A major goal of the International
Civil Aviation Organization (ICAO) is near-universal
use of a Global Navigation Satellite System
(GNSS). ICAO supports augmenting GNSS to provide
for increased civil aviation safety and capacity.
The FAA’s Wide Area Augmentation System
(WAAS) has become a critical component of
seamless satellite navigation system for civil
aviation. This system improves the accuracy,
availability, and integrity of GPS, thereby
improving the safety and capacity provided
by the U.S. National Airspace System (NAS).
Ultimately, WAAS will allow GPS to be used
as a primary means of navigation from takeoff
through Category I precision approach.
The WAAS signal-in-space
(SIS) will cover the entire Continental U.S.
and Alaska. This single system provides service
to most of the NAS through all phases of flight,
thus providing much of the same capability
that currently needs hundreds of ground-based
navaids (GBNA). The WAAS provides augmentation
information to GPS/WAAS receivers to enhance
the accuracy and reliability of GPS position
estimates. The signals from GPS satellites
are received across the NAS at many widely-spaced
wide area reference stations. Each reference
station relays the information, via a terrestrial
communication network, to WAAS wide area master
stations. The master stations use the information
collected by the reference stations to develop
corrections to the GPS position information.
These corrections are sent to a ground uplink
station where they are transmitted in the
form of a WAAS correction message to a Geostationary
Earth Orbit (GEO) satellite. These GEOs broadcast
the WAAS message to users across the U.S.,
and portions of Alaska, on the same frequency
as GPS. The WAAS broadcast message improves
GPS signal accuracy from 20 meters to approximately
1.5 - 2 meters in both the horizontal and
vertical dimensions, allowing more efficient
arrival, enroute, and departure operations
at low cost to an increased number of airports
throughout the U.S. WAAS also provides savings
from simplified cockpit avionics for all classes
of aircraft.
Additionally, WAAS 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.
The FAA commissioned WAAS
at 12:01AM on July 10, 2003. This step moves
the FAA directly to Lateral Navigation/Vertical
Navigation (LNAV/VNAV) and LPV capabilities
using WAAS. Both LNAV/VNAV and LPV approaches
use the accuracy of the WAAS signal to include
vertical (glide path) guidance capability.
This capability facilitates improved instrument
approaches to include vertical (glide path)
guidance to an expanded number of airports
throughout the U.S.
There are nearly 600 LNAV/VNAV
approaches available today at nearly 300 airports
throughout the NAS. The FAA is continuing
to develop these procedures, adding many more
each year.
The implementation of
LPV approaches further improves precision
approach capability to users. LPV approaches
are designed to fully exploit the tighter
satellite signal protection limits from the
WAAS. This approach combines the LNAV/VNAV
vertical accuracy with lateral guidance similar
to the typical Instrument Landing System.
The use of LPV approaches capitalizes on the
inherent accuracy of the WAAS signal and will
result in lower approach minimums. There are
currently seven LPV approach locations in
the U.S., and production will continue until
all qualified airports have an LPV approach
at each runway end.
WAAS is also a critical
component of Required Navigation Performance
(RNP). WAAS can provide the most stringent
RNP for Random Navigation/Area Navigation
(RNAV) to all classes of users throughout
the NAS. As air traffic management becomes
more global, the WAAS concept can be applied
to civil aviation infrastructure worldwide,
enabling global safety improvements. Also,
with more stringent RNP standards, inefficiencies
in airspace utilization could be reduced,
thus increasing traffic flows and avoiding
delays.
Although the WAAS was
designed for aviation users, it supports a
wide variety of non-aviation uses including
agriculture, surveying, recreation, and surface
transportation, just to name a few. The WAAS
signal has been available for non safety-of-life
applications since August 24, 2000, and numerous
manufacturers have developed WAAS-enabled
GPS receivers for the consumer market. Today,
there are millions of non-aviation WAAS-enabled
GPS receivers in use.
The FAA continues to work
toward Final Operational Capability (FOC)
for WAAS to include a full compliment of WAAS
GEOs, which will ensure that each receiver
sees at least two GEO satellites at all times
throughout all of the Continental U.S., and
most of Alaska. The FAA is also working with
Canada and Mexico to expand the WAAS coverage
area to support North American implementation
of WAAS.
The FAA is evaluating
the approach to achieve a Global Navigation
Satellite System (GNSS) Landing System (GLS)
capability in later years. As GPS is modernized,
WAAS will continue to evolve to take full
advantage of GPS modernization, including
the L5 frequency. In fact, the FAA is adding
L5 to the current WAAS GEOs. The FAA is also
involved in ICAO’s GNSS panel, which
supports the development of standards and
procedures for satellite navigation for civil
aviation applications worldwide.
