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6/17/21 

AIM 

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. 

5. 

The WAAS scaling is also different than GPS 

TSO

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. 

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.  General 

1. 

The GLS provides precision navigation 

guidance for exact alignment and descent of aircraft 
on approach to a runway. GBAS equipment provides 
localized differential augmentation to the Global 
Positioning System (GPS). 

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). 

2. 

GLS displays three

dimension vertical and 

horizontal navigation guidance to the pilot much like 
ILS. GLS navigation is based on GPS signals 
augmented by position correction, integrity parame-
ters, and approach path definition information 
transmitted over VHF from the local GBAS ground 
station. One GBAS station can support multiple GLS 
precision approaches to nearby runways within the 
GBAS’s maximum use distance. 

3. 

GLS provides guidance similar to ILS 

approaches for the final approach segment, though 
the approach service volume has different dimen-

Navigation Aids 

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