Plain English Meaning
A ground antenna site that beams WAAS correction signals up to a satellite, which then sends those corrections down to aircraft so their GPS position is more accurate.
Full Definition
A ground facility within the Wide Area Augmentation System (WAAS) that transmits the WAAS correction message to a geostationary satellite, which then rebroadcasts it to GPS receivers in aircraft. The GUS receives processed correction data from a WAAS Master Station and uplinks it on the appropriate frequency to the WAAS geostationary satellite for distribution to users.
Context Anchor
Seen in discussions of how WAAS works, especially the ground-to-satellite part of the system that supports WAAS-capable GPS navigation and .