The Wide Area Augmentation System (WAAS) is the most valuable navigation tool for pilots today. It's the most precise location-providing service available in North America.
The WAAS is part of the Federal Aviation Administration's (FAA's) ongoing upgrade to a Next Generation Air Transportation System, which is often referred to simply as NextGen. The WAAS became fully operational in 2003.
Improvements to Using GPS Alone
The WAAS is a type of satellite-based augmentation system (SBAS) that uses widely spaced ground stations to correct small errors in Global Positioning System (GPS) satellite data. The GPS data are vulnerable to timing errors in either a satellite clock or a GPS receiver clock; errors related to imprecisely locating a satellite's position; disturbances from the ionosphere; and delays caused by the lower atmosphere (the troposphere, tropopause, and stratosphere). These errors aren't likely to cause significant problems, but they're the reason that GPS signals on their own aren't accurate enough for use with precision instrument approaches when landing.
Difference in Accuracy
The main benefit of the WAAS is a greatly improved accuracy. Traditional GPS is accurate to 15 meters (about 50 feet). WAAS-enabled GPS is accurate to less than 3 meters 95 percent of the time.
Practical Consequences of the WAAS
Along with increased accuracy comes the ability to utilize Localizer Performance with Vertical Guidance (LPV) approaches in situations where visibility is low. Aircraft that might otherwise be required to fly to an alternate location in low visibility can land using an LPV approach. Increased use of the WAAS also means radio-based equipment for instrument landing system approaches in low visibility may no longer need to be replaced or maintained.
All of that means fewer delays, fewer headaches for passengers, and lower costs for airlines and airports. The WAAS also allows for more direct travel routes, and that makes the National Airspace System more efficient overall.
How the WAAS Works
The WAAS uses 38 Wide-Area Reference Stations located on the ground across North America to monitor GPS satellites. The satellite data are collected at the reference stations and sent to one of three WAAS Master Stations. At a master station, the GPS-only data are corrected and used to create WAAS augmentation messages. Six Ground Uplink Stations send the messages to three geostationary communications satellites, which broadcast the improved positioning data to WAAS-enabled GPS receivers. The system also includes two Operational Control Centers that monitor and maintain it.
As of June 2019, there were WAAS-enabled LPV approach procedures at 1,946 airports. You can check the FAA's GPS/WAAS Approaches webpage to see which airports are served by the WAAS and to find out when new airports have been added.
Europe, Japan, and India each have their own SBASs, and they're meant to be interactive with each other and with the WAAS. Europe has the European Geostationary Navigation Overlay Service, Japan has the MTSAT Satellite Augmentation System, and India has the GPS Aided Geo Augmented Navigation system, or GAGAN.
Russia and China are in the process of developing SBASs as well.