WAAS stands for Wide
Area Augmentation System. It is an FAA
enhancement to the Global Positioning System
allowing for much higher accuracy in
GPS uses a
constellation of 24 orbiting satellites.
WAAS monitors these satellites with a
series of ground stations in North America.
Ground processing calculates very high
quality updates to their timing. These
updates are relayed to WAAS-enabled
receivers via two satellites in
geosynchronous (stationary) orbit.
Because of the
increased accuracy, WAAS-enhanced GPS
receivers may be used for all phases of
flight. Additionally they can be used for
precision approaches in much the same
fashion that the radio-based ILS is used.
WAAS has been designed and is being built to
provide performance comparable to Category 1
To summarize, WAAS
provides several advantages. First, the
geosynchronous satellites provide additional
ranging signals into the WAAS enabled
receiver, increasing GPS system coverage and
availability. Since WAAS monitors and
corrects variations in the GPS positioning,
the system is much more accurate with
smaller alert limits. This smaller integrity
limit supports the current generation of GPS
approaches, Localizer Performance with
Vertical guidance (LPV). Another advantage
is that it allows WAAS-equipped users to
flight plan and file for alternate airfields
with GPS-based approaches.