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The National Aviation Road Map

Highway in the Sky emerged from the Advanced General Aviation Transports Experiment, a consortium of 70 organizations that includes aviation industry manufacturers, universities and the FAA. Since its 1994 inception, the effort has been spearheaded by NASA, under director Dan Goldin’s stated aim to establish a “national general aviation road map to enable doorstep-to-destination travel at four times highway speeds to virtually all of the nation’s suburban, rural and remote communities.” Though the original consortium effort is fading into the background, it has passed the mission on to several spinoffs, of which “Highway” is one. Another is the Small Aircraft Transportation System, a five-year, $69 million program that kicked off last year. Technically, the program is an umbrella effort that embraces Highway in the Sky and other small-aircraft technology efforts. In practice, it is focused on the other side of the equation: the airport and air traffic control infrastructure necessary to support high-tech small aircraft. After all, a fancy display system will be no better than the data it crunches; providing that data is the Small Aircraft Transportation System’s mission.

Currently, flying through low-visibility conditions requires maintaining radio contact with ground-based air traffic control stations. These outposts track aircraft via radar, enabling controllers to issue verbal instructions that keep pilots on course-and out of each other’s way. When approaching an airport through clouds, the pilot of a small aircraft tries to stay in the ordered pattern and zero in on the runway by repeatedly centering a needle tuned to a series of radio beacons. Every year, confused pilots fatally lose control of their aircraft. In theory, it would be far easier to track your approach on a GPS-driven display, available for $400 in handheld versions specially designed for pilots. But the standard GPS signal can be off by tens of meters-an error range that can all too easily place an approaching aircraft in a position to crash.

Part of the Small Aircraft Transportation System mission is to change all that, by providing displays with “differential” GPS signals corrected to an accuracy of as little as one meter. There are several schemes for achieving this goal, but the most significant for small aircraft is the Wide Area Augmentation System. The idea is to equip special ground stations with GPS receivers. By monitoring location information derived from the standard GPS signal and comparing that to the precisely known position of the station, it’s possible to calculate the moment-to-moment error in the transmission caused by interference. The ground station then beams the corrected information to a satellite, which relays an updated, far-more-accurate signal to onboard GPS displays. Much of the system is already in place in the United States, though it will likely take a few years to work out reliability issues and achieve FAA certification for small-aircraft landing approaches.

When it does go into practice, and especially when combined with forthcoming display technologies, the Wide Area Augmentation System promises to make it far easier to touch down at airports that currently rely on instrument landing-system aids (which require pilots to align aircraft with runways by following ground-based directional signals) for poor-visibility approaches. The real benefit for small planes, though, is that the vast majority of local airports that could never justify the $1 million-plus cost of installing an instrument landing system at one end of a single runway should be able to certify both ends of every runway for poor-visibility landings for free. (Aircraft can land from either end of a runway, depending on wind direction; so authorization of poor-visibility landing from both approaches requires two systems.)

In fact, the savings for most small airports will be substantially more than $1 million. The low angles and gentle descents of instrument landing-system approaches force airports to purchase-or at least acquire easements over-large tracts of land leading up to a runway. But the greater accuracy and coverage of Wide Area Augmentation System technology enable steeper approaches, allowing the airport to minimize, or even forego, these runway protection zones. “The most important aspect of [the Small Aircraft Transportation System] is that it extends instrument landing capabilities to virtually every runway, end to end, in the nation,” says McCrea, who for the next three years will be overseeing the first statewide test of the system.

In addition to providing the ability to stay on track through poor-visibility approaches into almost any runway, the Small Aircraft Transportation System also seeks to eliminate much of the need for conventional air traffic control procedures for separating planes. In their place will be a “self-separation” or “free flight” system, in which each aircraft broadcasts its GPS-determined location to every other plane within 250 kilometers; a Highway in the Sky display can then adjust a plane’s flight path to keep it out of everyone else’s way, eliminating the need for a manned control tower at smaller airports.

Over time, as more airports install increasingly precise and affordable weather radar, the Small Aircraft Transportation System initiative will also add technologies that provide pilots with “micro-weather” data about an airport, including warnings of dangerous runway turbulence created by the wingtips of larger aircraft landing ahead of them. By punching through a few menus on a cockpit display, pilots will even be able to schedule fuel and maintenance services while still aloft, signal the airport rental car agency to have a vehicle waiting, or make a reservation at a nearby hotel if an unscheduled layover is in the cards. In this way, aviation authorities hope to turn thousands of community airports into “smartports,” in most cases without large expenditures.

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