Unlike traditional ground-based
navigation aids, the WAAS will cover nearly all of
the National Airspace System (NAS). The WAAS provides
augmentation information to GPS receivers to enhance
the accuracy and reliability of position estimates.
The signals
from GPS satellites are received across the NAS at
many widely-spaced Wide Area Reference Stations (WRS)
sites. The WRS locations are precisely surveyed so
that any errors in the received GPS signals can be
detected.
The GPS information collected
by the WRS sites is forwarded to the WAAS Master Station
(WMS) via a terrestrial communications network. At
the WMS, the WAAS augmentation messages are generated.
These messages contain information that allows GPS
receivers to remove errors in the GPS signal, allowing
for a significant increase in location accuracy and
reliability.
The augmentation messages are
sent from the WMS to uplink stations to be transmitted
to navigation payloads on Geostationary communications
satellites.
The navigation payloads broadcast
the augmentation messages on a GPS-like signal. The
GPS/WAAS receiver processes the WAAS augmentation
message as part of estimating position. The GPS-like
signal from the navigation transponder can also be
used by the receiver as an additional source for calculation
of the user’s position.
WAAS also provides indications
to GPS/WAAS receivers of where the GPS system is unusable
due to system errors or other effects. Further, the
WAAS system was designed to the strictest of safety
standards – users are notified within six seconds
of any issuance of hazardously misleading information
that would cause an error in the GPS position estimate.
