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AOPA applauds decision to terminate GPS selective availability, predicts safety benefits from improved signal accuracyAOPA applauds decision to terminate GPS selective availability, predicts safety benefits from improved signal accuracy

AOPA applauds decision to terminate GPS selective availability, predicts safety benefits from improved signal accuracy

The Aircraft Owners and Pilots Association, the world's largest civil aviation organization, is applauding the decision to stop degrading the accuracy of the worldwide GPS satellite navigation system.

"GPS is now in use across a wide spectrum of civilian activities, from aircraft navigation to surveying to moving maps in cars," said AOPA President Phil Boyer. "It just doesn't make sense for the military to intentionally scramble the signal while other federal agencies and commercial companies are simultaneously improving accuracy with differential GPS systems."

The White House announced that the Department of Defense will turn off selective availability (SA) at 8 p.m. Eastern Daylight Time on May 1. SA is a "dithering signal" that intentionally degrades the accuracy of the Global Position System (GPS) for civilian users. With SA turned off, guaranteed GPS position accuracy improves from about 300 feet to better than 100 feet.

"GPS without SA could yield important aviation safety benefits, including better position information to help reduce runway incursions," said Boyer.

Pilot disorientation is a factor in some runway incursion incidents. Moving maps showing taxiways and runways are becoming more common in general aviation cockpits. GPS without SA will allow pilots to more accurately determine their position on the airport and, therefore, be less likely to taxi inadvertently onto the wrong runway.

New GPS-based terrain avoidance systems are now available for general aviation. But because of SA-induced inaccuracies, these systems are prone to false alarms when a pilot is approaching an airport and close to the ground. Consequently, pilots turn off the alert function to avoid unnecessary distractions, defeating the purpose of the system.

"Improved accuracy from SA-free GPS would reduce these false alarms," said Boyer. "However, to realize the highest level of both operational and safety benefits, we still need to enhance GPS with differential correction systems like WAAS."

WAAS (the Wide Area Augmentation System) is a Federal Aviation Administration system that will monitor GPS and broadcast a correction signal that will provide guaranteed position accuracy to within 20 feet. WAAS will provide vertically guided instrument approaches at most of the nation's 5,300 public-use airports, providing all-weather access.

Most importantly, WAAS will send an immediate warning to a pilot when the system cannot be used safely for instrument approaches. This so-called integrity function is not available from GPS alone, even with SA turned off. (For more information, see the AOPA Air Safety Foundation publication GPS Technology..)

AOPA was among the first to advocate GPS for civilian navigation. The association's pioneering 1990 report to Congress, "The Future Is Now," helped open GPS to universal use. AOPA has also been a longtime advocate of removing selective availability.

The 360,000-member Aircraft Owners and Pilots Association is the world's largest civil aviation organization. More than one half of the nation's pilots are AOPA members.

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May 1, 2000

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