December 17, 2007
New System Makes Bad-Weather Flying Easier for Pilots
installed FAA navigation system called the Wide Area Augmentation System (WAAS)
is making life much easier for general-aviation pilots in the United States, particularly when trying to land at small airports in bad weather.
flying aircraft equipped with the latest Global Positioning System (GPS)
equipment in the continental U.S. and much of Canada and Mexico are now able to pinpoint their location to within 25 feet, thanks to WAAS.
WAAS is a network
of 39 precisely surveyed ground
reference stations located across the U.S. and in parts of Canada and Mexico. The system also involves two master stations, one each on the west and east coasts,
and three data uplink stations.
reference stations in the WAAS network collect GPS satellite data and develop
messages to correct any signal errors, which are caused by ionosphere disturbances
and timing and satellite orbit errors.
messages are broadcast through communication satellites to GPS receivers on
aircraft, ships, and land vehicles equipped with a WAAS-GPS receiver. WAAS also
provides GPS-satellite-integrity information to the U.S. Air Force 50th Space
Wing, which manages the satellite constellation.
Global Positioning System
Positioning System comprises 31 satellites circling the earth at a medium orbit
of approximately 12,660 miles. The satellites transmit signals to GPS receivers
that have become common in aircraft, maritime vessels, and land vehicles. Certain
types of cell phones and watches also have a GPS
Using the data
transmitted via microwave signals from the satellites, GPS receivers determine their
location, speed, and direction, and display the information to users. The satellite
signal also includes the precise time.
GPS receivers provide positional fixes accurate to within approximately 50
feet. In aircraft, such receivers do not provide vertical guidance to pilots
flying instrument approaches.
provides the additional accuracy, integrity, and availability required to
navigate using GPS for all phases of flight, from enroute flying to instrument approaches
at qualified airports within the WAAS coverage area.
this, prominent aviation organizations such as the National
Business Aviation Association and the Aircraft Owners and Pilots
Association (AOPA) have been strong supporters of WAAS.
urged both Congress and the FAA to
press ahead with the program because it improves air safety by providing the
precision vertical guidance needed, especially in poor weather
conditions," said Phil Boyer, president of AOPA, in March 2006. "And
it makes better use of the nation's system of airports, because thousands that
currently may only be used in good weather can become all-weather capable."
precision, vertical guidance
provides lateral precision, vertical guidance (LPV) down to 200 feet above
runways using developed LPV instrument approaches. The advantage of WAAS to
pilots is that they can fly an instrument approach in poor weather at many more
airports than before, particularly at smaller, general aviation airports. As a
result, WAAS improves the efficiency and capacity of the U.S. National Airspace
some of the satellites that receive WAAS correction signals are over the equator,
GPS-WAAS users in North America who are in mountainous areas may experience
difficulty receiving the WAAS signal. Until more ground reference stations are
installed, signal reception is best in open land and marine environments.
The FAA and
Department of Transportation began developing WAAS in 1994. The system has been
available for visual flight rules (VFR) and recreational use since August 2001.
In July 2003, WAAS was approved for non-precision aircraft instrument
operations, and in March 2006, LPV approaches became available. The WAAS
project is expected to be completed in 2013, with a final price tag of $3.3
produces cost savings for FAA
It costs about
$50,000 to map and publish a new, WAAS-GPS LPV approach. By comparison, a
Category 1 instrument landing system (ILS), which is comprised of electronic
equipment installed at an airport that guides aircraft horizontally and vertically
down to 200 feet above a runway, costs from $1 million to $1.5 million per
savings are another advantage of WAAS, which allows redundant ground-based
navigation aids to be decommissioned. WAAS also provides a back-up capability in
the event of an ILS outage.
of other nations are developing navigation systems similar to WAAS. Japan is working on a Multi-Functional Satellite Augmentation System (MSAS), while Europe is
developing the European Geostationary Navigation Overlay Service (EGNOS).
One day, GPS-equipped
pilots anywhere in the world will be able to determine their positions precisely
and navigate using WAAS, MSAS, EGNOS, and other compatible systems.
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