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Magellan Meridian GPS Receiver 91
Appendix
Your Magellan GPS receiver has been upgraded to include the latest WAAS
(Wide Area Augmentation System) technology. With WAAS your GPS receiver
can compute position locations with an accuracy never before possible in a
commercial receiver.
What is WAAS? WAAS is a means of improving the accuracy of GPS beyond the
standard civilian signal. WAAS was developed by the FAA to provide the
accuracy needed for aircraft to perform approaches for landings. WAAS improves
the accuracy of GPS by calculating the errors in the GPS signal at several
monitoring stations around the country and then transmitting the error correc-
tions to special WAAS satellites. In turn, these satellites broadcast the error
corrections to WAAS-capable GPS receivers around the world.
WAAS is a free service and, with the small exception of two additional satellites
displayed on the Satellite Status screen, it will be transparent to you. There is
nothing you need do, it’s all ready to go.
How accurate will my GPS receiver be with WAAS? The FAA reports that
expected accuracy can be improved to around 7 meters vertically and horizon-
tally. Our own testing suggests that Magellan GPS receivers will typically
experience accuracy improvements to around 3 meters. You can expect to see this
level of accuracy for 95% of the time that you are receiving WAAS signals.
What are some of the problems with WAAS? The signal coverage with WAAS
is not global like GPS satellites. In some locations, receiving WAAS signals can
be difficult and intermittent. There are two satellites transmitting WAAS for
corrections in the Americas and two more over Africa and India providing similar
coverage for Europe. These satellites are geostationary, i.e., they stay in the same
spot over the Earth equator. The two satellites for the Americas are over the
Pacific and Atlantic Oceans. Chances are then, they will probably be near the
horizon from where you are using your GPS receiver. This increases the chance
that buildings, foliage, terrain and other obstacles could block the WAAS signal.
Also WAAS is a relatively new system and is still in its developmental stage. So
on occasion you may experience service outages. Even during these outages, your
GPS receiver will still be working, computing position fixes and helping you get
to where you want to go using the GPS signals. The only difference is you will
not be getting the precision that WAAS was providing. You can expect to see
these outages diminish as the system matures and stabilizes.
For more information on WAAS, you can visit the FAA website’s section on their
WAAS system: http://gps.faa.gov/programs/WAAS/waas.htm