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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
8/5/04 AIM
Air Navigation Aids 1−1−43
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
requests 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 cause the DA to be
raised.
6. A new method has been added for selecting
the final approach segment of an instrument
approach. Along with the current method used by
most receivers using 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 until the ATD reaches 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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