AIM 

4/20/23

 

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38

Navigation Aids

smaller integrity limits) are approved to fly the LPV line of minima. The lateral integrity changes dramatically
from the 0.3 NM (556 meter) limit for GPS, LNAV, and LNAV/VNAV approach mode, to 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 combination 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 annunciation 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

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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 receivers 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 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.

5.

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.