WAAS

a collection of ground stations that are used to detect and correct inaccuracies in the position information derived from the GPS. Using WAAS, the accuracy of vertical position information is increased to within three meters....aircraft must be equipped with an IFR-approved GPS receiver with WAAS signal reception that integrates WAAS error correction signals into its position determining processing.

Uses 2 additional geostationary satellites to which increases the position accuracy of WAAS-capable GPS navigator.