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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’.


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.


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.


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.


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.


The WAAS scaling is also different than GPS


−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