NavList:

There are more technical explanations but the ground stations know where they are, pick up the GPS signal, figure a correction, and broadcast it to local GPS receivers so they can do the same relative correction. It improves the accuracy by a factor of about 5.
I thought this was an early feature when there was a, rumored, intentional offset and this was a workaround for boats that needed more accuracy close to land. I think today that GPS accuracy just depends on how much money you are willing to spend.

Maybe it's still needed.
See:
http://www8.garmin.com/aboutGPS/waas.html

Forgive me Garmin, but from that site:
"You've heard the term WAAS, seen it on packaging and ads for Garmin products, and maybe even know it stands for Wide Area Augmentation System. So what is it?
Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy."

Regards, Noell
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