To prepare for the predicted twofold to threefold increase in air traffic by 2025, government and industry are embarking on an overhaul of the national air system. The challenge is immense: The changes have to be made without system interruption and with continued safety performance. Imagine a juggler who learns a difficult new routine while keeping plates in the air! The overhaul will require the cooperation of a diverse set of people: multiple government agencies, also aircraft manufacturers and operators, and international partners.

The Federal Aviation Administration has taken on this challenge. It set up the Joint Planning and Development Office (JPDO) to coordinate the development of this new system, which is called the Next Generation Air Traffic Control System (NextGen).

The air traffic control system is mostly financed by taxes on aviation gas and on passenger tickets. There is also a sizeable contribution from the Treasury general fund, with the justification that the general public receives indirect benefits. It happens that this year the FAA is due for reauthorization, and agency funding is a contentious issue in the reauthorization debate. Passenger airlines declare the ticket tax unfair because a large aircraft requires about the same amount of support from the air traffic control system as does a small aircraft. On the other hand, passenger airlines rely on busy hubs, which require more controller attention than do smaller airports.

NextGen is really a collection of modernization efforts. One component is the utilization of Global Positioning System capabilities to determine and broadcast air-craft location, enabling a move away from reliance on radar technology. Another component is the use of digital, Internet-like communication with aircraft, enabling a move away from voice communications. Weather forecasting tools will be improved. These and other advances will enable more decision making to be shifted from air traffic controllers to pilots. As a result, flight paths can be more flexible to avoid bad weather and improve efficiency.

NextGen will also place a greater reliance on automation, which may enable quicker, more reliable decision making. One expected benefit is reduced requirements for spatial separations between aircraft. Perhaps someday it will be routine to see arrays of aircraft flying overhead together.

Planners intend for the implementation to be complete by 2025, ready to manage the 300% growth in system use that is anticipated by that year. Although 2025 is a long way off, and there will probably be many unanticipated opportunities and challenges along the way that will require modifications to the plan. But in the meantime, a framework for change is being put into place, and some changes are already taking shape.

There are benefits to starting the incorporation of new technologies by making studies of limited scope. The use of GPS, for example, has worked well in recent tests in Alaska and in the Gulf of Mexico.

What are the toughest challenges? That is difficult to answer because there are so many tough challenges associated with NextGen!

There will have to be close cooperation between large, often unwieldy, government agencies. Development of the new weather forecasting technologies will be led by the National Oceanic and Atmospheric Administration within the Department of Commerce, while the digital communications technologies will be championed by the Air Force. The FAA, with the JPDO, is within the Department of Transportation. And NASA will also play a major role. Each agency has different priorities, approval channels, budget environments, and decision timetables.

Engineers are continually exasperated with the difficulty in establishing technical standards, and with so many involved parties, there will probably be some standardization challenges. Many aircraft manufacturers, airlines, and private pilots operate internationally. The more technical requirements for flying that can be harmonized around the world, the better.

But global technical harmonization efforts have had only limited success, as a trip abroad reveals. You need to look no further than those countries where automobile steering wheels are on the right side. Manufacturers often have to create different versions of their products to meet varying international requirements, which can discourage manufacturers from entering some markets. Aircraft manufacturing is somewhat unique, and probably more difficult, because airlines and private pilots routinely fly their aircraft between numerous countries.

While NextGen is being developed, the European Union is embarking on a similar program, named SESAR. NextGen planners are working with SESAR planners, but each team will have trouble enough reaching decisions with its own diverse constituency; making decisions in parallel with one another will certainly add another level of complexity!

Other major challenges will be those associated with NextGen's reliance on GPS technology, and the technology's vulnerability to sun activity. The air control navigation system cannot abide by periodic, and sometimes days-long, interruptions, while the sky is filled with aircraft. How extensive will the backup system need to be? Will air traffic controllers have to be trained in both systems?

The incorporation of unpiloted aircraft into the airspace presents another immense challenge for NextGen planners. At present, there isn't much formal ac-commodation of unmanned air vehicles (also called unmanned air systems). And it is far from obvious how they should be accommodated. Part of the challenge is that UAVs range from aircraft small enough to rest in the palm of a hand to aircraft the size of a large airliner.

It is reasonable to require very sophisticated equipment on larger UAVs that will enable these aircraft to detect other aircraft and avoid them. That equipment will not fit on the smaller UAVs; however, smaller UAVs still can serve many useful purposes. Firefighters toss UAVs up in the air to take a quick bird's-eye view at a fire, for exam-ple, and the Army uses UAVs with infrared sensors to find buried land mines.

But as the demand for UAVs has increased, the regulatory process has not kept up. The Middle East war theater has propelled quick development of UAVs, and there are now many envisioned uses for them. UAVs are already in use by border patrols, and climate-monitoring UAVs can cruise untiringly in low earth orbits.

The UAV industry warns that the lack of regulation is holding back development. To date, almost all UAVs have been used in empty airspace because it hasn't been determined how they should interact with other aircraft. Sensors can see better than pilots can, and computers can make quicker decisions. Will pilots be eliminated entirely someday?

Besides technical challenges to NextGen, there will be plenty of policy challenges. For example, rather than spending money to undertake an airport expansion, should a region be able to implement congestion pricing? This would discourage use—just as it has discouraged automobile use in London—while raising money. But if this pricing is heavy enough to discourage use, it may end up being a small-plane exclusion.

The federal government can upgrade the national airspace, but it has only indirect influence on airports. Big improvements to the airspace won't do much good if airports aren't developed to handle the increase in traffic. Even if there is a local intention to enlarge an airport, there isn't always space to do so.

Another challenge is aviation's environmental effects. Until recently, aircraft noise was the primary aircraft emissions concern. This concern has played a part in airport expansion decisions, and manufacturers have responded with substantial noise reductions in aircraft.

Noise remains a concern, but climate change has recently become an important issue, too. Aviation's contribution to greenhouse gases is probably only about 2–3% of that from all human sources. However, some researchers conjecture that because some of these emissions occur far up in the atmosphere they may have a magnified effect.

 Scientists and policymakers want a more definitive evaluation of aviation's impact on the environment. Manufacturers are watching this issue very closely. Their aircraft tend to have lives of 25–30 years, so any changes to aircraft technology will take time to move into use. There are a number of promising research efforts un-derway, many of them conducted by NASA.

Another policy question involves avionics equipage. How actively should the government promote the purchase of NextGen-enabling technologies by air-space users? Technologies, such as GPS equipment, could be required of all users of major airports. Alternatively, unequipped aircraft could use all airports but with lower priority. Some, however, will regard such penalties as unfair to "the little guy," which raises the policy question of whether the government could subsidize equipage based on the argument that all users would benefit.

As NextGen moves forward, all of these technical and policy issues—and many others—will need to be resolved. For engineers and the general public, the progress will certainly be interesting to watch. To keep abreast of developments, visit www.jpdo.gov.

 Tim Athan, P.E., is watching NextGen develop as a fellow for the American Association for the Advancement of Science. Sponsored by the American Society of Mechanical Engineers, Athan is assigned to the Space and Aeronautics Subcommittee of the House Science and Technology Committee. The views expressed in this article are his own and not necessarily those of his employer.