Not a member? Join today. Already a member? Please login for an enhanced experience. Login Now

Air Traffic Services Brief -- Wide Area Augmentation System (WAAS)Air Traffic Services Brief -- Wide Area Augmentation System (WAAS)

Air Traffic Services Brief: Wide Area Augmentation System (WAAS)

The Issue

The Wide Area Augmentation System (WAAS) is a navigation system composed of satellites and ground stations that improve the quality of the Global Positioning System (GPS). Although the WAAS signal is approved for instrument flight, and WAAS avionics are available, additional work is required before the full benefits of WAAS are realized by general aviation.

The Importance to Our Members

With WAAS on board the aircraft, pilots are authorized to fly throughout the United States under instrument flight rules (IFR) without reliance on ground-based navigation aids. Capable of supporting all phases of flight including precision instrument approaches, WAAS is a cost-effective navigation system that general aviation pilots can use to improve safety as well as increased access to airports in all weather conditions.


The Federal Aviation Administration (FAA) began developing WAAS in 1995. A 1998 study by Johns Hopkins University Applied Physics Laboratory concluded that WAAS would allow pilots to rely on GPS as a sole means of navigation. Also, an independent review board (IRB) administered by the Institute for Defense Analyses concluded in a January 2001 study that WAAS is critical to both aviation and other users. The FAA authorized pilots to use WAAS for IFR operations in July 2003. In September 2003, the first WAAS approaches were published with minimums as low as 250 feet above the airport.

AOPA's Position

AOPA has been a strong advocate of WAAS since the program's inception. It is AOPA's position that WAAS benefits will far outweigh the costs of the system's development, offering cost savings for both users and the FAA in the future. However, AOPA maintains that the FAA must remain aggressively focused on delivering the benefits that the AOPA members desire: precision approaches to every airport.

How WAAS Works

There are 24 GPS receivers throughout the United States, all networked into the WAAS system. The extremely accurate receivers evaluate the quality of the GPS signal and pass that information on to two master stations. They receive the information and determine what differential GPS information is needed to improve the quality of GPS to precision navigation quality.

The master stations then transmit the correction data through a ground transmitter up to geostationary satellites that "hover" over the United States. These satellites broadcast the GPS correction signal, which is received by a WAAS-capable satellite navigation receiver.

The WAAS receiver uses the WAAS signal to calculate the improved accuracy and integrity information, ultimately improving its known GPS position. Simultaneously, the receiver uses WAAS to ensure that the pilot will not be receiving false or misleading navigation information.

The Benefits of WAAS

Given its precision approach capability, access to airports will increase. The FAA is publishing WAAS LPV (localizer performance with vertical guidance) approaches to general aviation airports. They are frequently providing minimums of less than 300 feet and ¾ mile. The LPV approaches provide unprecedented access to general aviation airports, at a fraction of the cost of traditional instrument landing system (ILS) approaches. Click here for a list of WAAS approaches.

WAAS also supports "pseudo glideslope" capabilities to every runway served by a nonprecision GPS approach. The WAAS avionics system generates a virtual glidepath that the aircraft's navigation system presents to the pilot. The pilot follows the glidepath, reducing workload and eliminating the need to level off at intermediate step-down points along the final approach. There are over 3,000 straight-in GPS and RNAV (GPS) approaches published with straight-in minimums. Glideslope information is provided for each of these approaches.

WAAS promotes smart aviation policy. Because there are no ground navigation systems (e.g., ILS) to purchase or maintain, the cost of installing a WAAS approach is less than 10 percent of an ILS. And while the annual ILS maintenance cost can be as high as $85,000, the cost to maintain a WAAS approach is less than $3,000 every two years.

Because WAAS is permitted as a sole-means navigation system, general aviation reliance on very high frequency omnirange (VOR) for instrument flight is reduced. Over the next decade, the use of VOR will continue to decline, and the role of VOR will increasingly become an optional en route navigation backup.

WAAS Avionics

Currently, there are two manufacturers who use the WAAS navigation aid for instrument navigation.

  1. Chelton Flight System's Flight Logic uses WAAS navigation for lateral guidance and barometric technology for vertical guidance. Chelton reports that an upgrade to full LPV capability is expected in the near future.
  2. Garmin manufacturers the GNS 480 navigation system, which is authorized for WAAS LPV approaches. Garmin also plans to offer an upgrade to its popular GNS-400/500 line of navigation systems sometime in the third quarter of 2006.

Next Steps For WAAS

AOPA continues to advocate for WAAS LPV approach proliferation. The FAA needs to generate comprehensive strategy that results in LPV approaches at every suitable runway. This may require the FAA to modify existing policy in several different areas:

  • Survey data needs to be collected for thousands of runway ends. Besides the strong relationship with the National Oceanic and Atmospheric Administration (NOAA) National Geological Survey, the FAA should develop methods to permit survey data from approved surveyors to be used for WAAS LPV approach procedure development.
  • Airport design standards need to be flexible to ensure that WAAS can safely provide the best all-weather instrument access. Currently, airport standards require extremely large runway protection zones, designed for transport-size aircraft. In addition, a parallel taxiway is required if the LPV provides ¾-mile visibility access or lower.
  • The FAA should seek outside assistance in developing the WAAS approaches. Although the FAA may need to evaluate the work of the procedure development contractors, the FAA should become managers of the procedure production, instead of the producers themselves.


On July 10, 2003, the WAAS signal was activated for general aviation, covering 95 percent of the United States and portions of Alaska offering 350-foot minimums.

Simultaneously, the Chelton Flight Logic glass-cockpit system began using WAAS for lateral navigation guidance.

In September 2003, the FAA first published precision WAAS LPV approaches at seven airports. The number of LPV approaches has now grown to 292.

In September 2004, Garmin certified the first IFR approach-approved WAAS navigation system, the Garmin GNS-480. The GNS-480 is capable of providing precision approach guidance from LPV approaches.

In early April 2010, one of two satellites providing Wide Area Augmentation System (WAAS) coverage experienced technical issues and will soon discontinue its broadcast. For more information, follow this link.

Updated Friday, April 23, 2010 11:31:35 AM