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|Editors=GMV
|Authors=GMV.
|Level=Basic
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|YearOfPublication=2011
|YearOfPublication=2011
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The Wide Area Augmentation System ([[WAAS General Introduction|WAAS]]) is the United States [[SBAS General Introduction|Satellite Based Augmentation System]]. The programme, started in 1992, is being carried out by the [http://www.faa.gov/ Federal Aviation Agency (FAA)]<ref name="FAA_NAV_HISTORY">[http://www.faa.gov Navigation Services - History - Satellite Navigation,] [http://www.faa.gov/ FAA.]</ref> and is specially developed for the civil aviation community.<ref name="FAA_WAAS">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/ Navigation Services - Wide Area Augmentation System (WAAS)], [http://www.faa.gov/ FAA.]</ref> The system, which was declared operational in late 2003,<ref name="STANFORD_WAAS">[https://gps.stanford.edu/research/currentcontinuing-research/waas-sbas Wide Area Differential GPS (WADGPS), Stanford University]</ref> currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.<ref name="APPROACHES">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches/index.cfm GNSS - GPS/WAAS Approaches,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref> WAAS service area includes CONUS, Alaska, Canada and Mexico.<ref name="WAASExpanded">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/ WAAS Service Expanded into Canada and Mexico, September 28, 2007,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref> The WAAS programme is continuously in evolution; three development phases have been already covered, and there are on-going plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.<ref name="EXTENSION">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNav_March08.pdf SatNav News, Vol. 33, March 2008,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref>


The Wide Area Augmentation System (WAAS) is the United States [[SBAS General Introduction|Satellite Based Augmentation System]]. The programme, started in 1992, is being carried out by the [http://www.faa.gov/ Federal Aviation Agency (FAA)]<ref name="FAA_NAV_HISTORY">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/history/satnav/index.cfm Navigation Services - History - Satellite Navigation,] [http://www.faa.gov/ FAA.]</ref> and is specially developed for the civil aviation community.<ref name="FAA_WAAS">[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/ Navigation Services - Wide Area Augmentation System (WAAS)], [http://www.faa.gov/ FAA.]</ref> The system, which was declared operational in late 2003,<ref name="STANFORD_WAAS">[http://waas.stanford.edu/research/waas.htm Wide Area Differential GPS (WADGPS), Stanford University]</ref> currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches/index.cfm GNSS - GPS/WAAS Approaches,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref> WAAS service area includes CONUS, Alaska, Canada and Mexico.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/ WAAS Service Expanded into Canada and Mexico, September 28, 2007,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref> The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/library/satnav/media/SatNav_March08.pdf SatNav News, Vol. 33, March 2008,] [http://www.faa.gov/ Federal Aviation Agency (FAA).]</ref>
==WAAS Messages==


==WAAS Messages==
Every satellite-based wide area augmentation system ([[SBAS General Introduction|SBAS]]), as the Wide Area Augmentation System [[WAAS General Introduction|WAAS]], does provide ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:
* GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
* Wide Area Differential (WAD): differential corrections to the existing GPS and GEO navigation services computed in a wide area to improve navigation services performance.
* GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS and GEO safe navigation service.


The WAAS Messages are mainly divided in two types: fast and slow. In the WAAS receiver, the two types of WAAS 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 name=SpecWAAS> 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)'']. FAA-E- 2892b. August 13, 2001.</ref>
WAAS provides the following information:
* Satellite orbit and clock corrections to the existing satellite navigation services (GPS and GEO), as well as the estimation of errors associated to satellites (UDRE).
* Ionospheric corrections for a given grid of points, as well as the estimation of errors associated to ionosphere (GIVE).
* Tropospheric corrections. Satellite orbit/clock corrections and ionospheric corrections are dynamically modelled. The SBAS shall communicate the user the corrections that are available to be used by the receiver. The information of the models is packed on messages to be sent to the user.  On the other hand, tropospheric corrections are statically modelled, which means that corrections are tabulated and the information does not depend on any external behaviour but the user position (a mean troposphere is assumed). The algorithm for computing the tropospheric correction is available to the global community (section A.4.2.4 of MOPS).<ref name="MOPS">[http://www.rtca.org RTCA] MOPS DO-229, ''Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System airborne equipment''</ref>
In addition to this, navigation data for each GEO satellite supporting ranging service is also transmitted through SBAS.
SBAS interacts with the user via the Signal in Space (SiS). The way the SBAS delivers to the user the aforementioned corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) is through messages encoded in the signal. These messages are sent each second with a data rate of 250 bits.


For a detailed description of SBAS Messages, see article [[EGNOS Messages]]. This articles follows the standard described in RTCA MOPS DO-229-C “''Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System airborne equipment''” (particularly in its Appendix A “''Signal characteristics and format''”), applicable for every Satellite Based Augmentation Systems (SBAS).  In the article, SBAS Message specification is also compliant with the ICAO SARPs “Standards and Recommended Practices”, Appendix B “Detailed technical specifications for the Global Navigation Satellite System (GNSS)”. These two standards are nearly aligned in what regards to the SBAS SiS specification, but the augmentation of GLONASS or other GNSS constellations except for GPS is not specified in MOPS, being MOPS the standard that follows WAAS, as WAAS does not provides augmentation to GLONASS satellites.
For a detailed description of SBAS Messages, see article [[EGNOS Messages]]; this article is fully applicable to WAAS, taking into account the following differences:
* WAAS provides GEO ranging.<ref group="footnotes">EGNOS GEO ranging is planned to be provided in the near future.</ref>
* WAAS augments GPS and GEO satellites.
* The standard gives to the SBAS system the possibility to use Message Type 27 (''SBAS Service Message'') or Message Type 28 (''Clock-Ephemeris Covariance Matrix Message''), but not both, as a way to control a potential degradation of the service in different regions. Whilst EGNOS has chosen to use Message Type 27 for this purpose, WAAS broadcasts Message Type 28.


==Notes==
==Notes==

Latest revision as of 08:45, 16 February 2021


WAASWAAS
Title WAAS Messages
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA)[1] and is specially developed for the civil aviation community.[2] The system, which was declared operational in late 2003,[3] currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada.[4] WAAS service area includes CONUS, Alaska, Canada and Mexico.[5] The WAAS programme is continuously in evolution; three development phases have been already covered, and there are on-going plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.[6]


WAAS Messages

Every satellite-based wide area augmentation system (SBAS), as the Wide Area Augmentation System WAAS, does provide ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

  • GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
  • Wide Area Differential (WAD): differential corrections to the existing GPS and GEO navigation services computed in a wide area to improve navigation services performance.
  • GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS and GEO safe navigation service.

WAAS provides the following information:

  • Satellite orbit and clock corrections to the existing satellite navigation services (GPS and GEO), as well as the estimation of errors associated to satellites (UDRE).
  • Ionospheric corrections for a given grid of points, as well as the estimation of errors associated to ionosphere (GIVE).
  • Tropospheric corrections. Satellite orbit/clock corrections and ionospheric corrections are dynamically modelled. The SBAS shall communicate the user the corrections that are available to be used by the receiver. The information of the models is packed on messages to be sent to the user. On the other hand, tropospheric corrections are statically modelled, which means that corrections are tabulated and the information does not depend on any external behaviour but the user position (a mean troposphere is assumed). The algorithm for computing the tropospheric correction is available to the global community (section A.4.2.4 of MOPS).[7]

In addition to this, navigation data for each GEO satellite supporting ranging service is also transmitted through SBAS. SBAS interacts with the user via the Signal in Space (SiS). The way the SBAS delivers to the user the aforementioned corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) is through messages encoded in the signal. These messages are sent each second with a data rate of 250 bits.

For a detailed description of SBAS Messages, see article EGNOS Messages; this article is fully applicable to WAAS, taking into account the following differences:

  • WAAS provides GEO ranging.[footnotes 1]
  • WAAS augments GPS and GEO satellites.
  • The standard gives to the SBAS system the possibility to use Message Type 27 (SBAS Service Message) or Message Type 28 (Clock-Ephemeris Covariance Matrix Message), but not both, as a way to control a potential degradation of the service in different regions. Whilst EGNOS has chosen to use Message Type 27 for this purpose, WAAS broadcasts Message Type 28.

Notes

  1. ^ EGNOS GEO ranging is planned to be provided in the near future.

References

  1. ^ Navigation Services - History - Satellite Navigation, FAA.
  2. ^ Navigation Services - Wide Area Augmentation System (WAAS), FAA.
  3. ^ Wide Area Differential GPS (WADGPS), Stanford University
  4. ^ GNSS - GPS/WAAS Approaches, Federal Aviation Agency (FAA).
  5. ^ WAAS Service Expanded into Canada and Mexico, September 28, 2007, Federal Aviation Agency (FAA).
  6. ^ SatNav News, Vol. 33, March 2008, Federal Aviation Agency (FAA).
  7. ^ RTCA MOPS DO-229, Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System airborne equipment
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