeoff, the tower hands the flight to a Terminal Radar Approach Control facility. TRACON controllers typically work the airspace within roughly 30 to 50 miles of the airports they serve. They’re the ones sequencing departures out of a crowded area and lining up arrivals for final approach. One TRACON often covers several airports at once; the New York TRACON handles JFK, LaGuardia, Newark, and a long list of smaller fields.
The center. Once the flight climbs out of terminal airspace, it’s handed to an Air Route Traffic Control Center. The US has about twenty of these covering the continental states, each responsible for a large chunk of the country split into sectors. A cross-country flight moves from sector to sector and center to center, and each handoff is a brief radio exchange: new frequency, check in, carry on.
At the other end, the whole thing runs in reverse. Center hands the flight to the destination TRACON, the TRACON sets it up for approach, and the tower clears it to land.
Oceanic control. Over the ocean, there’s no radar. Centers like Oakland, New York, and Anchorage handle huge stretches of oceanic airspace using position reports, HF radio, satellite data links, and wider spacing between aircraft to make up for the lack of a radar picture. Space-based ADS-B has started to change that; more on this in a minute.
How Controllers See Aircraft#
Radar is still the backbone, and there are two kinds. Primary radar bounces radio waves off anything in the sky. It doesn’t need anything from the aircraft, but it also can’t tell you who the aircraft is or how high it’s flying. Secondary radar interrogates the aircraft’s transponder, which replies with an identity code and altitude. That reply is what puts a labeled data block next to each target on a controller’s display.
ADS-B flips the model around. Instead of the ground asking “where are you?”, the aircraft figures out its own position from GPS and broadcasts it, along with altitude, speed, and identity, about twice a second. Since January 1, 2020, aircraft flying in most controlled US airspace (Class A, B, and C airspace, and Class E above 10,000 feet) have been required to carry ADS-B Out.1 ADS-B doesn’t replace radar in the US; the FAA keeps radar as a backup and for tracking anything that isn’t broadcasting.
Aireon put ADS-B receivers in orbit on Iridium’s satellite constellation and launched its service in April 2019. Air navigation providers including NAV CANADA and the UK’s NATS, both investors in Aireon, use it to track aircraft over the North Atlantic.2 For oceanic airspace, that’s the first time controllers have had something close to a live picture.
How Controllers Talk to Pilots#
Most communication is still voice over VHF radio, the same basic method used for decades. It works, but a single frequency is shared by every aircraft in a sector, so at busy times the radio gets crowded, and a misheard number can cause real trouble.
The FAA’s Data Comm program sends some of that traffic as text instead: departure clearances, route changes, and frequency changes delivered straight to the cockpit, where the crew accepts them with a button press. As of 2025, Data Comm was in use at 65 airports and has been expanding into the en route centers.1 Over the ocean, pilots and controllers use HF radio and satellite data links, since VHF only reaches line of sight.
The Computers Behind the Scopes#
The en route centers run on ERAM (En Route Automation Modernization), which replaced the 1980s-era Host computer system; the FAA finished that transition in 2015.1 TRACONs run STARS, the Standard Terminal Automation Replacement System. These systems process radar and ADS-B data, track flight plans, and flag possible conflicts.
And then there’s paper. Many US towers still manage flights using paper flight strips, small printed strips of cardstock that controllers move around in holders to track each aircraft. (As someone who works in technology, I found this equal parts charming and alarming.) The FAA’s Terminal Flight Data Manager program is replacing them with electronic strips, airport by airport.
Navigation#
For decades, airplanes navigated between ground-based radio beacons called VORs, often paired with DME equipment that measures distance. Flying beacon to beacon means zig-zagging across the country instead of flying straight. GPS-based area navigation (RNAV) and Performance-Based Navigation (PBN) let aircraft fly precise, direct routes and approaches that don’t depend on where a beacon happens to be. The FAA is gradually shrinking the VOR network but keeping a backbone of them in case GPS isn’t available.
Managing the Whole System#
Individual controllers work their sectors, but somebody has to look at the whole country at once. That’s the FAA’s Air Traffic Control System Command Center in Virginia. When thunderstorms shut down routes or an airport’s capacity drops, the Command Center coordinates reroutes and ground delay programs, which hold flights at their departure airports rather than letting them circle at the destination. (If you’ve ever sat at the gate with the engines off waiting for a “wheels-up time,” this is why.)
Tools like Time-Based Flow Management schedule arrivals by time rather than by spacing aircraft out in miles, which helps keep traffic moving into busy airports without long holding patterns.
NextGen and SESAR#
Much of what I’ve described (ADS-B, Data Comm, PBN, time-based flow management) came from NextGen, the FAA’s long-running modernization program. Europe’s equivalent is SESAR, the Single European Sky ATM Research program, which is working on the same problems across dozens of countries’ airspace. Both programs get their own treatment in part four.
What Makes It Hard#
Even with all this equipment, the system runs on people. Busy airports run near capacity, weather is the biggest single cause of delays, digital systems bring cybersecurity concerns, and there aren’t enough fully certified controllers. That last issue comes up again and again in this series.
The technology is impressive. What impressed me more is how much of it depends on trained people making good decisions quickly, all day, every day.
Next up: The Complexity of the Air Traffic Control System, which looks at what happens when all these pieces have to work together on a bad day.
References#
Next Generation Air Transportation System. (n.d.). In Wikipedia. https://en.wikipedia.org/wiki/Next_Generation_Air_Transportation_System ↩︎ ↩︎ ↩︎
Aireon. (n.d.). In Wikipedia. https://en.wikipedia.org/wiki/Aireon ↩︎