This is part of IEEE Spectrum's special report: Critical Challenges 2002: Technology Takes On
Several projects aimed squarely at upgrading airplane travel, either by enhancing navigation or by improving communications between pilots and air traffic controllers, drew closer to full implementation last year. In mid-June, the U.S. Federal Aviation Administration (FAA), Washington, D.C., outlined its 10-year Operational Evolution Plan to improve air travel by rolling out new technology to air traffic control facilities.
All the systemwide projects target congestion in the skies and on the runways. For too long a time, too many airports were scheduling too many takeoffs or landings for any given period, driving the numbers of delays up and into the news again and again. After 11 September, safer air travel became the burning issue. But congestion will no doubt return to prominence once airlines resume full schedules. Meantime, the global positioning system (GPS) satellite constellation and improved situational awareness tools are beginning to enhance safety now, and will alleviate congestion when it returns.
The GPS difference
Two projects make heavy use of the GPS satellites' data: the wide-area and the local-area augmentation systems. With their aid, pilots will gain extremely precise information about where their own and nearby planes are, helping them to navigate better and increasing the accuracy of their instrument-based landings. The wide-area system is used for en route navigation and precision approaches for landing under Category I conditions (200-foot ceilings), while the local-area system supports aircraft operations in the terminal area, like taxiing, precision approach and landing, and taking off. Both achieve their precision through differential GPS, a technique that corrects a plain GPS signal to yield positioning accuracy within a meter.
Recall that the U.S. GPS constellation comprises 24 satellites circling the earth once every 12 sidereal hours at an altitude of 20 200 km. Four satellites define each of six orbital planes, which ensure signal capture from at least four satellites by receivers no matter where they are located--in this case, in the aircraft and ground stations. But the location information is somewhat in error because of propagation anomalies between each receiver and the transmitting satellites.
Currently, commercial GPS receivers deliver a positioning accuracy of about 20-30 meters. Inertial navigation systems on transoceanic aircraft, however, can be off course by several kilometers after several hours without an update. Aircraft flying over land do better. As they cross the sky, they navigate from one land-based radio signal to another and from VOR to VOR (VHF omnidirectional range localizer), but even this system lacks the accuracy of GPS-based navigation.
Still, by itself, GPS cannot provide accurate enough information for pilots to fly an exact course, especially when it comes to landing. (GPS does provide an "acceptable" course by FAA standards for an en route or oceanic flight path.) Even 10 meters can mean the difference between landing on the end of the runway or in a large body of water or a heavily residential neighborhood.
That's where the augmentation systems shine. The wide-area augmentation system (WAAS) uses a series of ground stations with known locations spread across the United States, plus a communications network, to determine the accuracy and integrity of the GPS signal data. (The European version is known as the satellite-based augmentation system, or SBAS.) Each ground station's location is compared with the location data transmitted by GPS, and any difference between them is calculated as the range error, or correction factor.
Using this factor, the wide-area system adjusts the GPS signal to produce a differential GPS location accurate to within a meter, which is broadcast via satellite to aircraft (and anyone else) having a WAAS-enabled receiver. The 25 WAAS ground stations in the United States are in place and operational, providing their signal around the clock to WAAS-equipped aircraft. The FAA's contractor on the project, Raytheon Command, Control, Communication and Information Systems, in Fullerton, Calif., is currently enhancing the software and expects commissioning of the system in 2003.
The FAA expects the WAAS network to expand to Canada, Iceland, Mexico, Panama, and other countries. Japan, Europe, and Australia are building systems compatible with existing WAAS avionics.
Landing requires precise position data
The local-area augmentation system (LAAS), used for takeoff, landing, and taxiing, also supplies an extremely precise GPS correction. But as its name suggests, a LAAS ground station lacks the reach of WAAS; its VHF radio broadcasts reach no more than 37 km in any direction from the airport where it is located, Bruce Solomon, a program manager in air traffic management systems for Raytheon, told IEEE Spectrum.
Inside the plane, little equipment is needed beyond the GPS receivers already in most aircraft, said Solomon. Commercial planes built over the last few years possess multimode receivers that receive the GPS signal and estimate a position. By adding a LAAS datalink receiver to that mix, signals can be received from LAAS ground stations, too.