Wide Area Augmentation System - Drawbacks and Limitations

Drawbacks and Limitations

For all its benefits, WAAS is not without drawbacks and critical limitations:

• The broadcasting satellites are geostationary, which causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal.
• To calculate an ionospheric grid point's delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated.
• Aircraft conducting WAAS approaches must possess certified GPS receivers, which are much more expensive than non-certified units. In 2006, Garmin's least expensive certified receiver, the GNS 430W, had a suggested retail price of US$10,750.
• WAAS is not capable of the accuracies required for Category II or III ILS approaches. Thus, WAAS is not a sole-solution and either existing ILS equipment must be maintained or it must be replaced by new systems, such as the Local Area Augmentation System (LAAS).
• WAAS Localizer Performance with Vertical guidance (LPV) approaches with 200-foot minimums will not be published for airports without medium intensity lighting, precision runway markings and a parallel taxiway. Smaller airports, which currently may not have these features, would have to upgrade their facilities or require pilots to use higher minimums.