What is WAAS?
WAAS stands for Wide Area Augmentation System and consists of a network of satellites and ground stations that improve GPS accuracy. It was developed for civil aviation and provides service for all classes of aircraft in all phases of flight including instrument approaches with vertical guidance similar to an ILS.
How does WAAS Work?
Basic GPS uses a network of satellites to triangulate positions anywhere on Earth. There are at least five satellites in view at all times. Your onboard navigator can provide three-dimensional position and altitude information if it is receiving signals from at least four satellites.
WAAS uses two additional geostationary satellites to improve the accuracy of GPS to less than three meters. These satellites receive and transmit a correction signal to WAAS-enabled navigators. The correction signals are calculated on the ground by two wide area master stations and uploaded to the stationary satellites. These master stations receive signal error data from a network of precisely surveyed reference stations. The reference stations constantly monitor the GPS satellite signals for errors.
What approaches require WAAS?
A class of approach procedures provides lateral and vertical guidance to support WAAS-capable GPS. These procedures are referred to as Approach with Vertical Guidance (APV). They include approaches such as the LNAV/VNAV and localizer performance with vertical guidance (LPV). WAAS is required for LP, LPV, and LNAV/VNAV (without baro-VNAV) approaches.
WAAS GPS receivers may fly to LPV minima and LNAV/VNAV minima, using a WAAS electronic glide path. LPV minima may have a decision altitude as low as 200 feet above touchdown with visibility minimums as low as 1/2 mile as published on RNAV (GPS) approach charts.
A different WAAS-based line of minima, called Localizer Performance (LP) is also available where the terrain or obstructions do not allow publication of vertically guided LPV minima. LP takes advantage of the smaller position errors provided by WAAS to provide a lateral only procedure similar to an ILS Localizer.
