AIM
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2.
The International Civil Aviation Organization (ICAO) has defined Standards and Recommended
Practices (SARPs) for satellite
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based augmentation systems (SBAS) such as WAAS. India and Europe are
building similar systems: EGNOS, the European Geostationary Navigation Overlay System; and India’s GPS
and Geo
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Augmented Navigation (GAGAN) system. 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
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based navigation aids, WAAS will cover a more extensive service area.
Precisely surveyed wide
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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
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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
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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 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 operations are planned.
6.
GNSS navigation, including GPS and WAAS, is referenced to the WGS
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84 coordinate system. It should
only be used where the Aeronautical Information Publications (including electronic data and aeronautical charts)
conform to WGS
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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 approaches 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
5, Instrument Approach Procedure
Charts).
3.
A different WAAS-based line of minima, called Localizer Performance (LP) is being added in locations
where the terrain or obstructions do not allow publication of vertically guided LPV minima. LP takes advantage
of the angular lateral guidance and smaller position errors provided by WAAS to provide a lateral only procedure
similar to an ILS Localizer. LP procedures may provide lower minima than a LNAV procedure due to the
narrower obstacle clearance surface.
NOTE
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WAAS receivers certified prior to TSO
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C145b and TSO
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C146b, even if they have LPV capability, do not contain LP
Navigation Aids
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