Free Flight concept to require host of new technologies

July 1, 1999
WASHINGTON - Crowded skies already are making air travel difficult over the U.S. and Europe, and air traffic demands are growing rapidly in Asia. The current U.S. National Airspace System (NAS) and global aviation environment are being pushed to their operational limits.

By J.R. Wilson

WASHINGTON - Crowded skies already are making air travel difficult over the U.S. and Europe, and air traffic demands are growing rapidly in Asia. The current U.S. National Airspace System (NAS) and global aviation environment are being pushed to their operational limits.

Officials of the U.S. Federal Aviation Administration (FAA) are evolving a solution, in cooperation with other nations, called "Free Flight." This technology-driven approach would take air traffic management from the sole purview of ground controllers and share it with pilots, who could largely choose their own routes based on speed, efficiency, and cost.

Some elements of Free Flight already are being tested. Some, in fact, already are in use on a regular basis over the Pacific Ocean. Still, some FAA officials do not believe it will be possible to bring all elements of the aviation community - primarily general aviation - into full compliance with the necessary technologies until sometime after 2020.

"I think you`ll see some equipment changes after 2002," says Ray Hilton, a former air traffic controller who now serves as director air traffic management for the Air Transport Association (ATA) in Washington, which comprises 23 U.S. and five non-U.S. commercial air carriers, representing more than 95 percent of all U.S. passenger and cargo traffic.

New avionics technologies are under consideration that could help make the transition to Free Flight. These technologies, in the near term, revolve around data communications and satellite navigation, aviation leaders say. One concept driving the move toward Free Flight is called the Future Air Navigation System - better known as FANS.

"Some airplanes already are equipped with FANS 1, and some with GPS," Hilton points out. "Data communications is a very important part of all this and initiatives are underway in that arena. That is probably where you will see some of the first new technologies."

FAA officials are working closely with RTCA Inc. in Washington to find the best ways to phase in Free Flight. That effort includes evaluating existing technologies, such as primary and secondary ground radars, new communications, navigation, surveillance, avionics, and decision-support systems.

Free Flight would allow aircraft to fly much closer together than they can today, select their own routes, and broaden options for fast aircraft to pass slower aircraft. Free Flight also could enable two aircraft to land simultaneously on offset runways in bad weather.

These steps could reduce flight times, shorten distances flown, and save billions of dollars in fuel. But saving time, fuel, and money must balance with safety concerns.

Maintaining safe aircraft separation involves two airspace zones - protected and alert - each based on the speed, performance, and communications equipment of each aircraft. The protected zone immediately surrounds the aircraft and can never overlap the protected zone of another.

The alert zone extends well beyond the protected zone. Aircraft can maneuver freely through the available airspace until alert zones touch. Then a controller may provide one or both pilots with course corrections or restrictions to ensure separation. But with onboard computers and GPS-based positioning, Free Flight would enable pilots to use airborne traffic displays to choose their own solutions, although those may require the concurrence of controllers.

Accomplishing those goals will require that everyone involved has real-time, accurate, detailed information for improved situational awareness, communications, and coordination.

"To get to that level, we must have very good communications between the pilots and controllers, which requires good datalinks," says John Scardina, the FAA`s director of architecture and investment analysis. "Another enabling technology is that the pilots and controllers have to see the same traffic picture. Today the pilots don`t have a picture at all - they get their directions from the controllers."

The primary enabling technology is called ADS-B - short for automatic dependent surveillance-broadcast - which basically means the airplane broadcasts its location to other aircraft and ground controllers. Members of the ATA are working with the FAA to test some of these technologies this year in what is called the Ohio Valley Project.

Leaders of cargo air carriers Airborne Express, FedEx, and UPS each is providing four specially equipped aircraft to test ADS-B and datalinks. An operational evaluation is to be at Airborne`s Wilmington, Ohio, hub this month involving all 12 cargo aircraft and several FAA aircraft. A few European carriers also may take part. FAA officials are procuring and installing five ground stations to support air-to-ground testing.

Robert Frenzel, ATA`s senior vice president for aviation safety and operations, calls Safe Flight 21 "an indispensable program for the development of what I will call `Free Flight enabling technology.`"

The objectives of the Safe Flight 21 Program, he says, are to:

- conduct an operational concept validation for the nine NAS enhancement areas identified by RTCA (e.g., enhanced capability to see and avoid adjacent traffic, surveillance coverage in non-radar airspace);

- demonstrate and quantify operational benefits (safety and efficiency);

- demonstrate capabilities and evolve procedures;

- collect data ADS-B datalinks: Mode S (1090), MITRE`s Universal Access Transmitter (UAT), and VHF Data Link-Mode 4; and

- change the way that FAA does business.

In flights over the continental United States today, two different sensors gather positioning information - independent surveillance and dependent surveillance. Independent surveillance consists of primary en route radars, a system already being phased out. Dependent surveillance, meanwhile, involves an interrogation pulse to which the aircraft responds.

For years pilots flying over the oceans have used a dependent system in which they report their position every hour over HF radio. More recently, an automatic dependent system has used the aircraft`s onboard inertial navigation system (INS) to report its position every five minutes.

ADS-B will use GPS to derive a precise position, then transmit the aircraft`s identification, position, velocity, and direction every second to surrounding aircraft and ground controllers. Because aircraft generally deviate little from planned tracks over the oceans, transmissions probably will be no more frequent than every one to five minutes over radio and satellite links.

"If the airplane can report ADS-B information that is INS-derived, that would be suitable for most situations," Scardina says. "But when the plane is on the ground at an airport, we need more accuracy."

Not everyone is satisfied with the current progress, which FAA officials admit could take until at least 2005 just to equip commercial airlines and make equipment and training changes in the air traffic control (ATC) system.

"All of the recent programs have faltered," Hilton says. "Right now we`re living with 1960s technology and the users are suffering the consequences. What we want to do is see existing technologies implemented as quickly as possible.

Free Flight Phase One is a subset of the FAA`s overall NAS architecture phase one upgrade and focuses on giving the controllers better tools by 2002, Scardina says. Part of the problem is funding for equipment and training.

"There also are things the FAA can do operationally and procedurally that don`t require any major procurement or equipment changes," Hilton argues. "In the case of the TRACON software upgrades, they require no equipment changes."

Those upgrades include TMA (traffic management advisor - single center), passive FAST (final approach spacing tool), URET (user request evaluation tool), CDM (collaborative decision making), SMA (surface movement advisor) and CPDLC (controller-pilot data link communications) Build 1.

"At some point, FAA decided to go with COTS (commercial-off-the-shelf) and NDI (non-development items), but you just don`t build an ATC system with COTS," Hilton says. "You can use some COTS elements, but you need new and specialized equipment and software to handle it."

For North America, one of the biggest concerns is general aviation, which comprises a major share of the hemisphere`s air traffic. "We have acknowledged the GA community will take a decade or two to implement ADS-B, so you would have a long period of mixed operations," Scardina says.

One problem for general aviation is finding the most suitable type of datalink. One that might be best for a mixed commercial and general aviation environment in North America might not necessarily be the best for Europe or Asia.

For example, nearly all new Traffic Collision Avoidance System (TCAS) units have a 112-bit pulse Mode S frequency (the TCAS squitter uses a 66 bit pulse). This unit would be compatible with either TCAS or ADS-B using Mode S (1090). On the other hand, Scardina says, the general aviation community seems to favor UAT, while Europe is leaning toward 1090 or Mode 4.

"If we have a desire for the GA community to go to ADS-B with everyone else, we would have to have a technology that is compatible with some GA needs, but there is no sympathy in the international community for those concerns because only North America really has a GA community," Scardina says.

"Our schedule for implementation of ADS-B is the same time that Europe intends to implement it there. The worst of all worlds would be if Europe picks a different approach to ADS-B, which would mean we would have to carry two different boxes to operate over the U.S. and over Europe."

The U.S. also has a problem with its extensive military aviation community, which shares general aviation`s dislike for a system that broadcasts aircraft identification for anyone to hear. The current system filters military information in the U.S., but not in most of the rest of the world. With ADS-B, that could become a serious problem for military air operations.

"Procedures are currently being worked out to allow airplanes to broadcast their actual ID ... with a random number generator that cannot be mapped into any database record," Scardina says. "So you would know the airplane`s position, velocity and intent - with intent meaning the next waypoint - but not the actual identity."

From the FAA`s standpoint, one of the biggest challenges is to determine the overall aviation community`s vision of where the aviation system should be - and when.

"We will keep the current radar architecture through 2015, which is about when you would transition to ADS-B," Scardina says. "Right now our secondary radar system, transponders aboard aircraft, would continue within the foreseeable future. We`re currently putting in new secondary radars that will be good for 20 years. At the terminal, we don`t see our primary radars ever being replaced, even in a fully ADS-B world in which secondary radars may be gone."

So while the en route primary radars probably will be decommissioned by 2005, what happens to the secondary radars is largely up to the general aviation community.

"If GA does not go to ADS-B, they could go on forever. But if the entire aviation community goes to ADS-B, we`ll probably decommission them, as well," Scardina predicts. "But we also have to answer such questions is whether only a dependent surveillance system en route is satisfactory. What if the airplane loses its GPS capability? We are looking at other approaches, such as triangulation, that would allow us to determine its position from the ground and direct it to a landing."

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