Garmin eTrex Legend Cx - Manual (Page 87)

eTrex Venture Cx and Legend Cx Owner’s Manual 77
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WAAS/EGNOS?
What Is WAAS/EGNOS?
The Wide Area Augmentation System (WAAS) is an
FAA funded service to improve the overall integrity
of the GPS signal for users in North America. The
European Geostationary Navigation Overlay Service
(EGNOS) is the European version of WAAS and
operates in the same manner.
WAAS is made up of satellites and ground reference
stations positioned across the United States that
monitor GPS satellite data. Master stations, located on
either coast, collect data from the reference stations
and create a GPS data correction message. Master
station uplinks the correction message to the WAAS
GEO satellites using a ground uplink station. Then
WAAS GEO satellites broadcast the WAAS corrected
signal to user in the service area.
According to the FAA’s Web site, testing of WAAS in
September 2002 confirmed an accuracy performance
of 1-2 meters horizontal and 2-3 meters vertical
throughout the majority of the continental United
States and portions of Alaska.
WAAS and EGNOS are just two service providers
that adhere to the MOPS (Minimum Operational
Performance Standard) for global satellite based
augmentation systems (SBAS.) Eventually there
are to be several more services of worldwide
geostationary communication satellites that broadcast
integrity information and differential correction data
as transmitted by ground reference stations.
All SBAS systems use the same receiver frequency;
therefore, any operational SBAS system should be
capable of providing your GPS unit with increased
accuracy at any location in the world.
Currently, enabling WAAS on your eTrex in regions
that are not supported by ground stations, may not
improve accuracy, even when receiving signals from
an SBAS satellite. In fact, it can degrade the accuracy
to less than that provided by GPS satellites alone.
For this reason, when you enable WAAS on your
Garmin GPS receiver, the receiver automatically
uses the method that achieves the best accuracy. For
more information, go to
http://gps.faa.gov/Programs/WAAS/waas.htm.