Aeronautical Information Manual (AIM), page 82

Index   81 -- Page 82 -- 83

1-1-18. Wide Area Augmentation System

a. General
1. The FAA developed the WAAS to improve
the accuracy, integrity and availability of GPS
signals. WAAS will allow GPS to be used, as the
aviation navigation system, from takeoff through
approach when it is complete. WAAS is a critical
component of the FAA's strategic objective for a
seamless satellite navigation system for civil
aviation, improving capacity and safety.
2. The International Civil Aviation Organiza-
tion (ICAO) has defined Standards and
Recommended Practices (SARPs) for satellite-based
augmentation systems (SBAS) such as WAAS.
Japan, India, and Europe are building similar
systems: EGNOS, the European Geostationary
Navigation Overlay System; India's GPS and
Geo-Augmented Navigation (GAGAN) system; and
Japan's Multi-functional Transport Satellite (MT-
SAT)-based Satellite Augmentation System
(MSAS). The merging of these systems will create an
expansive navigation capability similar to GPS, but
with greater accuracy, availability, and integrity.

3. Unlike traditional ground-based navigation
aids, WAAS will cover a more extensive service area.
Precisely surveyed wide-area reference stations
(WRS) are linked to form the U.S. WAAS network.
Signals from the GPS satellites are monitored by
these WRSs to determine satellite clock and
ephemeris corrections and to model the propagation
effects of the ionosphere. Each station in the network
relays the data to a wide-area master station (WMS)
where the correction information is computed. A
correction message is prepared and uplinked to a
geostationary earth orbit satellite (GEO) via a GEO
uplink subsystem (GUS) which is located at the
ground earth station (GES). The message is then
broadcast on the same frequency as GPS (L1,
1575.42 MHz) to WAAS receivers within the
broadcast coverage area of the WAAS GEO.

4. In addition to providing the correction signal,
the WAAS GEO provides an additional pseudorange
measurement to the aircraft receiver, improving the
availability of GPS by providing, in effect, an
additional GPS satellite in view. The integrity of GPS
is improved through real-time monitoring, and the
accuracy is improved by providing differential

corrections to reduce errors. The performance
improvement is sufficient to enable approach
procedures with GPS/WAAS glide paths (vertical
5. The FAA has completed installation of 3
GEO satellite links, 38 WRSs, 3 WMSs, 6 GES, and
the required terrestrial communications to support
the WAAS network including 2 operational control
centers. Prior to the commissioning of the WAAS for
public use, the FAA conducted a series of test and
validation activities. Future dual frequency opera-
tions are planned.
6. GNSS navigation, including GPS and
WAAS, is referenced to the WGS-84 coordinate
system. It should only be used where the Aeronautical
Information Publications (including electronic data
and aeronautical charts) conform to WGS-84 or
equivalent. Other countries' civil aviation authorities
may impose additional limitations on the use of their
SBAS systems.

b. Instrument Approach Capabilities

1. A class of approach procedures which
provide vertical guidance, but which do not meet the
ICAO Annex 10 requirements for precision ap-
proaches has been developed to support satellite
navigation use for aviation applications worldwide.
These procedures are not precision and are referred to
as Approach with Vertical Guidance (APV), are
defined in ICAO Annex 6, and include approaches
such as the LNAV/VNAV and localizer performance
with vertical guidance (LPV). These approaches
provide vertical guidance, but do not meet the more
stringent standards of a precision approach. Properly
certified WAAS receivers will be able to fly to LPV
minima and LNAV/VNAV minima, using a WAAS
electronic glide path, which eliminates the errors that
can be introduced by using Barometric altimetry.

2. LPV minima takes advantage of the high
accuracy guidance and increased integrity provided
by WAAS. This WAAS generated angular guidance
allows the use of the same TERPS approach criteria
used for ILS approaches. LPV minima may have a
decision altitude as low as 200 feet height above
touchdown with visibility minimums as low as 1/2
mile, when the terrain and airport infrastructure
support the lowest minima. LPV minima is published
on the RNAV (GPS) approach charts (see Paragraph
5-4-5, Instrument Approach Procedure Charts).

1-1-28 Navigation Aids

Page 82 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   81 -- Page 82 -- 83