Editing Wide Area Augmentation System (section)

===Ground segment===
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.<ref name="WAAS_INTL">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/index.cfm News release from FAA announcing WAAS expansion into Mexico and Canada]</ref><ref name="WAAS_CGSIC47">FAA presentation, [http://www.navcen.uscg.gov/pdf/cgsicMeetings/47/%5B12%5D%20WAAS-LAAS-CGSIC-07.pdf WAAS and LAAS status] {{webarchive|url=https://web.archive.org/web/20110614020313/http://www.navcen.uscg.gov/pdf/cgsicMeetings/47/%5B12%5D%20WAAS-LAAS-CGSIC-07.pdf |date=2011-06-14 }} at 47th meeting of the Civil Global Positioning System Service Interface Committee, September 25, 2007</ref>

Using the data from the WRS sites, the WMSs generate two different sets of corrections: fast and slow. The fast corrections are for errors which are changing rapidly and primarily concern the GPS satellites' instantaneous positions and clock errors. These corrections are considered user position-independent, which means they can be applied instantly by any receiver inside the WAAS broadcast [[Footprint (satellite)|footprint]]. The slow corrections include long-term [[ephemeris|ephemeric]] and clock error estimates, as well as [[ionospheric delay]] information. WAAS supplies delay corrections for a number of points (organized in a grid pattern) across the WAAS service area<ref name="FAA_WAAS_FAQ">Federal Aviation Administration (FAA) [http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/faq/waas FAQ for WAAS]</ref> (see [[#User segment|user segment]] below to understand how these corrections are used).

Once these correction messages are generated, the WMSs send them to two pairs of ground uplink stations (GUS), which then transmit to satellites in the space segment for rebroadcast to the user segment.<ref name="NAS_GUS">Federal Aviation Administration (FAA), National Airspace System Architecture, [http://www.nas-architecture.faa.gov/nas/mechanism/mech_data.cfm?mid=7054 Ground Uplink Stations] {{webarchive|url=https://web.archive.org/web/20070828225454/http://www.nas-architecture.faa.gov/nas/mechanism/mech_data.cfm?mid=7054 |date=2007-08-28 }}</ref>

====Reference stations====
Each FAA [[Air Route Traffic Control Center]] in the [[U.S. state|50 states]] has a WAAS reference station, except for [[Indianapolis]]. There are also stations positioned in Canada, Mexico and Puerto Rico.<ref name="FAA_WAAS_FAQ" /> See [[List of WAAS reference stations]] for the coordinates of the individual receiving antennas.<ref>{{Cite web
| url         = http://www.nstb.tc.faa.gov/reports/waaspan26.pdf
| title       = Wide-Area Augmentation System Performance Analysis Report #26
| access-date  = 2009-01-17
| author      = NSTB/WAAS T&E Team
|date=October 2008
| publisher   = FAA/William J. Hughes Technical Center
| location    = Atlantic City International Airport, New Jersey
| pages       = 93–95
}}</ref>