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GNSS Frequently Asked Questions - WAAS

Q. What is WAAS?

A. WAAS is an extremely accurate navigation system developed for civil aviation. Before WAAS, the U.S. National Airspace System (NAS) did not have the potential to provide horizontal and vertical navigation for approach operations for all users at all locations. With WAAS, this capability is a reality.

Q. How does WAAS work?

A. The Wide Area Augmentation System (WAAS) uses a system of ground stations to provide corrections to the Global Positioning System Standard Positioning Service (GPS SPS) navigation signal. A network of precisely surveyed ground-based WAAS wide-area reference stations is strategically positioned across the country including Alaska, Hawaii, Puerto Rico, Canada, and Mexico to collect GPS satellite data. Using this information, a message is developed to correct any signal errors. These correction messages are then broadcast through communication satellites to receivers onboard aircraft using the same frequency as GPS.

For more about how WAAS works, please visit our WAAS - How It Works page.

Q. What are the benefits of WAAS?

A. The WAAS is designed to provide the additional accuracy, integrity, and availability necessary to enable users to rely on GPS for all phases of flight, from en route through approach for all qualified airports within the WAAS coverage area.

WAAS also provides benefits beyond aviation to all modes of transportation, including maritime, highways, and railroads.

For more information, please visit our WAAS - Benefits page.

Q. What is meant by WAAS "integrity" and "availability"?

A. Integrity refers to usability of the satellite signal. Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation as a result of errors or failures in the system. WAAS improves upon the integrity of the basic GPS signal and detects much smaller errors more quickly. Availability refers to the percentage of time in a given period that the signal is expected to be received and usable.

Q. How does WAAS know that the correction it sends is valid to my particular location?

A. WAAS supplies two different sets of corrections:

  • Corrected GPS parameters (position, clock, etc.).
    This set of corrections is user-position independent and they apply to all users located within the WAAS service area.
  • Ionospheric parameters.
    This set of corrections is area-specific. WAAS supplies correction parameters for a number of points (organized in a grid pattern) across the WAAS service area. The user's receiver computes ionospheric corrections for the received GPS signals based on algorithms which use the appropriate grid points for where the user is located. Further, the appropriate grid points may differ for each GPS satellite received and processed by the user's receiver as the GPS satellites are located at various positions in the sky relative to the user.

The combination of the two sets of corrections allows for significantly increased accuracy and confidence of the user's position anywhere in the WAAS service area.

Q. How does WAAS performance compare to the Instrument Landing System (ILS)?

A. WAAS has been designed and built to provide performance comparable to Category I ILS. WAAS provides both vertical and lateral guidance to minimums as low as 200 feet.

Q. Is WAAS required for ADS-B?

A. WAAS is not required for the ADS-B mandate. ADS-B does require a precise positioning source, such as the Global Positioning System (GPS).

GPS supports the basic requirements associated with ADS-B. To gain the full capability of ADS-B, a system that is more precise than GPS is needed, such as the Wide Area Augmentation System (WAAS). WAAS meets the most demanding ADS-B requirements.

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