background image

AIM

10/12/17

1−1−33

Navigation Aids

40 meters for LPV. It also provides vertical integrity

monitoring, which bounds the vertical error to 50

meters for LNAV/VNAV and LPVs with minima of

250’ or above, and bounds the vertical error to 35

meters for LPVs with minima below 250’.

2. When an approach procedure is selected and

active, the receiver will notify the pilot of the most

accurate level of service supported by the combina-

tion of the WAAS signal, the receiver, and the

selected approach, using the naming conventions on

the minima lines of the selected approach procedure.

For example, if an approach is published with LPV

minima and the receiver is only certified for

LNAV/VNAV, the equipment would indicate

“LNAV/VNAV available,” even though the WAAS

signal would support LPV. If flying an existing

LNAV/VNAV procedure with no LPV minima, the

receiver will notify the pilot “LNAV/VNAV

available,” even if the receiver is certified for LPV

and the signal supports LPV. If the signal does not

support vertical guidance on procedures with LPV

and/or LNAV/VNAV minima, the receiver annunci-

ation will read “LNAV available.” On lateral only

procedures with LP and LNAV minima the receiver

will indicate “LP available” or “LNAV available”

based on the level of lateral service available. Once

the level of service notification has been given, the

receiver will operate in this mode for the duration of

the approach procedure, unless that level of service

becomes unavailable. The receiver cannot change

back to a more accurate level of service until the next

time an approach is activated.

NOTE−

Receivers do not “fail down” to lower levels of service

once the approach has been activated. If only the

vertical off flag appears, the pilot may elect to use the

LNAV minima if the rules under which the flight is

operating allow changing the type of approach being flown

after commencing the procedure. If the lateral integrity

limit is exceeded on an LP approach, a missed approach

will be necessary since there is no way to reset the lateral

alarm limit while the approach is active.

3. Another additional feature of WAAS receiv-

ers is the ability to exclude a bad GPS signal and

continue operating normally. This is normally

accomplished by the WAAS correction information.

Outside WAAS coverage or when WAAS is not

available, it is accomplished through a receiver

algorithm called FDE. In most 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.

4. 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 differ-

ences 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.

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