The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to 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 satellite-based augmentation system (SBAS). Europe and Asia are developing their own SBASs: the Indian 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, OmniSTAR, and Atlas.
WAAS objectives
Accuracy
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 landing approaches to be developed for any airport, even ones without any ground equipment. A Category I approach requires an accuracy of laterally and vertically.Federal Aviation Administration (FAA), Press Release FAA Announces Major Milestone for Wide Area Augmentation System (WAAS). March 24, 2006.
FAA fact sheets and guidance on WAAS note that it enables thousands of instrument approach procedures in the United States, including LPV and LP procedures, and is often used to provide vertically guided minima comparable in concept (though not identical in standards) to precision-approach operations at airports without an ILS.
To meet this goal, the WAAS specification requires it to provide a position accuracy of or less (for both lateral and vertical measurements), at least 95% of the time.FAA. Specification for the Wide Area Augmentation System(WAAS) . FAA-E- 2892b. August 13, 2001. Actual performance measurements of the system at specific locations have shown it typically provides better than laterally and vertically throughout most of the contiguous United States and large parts of Canada and Alaska.National Satellite Test Bed (NSTB), WAAS PAN Report (July 2006). Retrieved November 22nd, 2006.
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. 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 1×10−7, 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).US House of Representatives Committee on Transportation's Subcommittee on Aviation Hearing on Cost Overruns & Delays in the FAA's Wide Area Augmentation System (WAAS) & Related Radio Spectrum Issues. June 29, 2000
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). The WAAS specification mandates availability as 99.999% (five nines) throughout the service area, equivalent to a downtime of just over 5 minutes per year.
Operation
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.News release from FAA announcing WAAS expansion into Mexico and CanadaFAA presentation, WAAS and LAAS status at 47th meeting of the Civil Global Positioning System Service Interface Committee, September 25, 2007
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. The slow corrections include long-term 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 areaFederal Aviation Administration (FAA) FAQ for WAAS (see 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.Federal Aviation Administration (FAA), National Airspace System Architecture, Ground Uplink Stations
Reference stations Each Air Route Traffic Control Center in the contiguous United States has a WAAS reference station, with the exception of Indianapolis Air Route Traffic Control Center. There are also stations positioned in Alaska, Canada, Mexico and Puerto Rico. See List of WAAS reference stations for the coordinates of the individual receiving antennas.
Space segment The space segment consists of multiple 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.WAAS PRN 135 Resumes Normal Operation . March 18, 2011. Accessed November 21, 2011.
Satellite history The original two WAAS satellites, named Pacific Ocean Region (POR) and Atlantic Ocean Region-West (AOR-W), were leased space on 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.Federal Aviation Administration (FAA) Announcement March 2005
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.FAA: New WAAS GEO to Begin Broadcasting in Test Mode in March (2010). January 19, 2010. Accessed November 21, 2011. 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.WAAS Intelsat GEO Satellite Ceases Broadcast. December 16, 2010. Accessed November 21, 2011. 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). After approximately three weeks with four active WAAS satellites, operational WAAS transmissions on Anik F1-R were ended on May 17, 2022.
Satellite name and details PRN NMEADesignator LocationActive period (not in test mode)StatusSignal capabilityAtlantic Ocean Region-West12235AORW54°W, later moved to 142°WThe Satellite Encyclopedia - Inmarsat 3F4. Accessed October 28, 2013.July 10, 2003 – July 31, 2017Ceased operational WAAS transmissions on July 31, 2017L1 narrowbandPacific Ocean Region (POR)13447POR178°EJuly 10, 2003 – July 31, 2017Ceased operational WAAS transmissions on July 31, 2017L1Galaxy 1513548CRW133°WNovember 2006 – July 25, 2019Ceased operational WAAS transmissions on July 25, 2019.L1, L5 (test mode)Anik F1R13851CRE107.3°WJuly 2007 – May 17, 2022Ceased operational WAAS transmissions on May 17, 2022.L1, L5 (test mode)Inmarsat-4 F313346AMR98°WNovember 2010 – November 9, 2017Ceased operational WAAS transmissions as of November 9, 2017."NOTICE: GEO PRN 133 (AMR) was removed from the WAAS satellite mask on November 9th, 2017." Accessed December 4th, 2017.L1 narrowband, L5 (test mode)Eutelsat 117 West B13144SM9117°WMarch 2018 – presentOperationalL1, L5 (test mode)SES 1513346S15129°WJuly 15, 2019 – presentOperationalL1, L5 (test mode)Galaxy 3013548G30125°WApril 26, 2022 – presentOperationalL1, 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.