Navigation performance and the growth of GPS

Wide Area Augmentation System (WAAS)
Wide Area Augmentation System (WAAS) is a product of the FAA to enhance the global positioning system and is accomplished by improving accuracy, integrity, and availability. The intent is to enable aircraft to use GPS for all phases of flight, including precision approaches to landing.

As of March 2008, there are 31 actively broadcasting satellites with those above the original 24 intended to improve the precision of GPS receiver calculations by providing redundant measurements. The increased number of satellites changed the constellation to a non-uniform arrangement. This arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.

The system takes advantage of ground-based reference stations in North America and Hawaii to measure the satellites’ signals and determine position error. Information from the reference stations are routed to master stations, which queue the received deviation correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (at least every five seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.

ICAO calls this type of system a satellite-based augmentation system (SBAS). Europe and Asia are developing their own SBASs — the Indian GPS Aided Geo Augmented Navigation (GAGAN), the European Geostationary Navigation Overlay Service (EGNOS), and the Japanese multi-functional satellite augmentation system (MSAS), respectively. Commercial systems include StarFire and OmniStar.

The WAAS specification requires it to provide a position accuracy of 7.6 meters or better (for both lateral and vertical measurements), at least 95 percent of the time. Actual performance measurements of the system at specific locations show it typically provides better than 1.0 meters laterally and 1.5 meters vertically throughout most of North America. With these results, WAAS is capable of achieving the required Category I precision approach accuracy of 16 meters laterally and 4 meters vertically.

Upgrades needed
Having a stand-alone GPS receiver or even a GPS-equipped flight management system (FMS) does not automatically mean the system is WAAS capable. Most satellite navigation systems manufactured prior to 2007 will require upgrades to utilize maximum system capabilities. These changes can be as simple as a software upgrade or strapping change but may involve installation of a new GPS receiver and possibly a new antenna.

Although the physical alterations or modifications may not sound involved, the certification to satisfy Airworthiness Authorities may be more complicated. In many cases airframe manufacturers will issue service data that can be used as a basis for airworthiness approvals, but trying to upgrade an out of production model may become more of a challenge. Some satellite navigation receivers are self-contained within the navigation display unit and an upgrade may involve returning the device to the manufacturer or simply contacting an authorized representative to conduct the upgrade procedure.

Frequently in larger aircraft, the GPS receiver is an internal component of a FMS. Most changes in operating system software require an aircraft flight manual change or issuance of a supplement. Changes not approved by the original equipment manufacturer may influence the certification basis and knock the modified aircraft out of a “Group Certification Basis” which may potentially affect the aircraft resale value. Supporting a “one off product” can be an additional challenge, especially if the agency issuing the certification happens to go out of business.

WAAS up with that?

Jim Sparks has been in aviation for 30 years and is a licensed A&P. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets.

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