Editing Wide Area Augmentation System (section)

==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" />