View source for Wide Area Augmentation System

{{short description|System that enhances the accuracy of GPS receivers}}
{{redirect|WAAS|other uses of the acronym "WAAS"|WAAS (disambiguation)}}
{{Infobox navigation satellite system
|name = Wide Area Augmentation System (WAAS)
|image = File:Waas-logo.svg
|image_caption = 
|country = United States
|type = 
|status = Operational
|operator = [[Federal Aviation Administration|FAA]]
|coverage = United States, Canada, Mexico
|precision = {{convert|1.0|m|ft}}<ref name="WAAS_NSTB_PAN_Report_Jul06"/>
|satellites_nominal = <!-- 33 -->
|satellites_current = 3
|first_launch = {{start date and age|2003}}
|last_launch = 
|launch_total = 
|regime = [[Geostationary orbit|GEO]] (uses communication satellites)
|orbit_height = <!--20,180&nbsp;km (12,540 mi)-->
}}
{{Geodesy}}

[[File:FAA WAAS System Overview.jpg|thumb|right|WAAS system overview]]

The '''Wide Area Augmentation System''' ('''WAAS''') is an [[air navigation]] aid developed by the [[Federal Aviation Administration]] to [[GNSS augmentation|augment]] the [[Global Positioning System]] (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including approaches with vertical guidance to any airport within its coverage area. It may be further enhanced with the [[local-area augmentation system]] (LAAS) also known by the preferred ICAO term ''ground-based augmentation system'' (GBAS) in critical areas.

WAAS uses a network of ground-based reference stations, in [[North America]] and [[Hawaii]], to measure small variations in the GPS satellites' signals in the [[Western Hemisphere]]. Measurements from the reference stations are routed to master stations, which queue the received deviation correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.

The [[International Civil Aviation Organization]] (ICAO) calls this type of system a [[GNSS augmentation|satellite-based augmentation system]] (SBAS). Europe and Asia are developing their own SBASs: the Indian [[GPS-aided GEO augmented navigation|GPS aided GEO augmented navigation]] (GAGAN), the [[European Geostationary Navigation Overlay Service]] (EGNOS), the Japanese [[Multi-functional Satellite Augmentation System]] (MSAS) and the Russian [[System for Differential Corrections and Monitoring]] (SDCM), respectively. Commercial systems include [[StarFire (navigation system)|StarFire]], [[OmniSTAR]], and [[Hemisphere GNSS|Atlas]].
[[File:SBAS Service Areas.png|right|thumb|SBAS Service Areas]]
==WAAS objectives==
[[File:WAAS service area.png|right|thumb|Typical WAAS service area. Dark red indicates best WAAS coverage. The service contours change over time with satellite geometry and ionospheric conditions.]]

===Accuracy===
{{missing information|section|flight phases (En-route, Terminal, LNAV, LNAV/VNAV, LPV, and LPV-200) and associated precision requirements; LPV-200 is especially under-explained in the article|date=August 2023}}
A primary goal of WAAS was to allow aircraft to make a Category I approach without any equipment being installed at the airport. This would allow new GPS-based [[instrument approach|instrument landing approach]]es to be developed for any airport, even ones without any ground equipment. A Category I approach requires an accuracy of {{convert|16|m}} laterally and {{convert|4.0|m}} vertically.<ref name="faa.gov">Federal Aviation Administration (FAA), Press Release  [http://www.faa.gov/news/press_releases/news_story.cfm?contentKey=4006 FAA Announces Major Milestone for Wide Area Augmentation System (WAAS)]. March 24, 2006.</ref>

To meet this goal, the WAAS specification requires it to provide a position accuracy of {{convert|7.6|m}} or less (for both lateral and vertical measurements), at least 95% of the time.<ref name=WASSspec>FAA. [http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/documents/media/waas/2892bC2a.pdf Specification for the Wide Area Augmentation System(WAAS)] {{webarchive|url=https://web.archive.org/web/20081004122449/http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/documents/media/waas/2892bC2a.pdf |date=2008-10-04 }}. FAA-E- 2892b. August 13, 2001.</ref> Actual performance measurements of the system at specific locations have shown it typically provides better than {{convert|1.0|m}} laterally and {{convert|1.5|m}} vertically throughout most of the [[contiguous United States]] and large parts of [[Canada]] and [[Alaska]].<ref name="WAAS_NSTB_PAN_Report_Jul06">National Satellite Test Bed (NSTB), [http://www.nstb.tc.faa.gov/REPORTS/waaspan17.pdf WAAS PAN Report (July 2006)]. Retrieved November 22nd, 2006.</ref>

===Integrity===
Integrity of a navigation system includes the ability to provide timely warnings when its signal is providing misleading data that could potentially create hazards. The WAAS specification requires the system detect errors in the GPS or WAAS network and notify users within 6.2 seconds.<ref name=WASSspec/> Certifying that WAAS is safe for [[instrument flight rules]] (IFR) (i.e. flying in the clouds) requires proving there is only an extremely small probability that an error exceeding the requirements for accuracy will go undetected. Specifically, the probability is stated as 1×10<sup>−7</sup>, and is equivalent to no more than 3 seconds of bad data per year. This provides integrity information equivalent to or better than [[receiver autonomous integrity monitoring]] (RAIM).<ref name="House_Subcommitee_0600">US House of Representatives Committee on Transportation's Subcommittee on Aviation [http://commdocs.house.gov/committees/Trans/hpw106-100.000/hpw106-100_1.HTM Hearing on Cost Overruns & Delays in the FAA's Wide Area Augmentation System (WAAS) & Related Radio Spectrum Issues]. June 29, 2000</ref>

===Availability===
Availability is the probability that a navigation system meets the accuracy and integrity requirements. Before the advent of WAAS, GPS specifications allowed for system unavailability for as much as a total time of four days per year (99% availability).{{citation needed|date=May 2008}} The WAAS specification mandates availability as 99.999% ([[High availability#Percentage calculation|five nines]]) throughout the service area, equivalent to a downtime of just over 5 minutes per year.<ref name=WASSspec/><ref name="House_Subcommitee_0600" />

==Operation==
[[File:WAAS Reference Station Barrow Alaska.jpg|thumb|left|WAAS reference station in [[Utqiagvik, Alaska]]]]
WAAS is composed of three main segments: the [[ground segment]], [[space segment]], and user segment.

===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>

===Space segment===
<!-- [[File:WAAS GEO Footprint March 2010.jpg|thumb|right| Current WAAS satellite signal footprint]] This image indicates that WAAS coverage is not available in Canada or southern Mexico, which is inaccurate.  It needs to be changed before it can be put back here. -->The space segment consists of multiple [[artificial satellites|communication satellites]] which broadcast the correction messages generated by the WAAS master stations for reception by the user segment. The satellites also broadcast the same type of range information as normal GPS satellites, effectively increasing the number of satellites available for a position fix. The space segment currently consists of three commercial satellites: ''[[Eutelsat 117 West B]]'', ''[[SES-15]]'', and ''[[Galaxy 30]]''.<ref name="prn-135-resumes">[http://www.gpsworld.com/gnss-system/augmentation-assistance/news/waas-prn-135-resumes-normal-operation-11243 WAAS PRN 135 Resumes Normal Operation] {{webarchive|url=https://web.archive.org/web/20110727142321/http://www.gpsworld.com/gnss-system/augmentation-assistance/news/waas-prn-135-resumes-normal-operation-11243 |date=2011-07-27 }}. March 18, 2011. Accessed November 21, 2011.</ref><ref>{{Cite web|title=SES-15 Enters Commercial Service to Serve the Americas|url=https://www.ses.com/press-release/ses-15-enters-commercial-service-serve-americas|website=SES|date=15 January 2018 |language=en|access-date=2020-05-24}}</ref><ref>{{Cite web|title=FAA Tasks Intelsat with Navigation Satellite WAAS Payload|url=https://www.aviationtoday.com/2018/04/18/faa-tasks-intelsat-navigation-satellite-waas-payload/|date=2018-04-18|website=Avionics|language=en|access-date=2020-05-24}}</ref>

==== Satellite history ====
The original two WAAS satellites, named ''Pacific Ocean Region'' (POR) and ''Atlantic Ocean Region-West'' (AOR-W), were leased space on [[Inmarsat#Inmarsat-3 satellites|Inmarsat III]] satellites. These satellites ceased WAAS transmissions on July 31, 2007. With the end of the Inmarsat lease approaching, two new satellites ([[Galaxy 15]] and [[Anik F1R]]) were launched in late 2005. Galaxy 15 is a [[PanAmSat]] and Anik F1R is a [[Telesat]]. As with the previous satellites, these are leased services under the FAA's Geostationary Satellite Communications Control Segment contract with [[Lockheed Martin]] for WAAS geostationary satellite leased services, who were contracted to provide up to three satellites through the year 2016.<ref>Federal Aviation Administration (FAA) Announcement [http://gps.faa.gov/Library/Data/waas/March_2005.doc March 2005] {{webarchive|url=https://web.archive.org/web/20061208030033/http://gps.faa.gov/Library/Data/waas/March_2005.doc |date=2006-12-08 }}</ref>

A third satellite was later added to the system. From March to November 2010, the FAA broadcast a WAAS test signal on a leased transponder on the Inmarsat-4 F3 satellite.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/ FAA: New WAAS GEO to Begin Broadcasting in Test Mode in March (2010)]. January 19, 2010. Accessed November 21, 2011.</ref> The test signal was not usable for navigation, but could be received and was reported with the identification numbers PRN 133 (NMEA #46). In November 2010, the signal was certified as operational and made available for navigation.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/ WAAS Intelsat GEO Satellite Ceases Broadcast]. December 16, 2010. Accessed November 21, 2011.</ref> Following in orbit testing, Eutelsat 117 West B, broadcasting signal on PRN 131 (NMEA #44), was certified as operational and made available for navigation on March 27, 2018. The SES 15 satellite was launched on May 18, 2017, and following an in-orbit test of several months, was set operational on July 15, 2019. In 2018, a contract was awarded to place a WAAS L-band payload on the Galaxy 30 satellite. The satellite was successfully launched on August 15, 2020, and the WAAS transmissions were set operational on April 26, 2022, re-using PRN 135 (NMEA #48).<ref>{{Cite web|url=https://investors.leidos.com/news-and-events/news-releases/press-release-details/2018/Leidos-Awarded-GEO-7-Task-Order-to-Enhance-US-Air-Traffic-System/default.aspx|title=Leidos Awarded GEO 7 Task Order to Enhance U.S. Air Traffic System|website=investors.leidos.com|language=en-US|access-date=2019-03-26}}</ref><ref name=":0">{{Cite web |last=Miller |first=Dan |date=2022-05-14 |title=FAA Shutdown of Geostationary Satellite on Tuesday Could Affect Some GPS Farming Systems |url=https://www.dtnpf.com/agriculture/web/ag/equipment/article/2022/05/14/faa-shutdown-geostationary-satellite |access-date=2022-06-04 |website=DTN Progressive Farmer |language=en-US}}</ref> After approximately three weeks with four active WAAS satellites, operational WAAS transmissions on Anik F1-R were ended on May 17, 2022.<ref name=":0" />

{| class="wikitable"
! Satellite name and details
! PRN
! NMEA
!Designator
! Location
!Active period (not in test mode)
!Status
!Signal capability
|-
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1997-027A Atlantic Ocean Region-West]
|122
|35
|AORW
|54°W, later moved to 142°W<ref>[http://www.tbs-satellite.com/tse/online/sat_inmarsat_3f4.html The Satellite Encyclopedia - Inmarsat 3F4]. Accessed October 28, 2013.</ref>
|July 10, 2003 – July 31, 2017
|''Ceased operational WAAS transmissions on July 31, 2017''
|L1 narrowband
|-
|[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1996-070A Pacific Ocean Region (POR)]
|134
|47
|POR
|178°E
|July 10, 2003 – July 31, 2017
|''Ceased operational WAAS transmissions on July 31, 2017''
|L1
|-
|[[Galaxy 15]]
|135
|48
|CRW
|133°W
|November 2006 – July 25, 2019
|''Ceased operational WAAS transmissions on July 25, 2019.''
|L1, L5 (test mode)
|-
|[[Anik F1R]]
|138
|51
|CRE
|107.3°W
|July 2007 – May 17, 2022
|''Ceased operational WAAS transmissions on May 17, 2022.''<ref name=":0" />
|L1, L5 (test mode)
|-
|[[Inmarsat-4 F3]]
|133
|46
|AMR
|98°W
|November 2010 – November 9, 2017
|''Ceased operational WAAS transmissions as of November 9, 2017.''<ref>[http://www.nstb.tc.faa.gov/ "NOTICE: GEO PRN 133 (AMR) was removed from the WAAS satellite mask on November 9th, 2017."] Accessed December 4th, 2017.</ref>
|L1 narrowband, L5 (test mode)
|-
|[[Eutelsat 117 West B]]
|131
|44
|SM9
|117°W
|March 2018 – present
|Operational
|L1, L5 (test mode)
|-
|[[SES-15|SES 15]]
|133
|46
|S15
|129°W
|July 15, 2019 – present
|Operational
|L1, L5 (test mode)
|-
|[[Galaxy 30]]
|135
|48
|G30
|125°W
|April 26, 2022 – present
|Operational
|L1, L5 (test mode)
|}
In the table above, PRN is the satellite's actual pseudo-random number code. NMEA is the satellite number sent by some receivers when outputting satellite information (NMEA = PRN - 87).

===User segment===
The user segment is the GPS and WAAS receiver, which uses the information broadcast from each GPS satellite to determine its location and the current time, and receives the WAAS corrections from the Space segment. The two types of correction messages received (fast and slow) are used in different ways.

The GPS receiver can immediately apply the fast type of correction data, which includes the corrected satellite position and clock data, and determines its current location using normal GPS calculations. Once an approximate position fix is obtained the receiver begins to use the slow corrections to improve its accuracy. Among the slow correction data is the ionospheric delay. As the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received an ionospheric delay value for that location, corrects for the error the ionosphere created.

While the slow data can be updated every minute if necessary, [[ephemeris]] errors and ionosphere errors do not change this frequently, so they are only updated every two minutes and are considered valid for up to six minutes.<ref>{{cite web |title=DGPS on Garmin Receivers |url=http://www.gpsinformation.org/dale/dgps.htm#waas |access-date=2007-04-13 }}</ref>

==History and development==
{{More citations needed section|date=December 2006}}
The WAAS was jointly developed by the United States Department of Transportation (DOT) and the Federal Aviation Administration (FAA) as part of the [http://gauss.gge.unb.ca/us1996frp.pdf Federal Radionavigation Program] (DOT-VNTSC-RSPA-95-1/DOD-4650.5), beginning in 1994, to provide performance comparable to category 1 [[instrument landing system]] (ILS) for all aircraft possessing the appropriately certified equipment.<ref name="FAA_WAAS_FAQ" /> Without WAAS, ionospheric disturbances, [[clock drift]], and satellite orbit errors create too much error and uncertainty in the GPS signal to meet the requirements for a [[precision approach]] (see [[Gps#Accuracy and error sources|GPS sources of error]]). A precision approach includes altitude information and provides course guidance, distance from the runway, and elevation information at all points along the approach, usually down to lower altitudes and weather minimums than non-precision approaches.

Prior to the WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide lateral and vertical navigation for precision approaches for all users at all locations. The traditional system for precision approaches is the [[instrument landing system]] (ILS), which used a series of radio transmitters each broadcasting a single signal to the aircraft. This complex series of radios needs to be installed at every runway end, some offsite, along a line extended from the runway centerline, making the implementation of a precision approach both difficult and very expensive. The ILS system is composed of 180 different transmitting antennas at each point built. 

For some time the FAA and [[NASA]] developed a much improved system, the [[microwave landing system]] (MLS). The entire MLS system for a particular approach was isolated in one or two boxes located beside the runway, dramatically reducing the cost of implementation. MLS also offered a number of practical advantages that eased traffic considerations, both for aircraft and radio channels. Unfortunately, MLS would also require every airport and aircraft to upgrade their equipment.

During the development of MLS, consumer GPS receivers of various quality started appearing. GPS offered a huge number of advantages to the pilot, combining all of an aircraft's long-distance navigation systems into a single easy-to-use system, often small enough to be hand held. Deploying an aircraft navigation system based on GPS was largely a problem of developing new techniques and standards, as opposed to new equipment. The FAA started planning to shut down their existing long-distance systems ([[VHF omnidirectional range|VOR]] and [[Non-directional beacon|NDB]]s) in favor of GPS. This left the problem of approaches, however. GPS is simply not accurate enough to replace ILS systems. Typical accuracy is about {{convert|15|m}}, whereas even a "CAT I" approach, the least demanding, requires a vertical accuracy of {{convert|4|m}}.

This inaccuracy in GPS is mostly due to large "billows" in the [[ionosphere]], which slow the radio signal from the satellites by a random amount. Since GPS relies on timing the signals to measure distances, this slowing of the signal makes the satellite appear farther away. The billows move slowly, and can be characterized using a variety of methods from the ground, or by examining the GPS signals themselves. By broadcasting this information to GPS receivers every minute or so, this source of error can be significantly reduced.
<!-- The following paragraph is subject to deletion --><!-- Why? Explain on talk page -->
This led to the concept of [[Differential GPS]], which used separate radio systems to broadcast the correction signal to receivers. Aircraft could then install a receiver which would be plugged into the GPS unit, the signal being broadcast on a variety of frequencies for different users (FM radio for cars, longwave for ships, etc.). Broadcasters of the required power generally cluster around larger cities, making such DGPS systems less useful for wide-area navigation. Additionally, most radio signals are either line-of-sight, or can be distorted by the ground, which made DGPS difficult to use as a precision approach system or when flying low for other reasons.

The FAA considered systems that could allow the same correction signals to be broadcast over a much wider area, such as from a satellite, leading directly to WAAS. Since a GPS unit already consists of a satellite receiver, it made much more sense to send out the correction signals on the same frequencies used by GPS units, than to use an entirely separate system and thereby double the probability of failure. In addition to lowering implementation costs by "piggybacking" on a planned satellite launch, this also allowed the signal to be broadcast from [[geostationary orbit]], which meant a small number of satellites could cover all of North America.

On July 10, 2003, the WAAS signal was activated for general aviation, covering 95% of the United States, and portions of Alaska offering {{convert|350|ft}} minimums.

On January 17, 2008, Alabama-based Hickok & Associates became the first designer of helicopter WAAS with Localizer Performance (LP) and [[Localizer Performance with Vertical guidance]] (LPV) approaches, and the only entity with FAA-approved criteria (which even FAA has yet to develop).<ref>{{Cite web |url=http://www.ainonline.com/news/single-news-page/article/waas-approaches-coming-to-heliports/?no_cache=1&cHash=a7ee70cd1a |title = WAAS approaches coming to heliports |website=AINonline |archive-url=https://archive.today/20110616214831/http://www.ainonline.com/news/single-news-page/article/waas-approaches-coming-to-heliports/?no_cache=1&cHash=a7ee70cd1a |archive-date=16 June 2011 |url-status=dead}}</ref><ref>{{cite web | url=http://www.flttechonline.com/Current/Hickok%20and%20Associates%20Developing%20WAAS%20Approaches%20for%20Helicopters.htm | title= Hickok & Associates Developing WAAS Approaches for Helicopters | access-date=2025-09-25 | archive-date=2008-09-08 | archive-url=https://web.archive.org/web/20080908083701/http://www.flttechonline.com/Current/Hickok%20and%20Associates%20Developing%20WAAS%20Approaches%20for%20Helicopters.htm | url-status=usurped | website=Flt Tech Online }}</ref><ref>{{cite news |url=http://www.ainonline.com/ain-and-ainalerts/aviation-international-news/single-publication-story/browse/0/article/owners-responsible-for-private-helo-approaches/?no_cache=1&tx_ttnews |archive-url=https://archive.today/20110707100623/http://www.ainonline.com/ain-and-ainalerts/aviation-international-news/single-publication-story/browse/0/article/owners-responsible-for-private-helo-approaches/?no_cache=1&tx_ttnews |archive-date=2011-07-07 |title=Owners responsible for private helo approaches |website=AINOnline |date=February 2009 |last=Huber |first=Mark}}</ref> This helicopter WAAS criteria offers as low as 250 foot minimums and decreased visibility requirements to enable missions previously not possible. On April 1, 2009, FAA AFS-400 approved the first three helicopter WAAS GPS approach procedures for Hickok & Associates' customer California Shock/Trauma Air Rescue (CALSTAR). Since then they have designed many approved WAAS helicopter approaches for various EMS hospitals and air providers, within the United States as well as in other countries and continents.

On December 30, 2009, Seattle-based Horizon Air flew the first scheduled-passenger service flight<ref>{{cite web|url=http://www.alaskasworld.com/newsroom/QXnews/QXstories/QX_20100108_104108.asp|title=Horizon Makes Aviation History with First WAAS Flight|access-date=2010-01-13|archive-date=2010-01-12|archive-url=https://web.archive.org/web/20100112113104/http://www.alaskasworld.com/newsroom/QXnews/QXstories/QX_20100108_104108.asp|url-status=dead}}</ref> using WAAS with LPV on flight 2014, a Portland to Seattle flight operated by a Bombardier Q400 with a WAAS FMS from Universal Avionics. The airline, in partnership with the FAA, will outfit seven Q400-aircraft with WAAS and share flight data to better determine the suitability of WAAS in scheduled air service applications.{{update inline|date=May 2024}}

===Timeline===
'''Wide-Area Augmentation System (WAAS) timeline'''
<timeline>
ImageSize  = width:700 height:1000
PlotArea   = left:40 right:30 top:10 bottom:20 
DateFormat = mm/dd/yyyy
TimeAxis   = orientation:vertical order:normal format:yyyy
Period     = from:1995 till:2022
AlignBars  = early
ScaleMajor = unit:year increment:1 start:1995
ScaleMinor = unit:month increment:6 start:06/01/1995
Colors = 
  id:gray value:gray(0.7)

# there is no automatic collision detection,
# so shift texts up or down manually to avoid overlap

Define $dx = 25 # shift text to right side of bar

PlotData =
  bar:event width:20 color:blue shift:($dx,-4) 
  
  from:start till:end color:blue
  
  mark:(line, white)

  at:08/01/1995 text:"August, 1995: Wilcox Electric contracted to deliver WAAS."
  at:02/01/1996 text:"February, 1996: WAAS Architecture Version 1.5 Released."
  at:04/01/1996 shift:($dx,1.5) text:"April, 1996: Wilcox contract terminated due to inadequate technical capability by Wilcox."
  at:10/01/1996 text:"October, 1996: Hughes Aircraft contracted to deliver Phase 1 WAAS by April 1, 1999."
  at:12/18/1996 shift:($dx,0.5) text:"December, 1996: Inmarsat's POR (NMEA #47) is launched."
  at:06/03/1997 text:"June, 1997: Inmarsat's AOR-W (NMEA #35) is launched."
  at:01/01/1998 text:"January, 1998: Raytheon Systems purchases Hughes Aircraft, assuming control of WAAS contract."
  at:12/01/1999 text:"December, 1999: WAAS signal being transmitted from satellites for testing purposes."
  at:03/31/2003 text:"March 31, 2003: Capstone conducts the first commercial flight with a TSO-145 GPS/WAAS receiver."
  at:07/10/2003 text:"July 10, 2003: The FAA commissions the Wide Area Augmentation System (WAAS) for aviation use."
  at:09/01/2004 shift:($dx,-15) text:"September, 2004: Site surveys for new WAAS reference stations (WRS) in Alaska and Canada are completed."
  at:10/01/2004 text:"October, 2004: The FAA approves the Garmin 480 as the first WAAS-equipped avionics for LPV approaches."
  at:03/01/2005 text:"March, 2005: The FAA selects Lockheed Martin as new Ground Control Contractor."
  at:06/01/2005 text:"June, 2005: First international Wide-area Reference Station Installed in Gander, Newfoundland & Labrador, Canada."
  at:09/09/2005 text:"September 9, 2005: Telesat's Anik F1R (NMEA #51) is launched."
  at:10/13/2005 shift:($dx,5) text:"October 13, 2005: PanAmSat's Galaxy XV (NMEA #48) is launched."
  at:02/01/2006 shift:($dx,2) text:"February, 2006: Inmarsat's AOR-W (NMEA #35) moved from 54°W to 142°W, interrupting service for the northeastern United States."
  at:03/01/2006 shift:($dx,10) text:"March, 2006: WAAS approved to provide guidance down to 200 feet above an airport’s surface for LPV instrument approaches."
  at:11/09/2006 text:"Galaxy XV (NMEA #48) begins broadcasting certified correction messages, restoring service for the northeastern United States."
  at:09/27/2007 text:"New Wide-area Reference Stations in Mexico and Canada come online, expanding WAAS service area."
  at:06/15/2016 text:"June 15, 2016: Eutelsat 117 West B (Satmex 9; NMEA #44) is launched by SpaceX."
  at:05/18/2017 text:"May 18, 2017: Boeing SES-15 is launched by Arianespace."
  at:11/09/2017 text:"November, 2017: Inmarsat 4-F3 (AMR; NMEA #46) removed from WAAS satellite mask."
  at:08/15/2020 text:"August 15, 2020: Galaxy 30 is launched by Arianespace."
  at:04/26/2022 text:"April 26, 2022: Galaxy 30 (NMEA #48) added to WAAS satellite mask."
  at:05/17/2022 shift:($dx, 4.0) text:"May 17, 2022: Anik F1R (NMEA #51) removed from WAAS satellite mask."
</timeline><ref>* [http://www.defensedaily.com/cgi/rw/show_mag.cgi?pub=av&mon=0303&file=0303capstone.htm Capstone program testing] {{Webarchive|url=https://web.archive.org/web/20120206011224/http://www.defensedaily.com/cgi/rw/show_mag.cgi?pub=av&mon=0303&file=0303capstone.htm |date=2012-02-06 }}
* Inmarsat moves AOR-W Satellite #35 east Federal Aviation Administration. [http://gps.faa.gov/programs/waas/for_pilots.htm Information for Pilots]. Accessed 12 June 2006.
* Contract with [[Hughes Aircraft]] finalized [http://www.hq.nasa.gov/office/pao/History/presrep96/Presrp96/ch6b.htm First reference] {{Webarchive|url=https://web.archive.org/web/20150922214905/http://www.hq.nasa.gov/office/pao/History/presrep96/Presrp96/ch6b.htm |date=2015-09-22 }},  [http://www.fas.org/spp/military/gao/rced98012.htm Second reference] {{Webarchive|url=https://web.archive.org/web/20160311202204/http://fas.org/spp/military/gao/rced98012.htm |date=2016-03-11 }} 
* [http://www.hq.nasa.gov/office/pao/History/presrep96/Presrp96/ch6b.htm Version 1.5 Released] {{Webarchive|url=https://web.archive.org/web/20150922214905/http://www.hq.nasa.gov/office/pao/History/presrep96/Presrp96/ch6b.htm |date=2015-09-22 }} 
* General source: Federal Aviation Administration. [http://gps.faa.gov/programs/waas/currentnews-text.htm WAAS Current news]. Accessed June 12, 2006.</ref>

==Comparison of accuracy==
{| class="wikitable"
|+ A comparison of various radionavigation system accuracies
! System
! 95% accuracy<br /> (lateral / vertical)
! Details
|-
|[[LORAN-C]] specification
|460&nbsp;[[metre|m]] / 460&nbsp;m
|The specified absolute accuracy of the LORAN-C system.
|-
|[[Distance measuring equipment]] (DME) specification
|185&nbsp;m (Linear)
|DME is a radionavigation aid that can calculate the linear distance from an aircraft to ground equipment.
|-
|[[GPS]] specification
|100&nbsp;m / 150&nbsp;m
|The specified accuracy of the GPS system with the [[Selective Availability]] (SA) option turned on. SA was employed by the U.S. Government until May 1, 2000.
|-
|[[LORAN-C]] measured repeatability
|50&nbsp;m / 50&nbsp;m
|The U.S. Coast Guard reports "return to position" accuracies of 50 meters in time difference mode.
|-
|[[eLORAN]] repeatability{{citation needed|date=August 2018}}
|
|Modern LORAN-C receivers, which use all the available signals simultaneously and H-field antennas. <!-- Potential source: http://www.eu-gloria.org/2003/GNSS2003%20Loran-C%20Challenges%20GNSS.pdf -->
|-
|[[Differential GPS]] (DGPS)
|10&nbsp;m / 10&nbsp;m
|This is the [[Differential GPS]] (DGPS) worst-case accuracy. According to the 2001 Federal Radionavigation Systems (FRS) report published jointly by the U.S. DOT and [[United States Department of Defense|Department of Defense]] (DoD), accuracy degrades with distance from the facility; it can be < 1 m but will normally be < 10 m.
|-
|Wide-area augmentation system (WAAS) specification
|7.6&nbsp;m / 7.6&nbsp;m
|The worst-case accuracy that the WAAS must provide to be used in precision approaches.
|-
|[[GPS]] measured
|2.5&nbsp;m / 4.7&nbsp;m
|The actual measured accuracy of the system (excluding receiver errors), with SA turned off, based on the findings of the FAA's National Satellite Test Bed, or NSTB.<!-- Not a typo: This is NOT the NTSB! -->
|-
|WAAS measured
|0.9&nbsp;m / 1.3&nbsp;m
|The actual measured accuracy of the system (excluding receiver errors), based on the NSTB's findings.
|-
|[[Local-area augmentation system]] (LAAS) specification
|
|The goal of the LAAS program is to provide [[Instrument landing system#ILS categories|Category IIIC ILS]] capability. This will allow aircraft to land with zero visibility utilizing '[[autoland]]' systems and will indicate a very high accuracy of < 1 m.<ref>[https://books.google.com/books?id=zwmJI0I3qCMC&pg=PA279&dq=%22laas%22+autoland&hl=en Aircraft Instrumentation and Systems]. page 279 chapter "9. Aircraft Navigation Systems" section "2 Ground Based Augmentation Systems"</ref>
|}

==Benefits==
[[File:Napa GUS facility.jpg|thumb|WAAS ground uplink station (GUS) in [[Napa, California]]]]
WAAS addresses all of the "navigation problem", providing highly accurate positioning that is extremely easy to use, for the cost of a single receiver installed on the aircraft. Ground- and space-based infrastructure is relatively limited, and no on-airport system is needed. WAAS allows a precision approach to be published for any airport, for the cost of developing the procedures and publishing the new approach plates. This means that almost any airport can have a precision approach and the cost of implementation is drastically reduced.

Additionally WAAS works just as well between airports. This allows the aircraft to fly directly from one airport to another, as opposed to following routes based on ground-based signals. This can cut route distances considerably in some cases, saving both time and fuel. In addition, because of its ability to provide information on the accuracy of each GPS satellite's information, aircraft equipped with WAAS are permitted to fly at lower en-route altitudes than was possible with ground-based systems, which were often blocked by terrain of varying elevation. This enables pilots to safely fly at lower altitudes, not having to rely on ground-based systems. For unpressurized aircraft, this conserves oxygen and enhances safety.

The above benefits create not only convenience, but also have the potential to generate significant cost savings. The cost to provide the WAAS signal, serving all 5,400 public use airports, is just under [[US$]]50 million per year. In comparison, the current ground based systems such as the Instrument Landing System (ILS), installed at only 600 airports, cost US$82 million in annual maintenance.{{citation needed|date=January 2008}} Without ground navigation hardware to purchase, the total cost of publishing a runway's WAAS approach is approximately US$50,000; compared to the $1,000,000 to $1,500,000 cost to install an ILS radio system.<ref name="AOPA_welcomes_WAAS">[[Aircraft Owners and Pilots Association]],  [http://www.aopa.org/whatsnew/newsitems/2006/060307waas.html AOPA welcomes improved WAAS minimums]. March 7, 2006. Accessed January 6, 2008.</ref>

==Drawbacks and limitations==
For all its benefits, WAAS is not without drawbacks and critical limitations:

* [[Space weather]]. All man-made satellite systems are subject to space weather and space debris threats. For example, a solar super-storm event composed of an extremely large and fast earthbound [[coronal mass ejection]] (CME) could disable the geosynchronous or GPS satellite elements of WAAS.
* The broadcasting satellites are geostationary, which causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of [[Alaska]] or [[northern Canada]] may have difficulty maintaining a lock on the WAAS signal.<ref>Department of Aeronautics and Astronautics, Stanford University. [http://waas.stanford.edu/~wwu/papers/gps/PDF/LoPLANS02.pdf WAAS Performance in the 2001 Alaska Flight Trials of the High Speed Loran Data Channel] {{webarchive|url=https://web.archive.org/web/20060427104555/http://waas.stanford.edu/~wwu/papers/gps/PDF/LoPLANS02.pdf |date=2006-04-27 }}. Accessed June 12, 2006.</ref>
* To calculate an ionospheric grid point's delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated.
* Aircraft conducting WAAS approaches use certified GPS receivers, which are much more expensive than non-certified units. In 2024, Garmin's least expensive certified receiver, the GPS 175, had a suggested retail price of US$5,895.
* WAAS is not capable of the accuracies required for Category II or III ILS approaches. Thus, WAAS is not a sole-solution and either existing ILS equipment must be maintained or it must be replaced by new systems, such as the [[local-area augmentation system]] (LAAS).<ref>Federal Aviation Administration. [http://gps.faa.gov/FAQ/faq-waas.htm WAAS FAQ] {{webarchive|url=https://web.archive.org/web/20060517035105/http://gps.faa.gov/FAQ/faq-waas.htm |date=2006-05-17 }}. Accessed June 12, 2006.</ref>
* WAAS Localizer Performance with Vertical guidance (LPV) approaches with 200-foot minimums (LPV-200) will not be published for airports without medium intensity lighting, precision runway markings and a parallel taxiway. Smaller airports, which currently may not have these features, would have to upgrade their facilities or require pilots to use higher minimums.<ref name="AOPA_welcomes_WAAS" />
* As precision increases and error approaches zero, the [[navigation paradox]] states that there is an increased collision risk, as the likelihood of two craft occupying the same space on the shortest distance line between two navigational points has increased.

==Future of WAAS==

===Improvement to aviation operations===
In 2007, WAAS vertical guidance was projected to be available nearly all the time (greater than 99%), and its coverage encompasses the full continental U.S., most of Alaska, northern Mexico, and southern Canada.<ref>Federal Aviation Administration. [http://gps.faa.gov/programs/waas/questionsanswers.htm WAAS 200ft Minimum Related Questions and Answers] {{webarchive|url=https://web.archive.org/web/20060925215639/http://gps.faa.gov/programs/waas/questionsanswers.htm |date=2006-09-25 }}. Accessed June 12, 2006.</ref> At that time, the accuracy of WAAS would meet or exceed the requirements for Category 1 [[Instrument Landing System|ILS]] approaches, namely, three-dimensional position information down to 200 feet (60 m) above touchdown zone elevation.<ref name="faa.gov" />

===Software improvements===
Software improvements, to be implemented by September 2008, significantly improve signal availability of vertical guidance throughout the CONUS and Alaska. Area covered by the 95% available LPV solution in Alaska improves from 62% to 86%. And in the CONUS, the 100% availability LPV-200 coverage rises from 48% to 84%, with 100% coverage of the LPV solution.<ref name="WAAS_CGSIC47" />

===Space segment upgrades===
Both Galaxy XV (PRN #135) and Anik F1R (PRN #138) contain an L1 & L5 GPS payload. This means they will potentially be usable with the [[GPS modernization|L5 modernized GPS signals]] when the new signals and receivers become available. With L5, avionics will be able to use a combination of signals to provide the most accurate service possible, thereby increasing availability of the service. These avionics systems will use ionospheric corrections broadcast by WAAS, or self-generated onboard dual frequency corrections, depending on which one is more accurate.<ref name="FAA_GPS_Modern">Federal Aviation Administration (FAA), [http://gps.faa.gov/gpsbasics/GPSmodernization-text.htm GPS Modernization] {{webarchive|url=https://web.archive.org/web/20060926023328/http://gps.faa.gov/GPSbasics/GPSmodernization-text.htm |date=2006-09-26 }} Page. Accessed 29 November 2006.</ref>

==See also==
* [[GNSS augmentation|Satellite-based augmentation system]] (SBAS)
* [[EGNOS]]—the European operational SBAS
* [[MTSAT Satellite Augmentation System|MSAS]]—the Japanese operational SBAS
* [[GPS·C|CDGPS]] Canadian differential GPS
* [[Local-area augmentation system]] (LAAS)
* [[Joint Precision Approach and Landing System]] (JPALS)
* [[Distance measuring equipment]] (DME)
* [[Instrument flight rules]] (IFR)
* [[Instrument landing system]] (ILS)
* [[Localizer performance with vertical guidance]] (LPV)
* [[LORAN|Long-range radio navigation]] (LORAN)
* [[Microwave landing system]] (MLS)
* [[Non-directional beacon]] (NDB)
* [[Tactical air navigation system]] (TACAN)
* [[Transponder landing system]] (TLS)
* [[VHF omnidirectional range]] (VOR)

==References==
* U.S. Department Of Transportation & Federal Aviation Administration, [https://web.archive.org/web/20081004122449/http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/documents/media/waas/2892bC2a.pdf Specification for the Wide Area Augmentation System (WAAS)]
{{reflist}}

==External links==
{{GeoGroup}}
* FAA WJHTC's [http://www.nstb.tc.faa.gov/sms Real-Time Interactive WAAS Performance Display ]
* FAA's [http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/ WAAS program]
* Garmin's [https://web.archive.org/web/20181212031825/https://www8.garmin.com/aboutGPS/waas.html What is WAAS?]
* US Government's [http://www.navcen.uscg.gov/pdf/frp/frp2005/default.htm 2005 Federal Radionavigation Plan (FRP)]{{dead link|date=March 2018 |bot=InternetArchiveBot |fix-attempted=yes }}
* [http://members.shaw.ca/pdops/WAAS.html WAAS coverage in Canada]

{{Satellite navigation systems}}

[[Category:Global Positioning System]]
[[Category:2003 in aviation]]
[[Category:Navigational aids]]
[[Category:Satellite-based augmentation systems]]