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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. 


The International Civil Aviation Organiza-

tion (ICAO) has defined Standards and 
Recommended Practices (SARPs) for satellite


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


Navigation (GAGAN) system. The merging of these 
systems will create an expansive navigation 
capability similar to GPS, but with greater accuracy, 
availability, and integrity. 


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. 


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 


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. 


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 


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. 


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 



mile, when the terrain and airport infrastructure 
support the lowest minima. LPV minima is published 
on the RNAV (GPS) approach charts (see Paragraph 


5, Instrument Approach Procedure Charts). 


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. 

Navigation Aids