Wide Area Augmentation System
The Wide Area Augmentation System is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System, 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 also known by the preferred ICAO term ground-based augmentation system 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 and send the correction messages to geostationary WAAS satellites in a timely manner. 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 calls this type of system a satellite-based augmentation system. Europe and Asia are developing their own SBASs: the Indian GPS aided GEO augmented navigation, the European Geostationary Navigation Overlay Service, the Japanese Multi-functional Satellite Augmentation System and the Russian System for Differential Corrections and Monitoring, 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.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 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, at least 95% of the time. 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.
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 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.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. The WAAS specification mandates availability as 99.999% 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. These precisely surveyed ground stations monitor and collect information on the GPS signals, then send their data to three wide-area master stations 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, seven in Alaska, one in Hawaii, one in Puerto Rico, five in Mexico, and four in Canada.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 across the WAAS service area.
Once these correction messages are generated, the WMSs send them to two pairs of ground uplink stations, which then transmit to satellites in the space segment for rebroadcast to the user segment.
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.Satellite history
The original two WAAS satellites, named Pacific Ocean Region and Atlantic Ocean Region-West, 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 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.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. The test signal was not usable for navigation, but could be received and was reported with the identification numbers PRN 133. In November 2010, the signal was certified as operational and made available for navigation. Following in orbit testing, Eutelsat 117 West B, broadcasting signal on PRN 131, 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. 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 | NMEA | Designator | Location | Active period | Status | Signal capability |
| 122 | 35 | AORW | 54°W, later moved to 142°W | July 10, 2003 – July 31, 2017 | Ceased operational WAAS transmissions on July 31, 2017 | L1 narrowband | |
| 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 |
| Anik F1R | 138 | 51 | CRE | 107.3°W | July 2007 – May 17, 2022 | Ceased operational WAAS transmissions on May 17, 2022. | L1, L5 |
| Inmarsat-4 F3 | 133 | 46 | AMR | 98°W | November 2010 – November 9, 2017 | Ceased operational WAAS transmissions as of November 9, 2017. | L1 narrowband, L5 |
| Eutelsat 117 West B | 131 | 44 | SM9 | 117°W | March 2018 – present | Operational | L1, L5 |
| SES 15 | 133 | 46 | S15 | 129°W | July 15, 2019 – present | Operational | L1, L5 |
| Galaxy 30 | 135 | 48 | G30 | 125°W | April 26, 2022 – present | Operational | L1, L5 |
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.
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 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.