AIM

4/3/14

1−1−32

Navigation Aids

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 ap-

proaches 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 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. The resulting approach procedure

minima, titled LPV (localizer performance with

vertical guidance), 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).

3.

A new nonprecision WAAS approach, called

Localizer Performance (LP) is being added in

locations where the terrain or obstructions do not

allow publication of vertically guided LPV proced-

ures. This new approach 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

−

WAAS receivers certified prior to TSO

−C145b and

TSO

−C146b, even if they have LPV capability, do not

contain LP capability unless the receiver has been
upgraded. Receivers capable of flying LP procedures must
contain a statement in the Flight Manual Supplement or
Approved Supplemental Flight Manual stating that the
receiver has LP capability, as well as the capability for the
other WAAS and GPS approach procedure types.

4.

WAAS provides a level of service that

supports all phases of flight, including RNAV (GPS)

approaches to LNAV, LP, LNAV/VNAV and LPV

lines of minima, within system coverage. Some