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. 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.