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The Wikipedia article on WAAS says, 

> The ground segment is composed of multiple Wide-area Reference Stations 
(WRS). These precisely surveyed ground stations monitor and collect 
information on the GPS signals, then send their data to three Wide-area 
Master Stations (WMS) using a terrestrial communications network. The 
reference stations also monitor signals from WAAS geostationary satellites, 
providing integrity information regarding them as well. As of October 2007 
there were 38 WRSs: twenty in the contiguous United States (CONUS), seven in 
Alaska, one in Hawaii, one in Puerto Rico, five in Mexico, and four in 
Canada.[6][7]

Originally, civilian GPS signals were degraded.  Some of the history of that is unclear to me.

However, what is relevant to this board is the "precisely surveyed ground 
stations."  Clearly, that has to be done using optical methods.  While we are 
primarily a maritime community, necessitating use of a sextant most of the 
time, with attendant accuracy limitations, the question of how one would go 
about "precisely surveying" a WAAS ground station is interesting.

Fred Hebard
mbiew@comcast.net



On May 15, 2014, at 7:31 AM, Noell Wilson wrote:

> 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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