Digital Data Links and Future Air Traffic Control.

Air Traffic Control in the United States is generally facilitated by the FAA and its contractors through radar surveillance, Secondary Surveillance Radars that use the transponders aboard aircraft, and voice communications. The precise navigation and separation of aircraft is done through standardized procedures, and complex avionics both radio based and GPS based. The problem is, the actual control of the  aircraft is done via voice communications. And voice communications are an awful way to share information.


For one thing, one controller is typically controlling several aircraft. The controller has responsibility for a certain slice of the airspace, and all aircraft flying through his particular piece are his responsibility. As you can grasp from the tower controller at JFK airport above, there’s only a certain amount of time available on a voice channel for each aircraft to actually talk with the controller. That doesn’t even count the time the controller is coordinating his actions with other controllers for when he hands them off to another.

Outside of air traffic control, airliners also need to conduct extensive communications with their dispatchers back at the home office.  Weather updates, information on connecting flights, and optimal routes for economy are discussed, among other things.  One of the most important matters is OOOI, or “Out of the Gate,” “Off the Ground,” “On the Ground,” and “In the Gate.” You see, airline crews and flight attendants are only paid for time spent flying, not time spent sitting on the ground.  In the old days, airline crews would use a second radio to call in these critical points. And it occurred to some airlines that maybe, just maybe, the crews might fudge just a shade in their favor on some events. And so the airlines turned to the avionics industry for a technical solution. The result was ACARS, Aircraft Communications Addressing and Reporting System. Sensors around the aircraft would be able, for instance, to tell if the cabin doors were all closed, and the engines running. A very brief digital message could be sent over the radio (it sounds like a chirp followed by about a 1/4 second of static) updating the aircraft status to the dispatcher and airline operations center. In essence, it began as a time clock, letting the airlines know when they had to start paying the crew.

Piedmont was the first airline to adopt ACARS, but by the mid 1980s, it was in widespread use. And the airlines figured out a few other things as well. ACARS could send a whole host of information back to home. It also was a handy way of sending information to the airliner in flight. An airliner might be sending back its position, speed, altitude, route, fuel on board, engine performance at regular intervals, while dispatch regularly uplinked weather updates, pilot reports of conditions along the route, information concerning any delays, and  connecting flight information. All this could be done using extremely brief digital messages.

The FAA hasn’t been totally sitting on its hands. There’s a vast wealth of online tools for pilots to plan flights. The days of a huge Jeppeson case stuffed with paper charts are gone. Virtually every pilot today gets absolutely up to date charts, approach plates and airport diagrams on his iPad, either via the FAA or any number of online resources. But those resources are for planning the flight, or looking up information during the flight. Again, the air traffic controller still interacts with the flight crew via voice radio.

That’s sl0wly starting to change, particularly on long oceanic flights. VHF radio is essentially line of sight, with a range of about 200 miles for a high flying aircraft. Jets on long overwater flight instead use HF radio. But often times, HF radio audio quality is poor.  Instead, HF radio can be used to send data, or alternately, satellite communications through InMarSat can be used to transmit ATC message both ways.

The generic term is Controller-Pilot Datalink Communications (CPDLC). Boeing and Airbus both are fielding systems under the name FANS, Future Air Navigation System, and in addition to oceanic routes, its being adopted for use over parts of continental Europe. As yet, it is not in use in the continental US airspace. One suspects that it inevitably will be adopted at least in part. One great advantage that testing of CPDLC has shown is that it greatly reduces the volume of voice traffic for a given controller, by as much as 75%.  One example was the handoff from one controller to the next as an aircraft moves from one slice of the airspace to another.

Here’s a notional handoff as currently done by voice from an Air Route Traffic Control Center:

Oakland Center: United 345, contact Los Angeles Center on 134.45.

United 345: Los Angeles on 134.45, good day.

United 345://tunes radio to 134.45

United 345: Los Angeles, United 345 with you.

Los Angeles Center: United 345, roger.

With a CPDLC system, as the aircraft approaches the airspace boundary, the controller (or even the computer automatically) would  generate a digital message to the aircraft that would automatically interface with the airliner’s Flight Management Computer, telling the crew, automatically tuning the radio, and automatically generating a check in message to the new controller at Los Angeles Center, all reducing the workload of the flight crew, the controller, and reducing the volume of traffic on the voice network, allowing controllers to focus more on higher priority issues.

What’s interesting is this trend toward datalink control of air traffic is using modern technology to implement techniques that were first established as far back as the 1950s when the North American Aerospace Defense Command used the Semi-Automatic Ground Environment to control interceptor aircraft defending against potential Soviet bomber attacks.