owned and operated by the FAA and provided free of direct  user charges to users across the United States and most of  Canada and Mexico.  Like flying any other IAP, the pilot must see and avoid any  obstacles in the visual segment during transition to landing.  A constant-rate descent has many safety advantages over  non-precision approaches that require multiple level-offs at  stepdown fixes or manually calculating rates of descent. A  stabilized approach can be maintained from the FAF to the  landing when a constant-rate descent is used. Additionally,  the use of an electronic vertical path produced by onboard  avionics can serve to reduce CFIT, and minimize the  effects of visual illusions on approach and landing. Some  countries even mandate the use of continuous descent  final approaches (CDFAs) on non-precision approaches.  The Wide Area Augmentation System (WAAS) offers  an opportunity for airports to gain ILS like approach  capability without the purchase or installation of any  ground-based navigation equipment at the airport.  Today, WAAS is already being used at more than 900  runways across the United States to achieve minimums  as low as 200 feet height above HAT/one-half mile  visibility.  WAAS is a navigation service using a combination of GPS  satellites and the WAAS geostationary satellites to improve  the navigational service provided by GPS. WAAS achieved  initial operating capability (IOC) in 2003. The system is  The advantages of WAAS enabled LPV approaches include:  WAAS improves the navigational system accuracy for  en route, terminal, and approach operations over all the  continental United States and significant portions of Alaska,  Canada, and Mexico. This new navigational technology  supports vertically-guided instrument approaches to all  qualifying runways in the United States. Vertically-guided  approaches reduce pilot workload and provide safety  benefits compared to non-precision approaches. The WAAS  enabled vertically guided approach procedures are called  LPV, which stands for “localizer performance with vertical  guidance,”and provide ILS equivalent approach minimums  as low as 200 feet at qualifying airports. Actual minimums  are based on an airport’s current infrastructure, as well as  an evaluation of any existing obstructions. The FAA plans to  publish 300 WAAS approach procedures per year to provide  service to all qualifying instrument runways within the NAS.  Wide Area Augmentation System  Benefits Of WAAS In The Airport Environment  Advantages Of WAAS Enabled LPV Approaches  • LPV procedures have no requirement for ground- based transmitters at the airport. • No consideration needs to be given to the placement  of navigation facility, maintenance of clear zones Figure 4-13. GBAS architecture.  4-25 Ranging sources  Status information  GBAS reference receivers  GBAS ground facility  Omnidirectional VHF data broadcast (VDB) signal  GPS satellites  Differential corrections, integrity  data and path definition 