Aeronautical Information Manual

3/21/24

AIM

cases this operation will be invisible to the pilot since the receiver will continue to operate with other available

satellites after excluding the “bad” signal. This capability increases the reliability of navigation.

Both lateral and vertical scaling for the LNAV/VNAV and LPV approach procedures are different than

the linear scaling of basic GPS. When the complete published procedure is flown,

1 NM linear scaling is

provided until two (2) NM prior to the FAF, where the sensitivity increases to be similar to the angular scaling

of an ILS. There are two differences in the WAAS scaling and ILS: 1) on long final approach segments, the initial

scaling will be

0.3 NM to achieve equivalent performance to GPS (and better than ILS, which is less sensitive

far from the runway); 2) close to the runway threshold, the scaling changes to linear instead of continuing to

become more sensitive. The width of the final approach course is tailored so that the total width is usually 700

feet at the runway threshold. Since the origin point of the lateral splay for the angular portion of the final is not

fixed due to antenna placement like localizer, the splay angle can remain fixed, making a consistent width of final

for aircraft being vectored onto the final approach course on different length runways. When the complete

published procedure is not flown, and instead the aircraft needs to capture the extended final approach course

similar to ILS, the vector to final (VTF) mode is used. Under VTF, the scaling is linear at

±1

NM until the point

where the ILS angular splay reaches a width of

1 NM regardless of the distance from the FAWP.

The WAAS scaling is also different than GPS TSO

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C129() in the initial portion of the missed approach.

Two differences occur here. First, the scaling abruptly changes from the approach scaling to the missed approach

scaling, at approximately the departure end of the runway or when the pilot selects missed approach guidance

rather than ramping as GPS does. Second, when the first leg of the missed approach is a Track to Fix (TF) leg

aligned within 3 degrees of the inbound course, the receiver will change to 0.3 NM linear sensitivity until the

turn initiation point for the first waypoint in the missed approach procedure, at which time it will abruptly change

to terminal (

1 NM) sensitivity. This allows the elimination of close in obstacles in the early part of the missed

approach that may otherwise cause the DA to be raised.

There are two ways to select the final approach segment of an instrument approach. Most receivers use

menus where the pilot selects the airport, the runway, the specific approach procedure and finally the IAF, there

is also a channel number selection method. The pilot enters a unique 5

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digit number provided on the approach

chart, and the receiver recalls the matching final approach segment from the aircraft database. A list of

information including the available IAFs is displayed and the pilot selects the appropriate IAF. The pilot should

confirm that the correct final approach segment was loaded by cross checking the Approach ID, which is also

provided on the approach chart.

The Along

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Track Distance (ATD) during the final approach segment of an LNAV procedure (with a

minimum descent altitude) will be to the MAWP. On LNAV/VNAV and LPV approaches to a decision altitude,

there is no missed approach waypoint so the along

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track distance is displayed to a point normally located at the

runway threshold. In most cases, the MAWP for the LNAV approach is located on the runway threshold at the

centerline, so these distances will be the same. This distance will always vary slightly from any ILS DME that

may be present, since the ILS DME is located further down the runway. Initiation of the missed approach on the

LNAV/VNAV and LPV approaches is still based on reaching the decision altitude without any of the items listed

in 14 CFR Section 91.175 being visible, and must not be delayed while waiting for the ATD to reach zero. The

WAAS receiver, unlike a GPS receiver, will automatically sequence past the MAWP if the missed approach

procedure has been designed for RNAV. The pilot may also select missed approach prior to the MAWP; however,

navigation will continue to the MAWP prior to waypoint sequencing taking place.

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19. Ground Based Augmentation System (GBAS) Landing System (GLS)

A GBAS ground installation at an airport can provide localized, differential augmentation to the Global

Positioning System (GPS) signal

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space enabling an aircraft’s GLS precision approach capability. Through

the GBAS service and the aircraft’s GLS installation a pilot may complete an instrument approach offering

three

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dimensional angular, lateral, and vertical guidance for exact alignment and descent to a runway. The

operational benefits of a GLS approach are similar to the benefits of an ILS or LPV approach operation.

Navigation Aids

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