WAAS consists of multiple ground reference stations positioned across the U.S. that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through 1 of 2 geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.
WAAS is a satellite based augmentation system
EGNOS is a satellite based augmentation system (ICAO definition)
SBAS is a wide coverage augmentation system in which the user receives augmentation information from a satellite based transmitter (ICAO definition)