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AIM

4/20/23

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39

Navigation Aids

6.

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

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.

7.

The Along

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

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.

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

Positioning System (GPS) signal

in

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

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.

NOTE

To remain consistent with international terminology, the FAA will use the term GBAS in place of the former term Local Area
Augmentation System (LAAS).

b.

An aircraft’s GLS approach capability relies on the broadcast from a GBAS Ground Facility (GGF)

installation. The GGF installation includes at least four ground reference stations near the airport’s runway(s),
a corrections processor, and a VHF Data Broadcast (VDB) uplink antenna. To use the GBAS GGF output and
be eligible to conduct a GLS approach, the aircraft requires eligibility to conduct RNP approach (RNP APCH)
operations and must meet the additional, specific airworthiness requirements for installation of a GBAS receiver
intended to support GLS approach operations. When the aircraft achieves GLS approach eligibility, the aircraft’s
onboard navigation database may then contain published GLS instrument approach procedures.

c.

During a GLS instrument approach procedure, the installation of an aircraft’s GLS capability provides the

pilot three

dimensional (3D) lateral and vertical navigation guidance much like an ILS instrument approach.

GBAS corrections augment the GPS signal

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space by offering position corrections, ensures the availability

of enhanced integrity parameters, and then transmits the actual approach path definition over the VDB uplink
antenna. A single GBAS ground station can support multiple GLS approaches to one or more runways.

d.

Through the GBAS ground station, a GLS approach offers a unique operational service volume distinct

from the traditional ILS approach service volume (see FIG 1

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9). However, despite the unique service volume,

in the final approach segment, a GLS approach provides precise 3D angular lateral and vertical guidance
mimicking the precision guidance of an ILS approach.

e.

Transitions to and segments of the published GLS instrument approach procedures may rely on use of

RNAV 1 or RNP 1 prior to an IAF. Then, during the approach procedure, prior to the aircraft entering the GLS
approach mode, a GLS approach procedure design uses the RNP APCH procedure design criteria to construct
the procedural path (the criteria used to publish procedures titled “RNAV (GPS)” in the US). Thus, a GLS
approach procedure may include paths requiring turns after the aircraft crosses the IAF, prior to the aircraft’s