Evolution of the United States National Airspace: The Move Towards Performance-Based Navigation
Without a doubt, the most significant advance in navigation technology is the advent of satellite navigation. Global navigation Satellite System (GNSS) in the form of the U.S. Global Positioning System (GPS) was first introduced to the world in 1983. Following the crash of Korean Air Lines flight 007, then President Reagan offered the use of the GPS to the international civil community. GPS is a very precise positioning, navigation, and timing signal which can be used to pinpoint a location anywhere in the world to within several meters. The international aviation community quickly realized the potential of GPS for navigation use. In 1988, the International Civil Aviation Organization officially recognized GPS as a component of the future air navigation system. GPS rapidly became a crucial navigation component within the enroute and oceanic phases of flight. Internationally, ICAO recognized the Russian satellite navigation system, GLONASS, as a component of GNSS. The European Union and other countries have begun or plan to begin development of satellite navigation systems. All of these systems promise to contribute to the overall global navigation satellite system or GNSS.
As accurate as GNSS is, it is currently incapable of supplying enough integrity to be used during the most strenuous phase of flight, precision instrument approach. Integrity is defined as the probability that the information provided by a system will not be hazardously misleading. In other words, if the system breaks, what is the likelihood that this system could send a user bad data that could cause a dangerous situation? For a new navigation system in the United States, this situation can only have a one in ten million chance of happening.
The FAA has implemented GNSS, SBAS, and GBAS in the form of GPS, WAAS, and LAAS. The FAA created the WAAS program in 1992 to primarily provide the necessary integrity to utilize GPS signals for precision approach. The WAAS consists of a network of precisely surveyed wide area reference stations (WRS). These reference stations monitor GPS satellites to determine errors in the GPS satellite signal. Each reference station relays the information about the GPS satellites to the WAAS wide area master stations (WMS). There are two WMS, one located on the East Coast, one on the West Coast. The master station uses the information collected by the reference stations to develop corrections to the GPS position information and provides timely notification of unreliable GPS data. These corrections are sent to ground uplink stations (GUS) where they are transmitted in the form of a WAAS correction message to a Geostationary Earth Orbit (GEO) satellite. The WAAS signal is then broadcast to users across the U.S. and Caribbean on the same frequency as GPS. This WAAS corrected signal provides three-dimensional guidance to aircraft. The WAAS became the world’s first SBAS system approved for instrument flight in July 2003.
The Local Area Augmentation System or LAAS is the FAA’s ground based augmentation system. The LAAS is an augmentation to GPS that focuses its service on the airport area (approximately a 20-30 mile radius). It broadcasts its correction message via a very high frequency (VHF) radio data link from a ground-based transmitter. LAAS will yield the extremely high accuracy, availability, and integrity necessary for Category I, II, and III precision approaches, and will provide the ability for more flexible, curved approach paths. In flight tests, LAAS demonstrated accuracy of less than 1 meter in both the horizontal and vertical axis. The LAAS program is currently in the development stage.