The FAA's Wide Area Augmentation System (WAAS) GPS-based navigation network consists of 39 ground reference stations in the continental U.S. and parts of Canada and Mexico, as well as two master stations and three data uplink stations. WAAS provides pilots with positions accurate to within 25 feet, and is allowed to be used to guide descending aircraft to a vertical height of 200 feet before landing, making instrument approaches in bad weather possible to many more small U.S. airports than previously. Credit: Federal Aviation Administration
