Os modos são WAAS / EGNOS e Normal
WAAS e EGNOS é a mesma coisa, só que têm nomes diferentes conforme a zona, os americas chamam waas os europeus egnos
"WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. "
"Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems."
Mais info:
http://www8.garmin.com/aboutGPS/waas.html
ps. de longe sou entendido neste assunto, penso que por aqui no forum já vi explicações mais profundas