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radar vectors to the intermediate segment;
11. Indication of the actions required for RAIM
failure both before and after the FAWP; and
12. Programming a radial and distance from a
VOR (often used in departure instructions).
1−1−20. Wide Area Augmentation System
(WAAS)
a. General
1. The FAA developed the Wide Area Augmentation
System (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 Category I precision
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 Organization
(ICAO) has defined Standards and Recommended
Practices (SARPs) for satellite−based
augmentation systems (SBAS) such as WAAS. Japan
and Europe are building similar systems that are
AIM 8/5/04
1−1−40 Air Navigation Aids
planned to be interoperable with WAAS: EGNOS,
the European Geostationary Navigation Overlay
System, and MSAS, the Japan Multifunctional
Transport Satellite (MTSAT) Satellite−based Augmentation
System. The merging of these systems will
create a worldwide seamless 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 ground 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 satellite (GEO) via a
ground uplink station (GUS). 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
guidance).
5. The FAA has completed installation of
25 WRSs, 2 WMSs, 4 GUSs, and the required
terrestrial communications to support the WAAS
network. Prior to the commissioning of the WAAS for
public use, the FAA has been conducting a series of
test and validation activities. Enhancements to the
initial phase of WAAS will include additional master
and reference stations, communication satellites, and
transmission frequencies as needed.
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 new class of approach procedures which
provide vertical guidance, but which do not meet the
ICAO Annex 10 requirements for precision
approaches has been developed to support satellite
navigation use for aviation applications worldwide.
These new procedures called Approach with Vertical
Guidance (APV), are defined in ICAO Annex 6, and
include approaches such as the LNAV/VNAV
procedures presently being flown with barometric
vertical navigation (Baro−VNAV). 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 these
LNAV/VNAV procedures using a WAAS electronic
glide path, which eliminates the errors that can be
introduced by using Barometric altimetery.
2. A new type of APV approach procedure, in
addition to LNAV/VNAV, is being implemented to
take advantage of the lateral precision provided by
WAAS. This lateral precision, combined with an
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