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approach functions and provide additional capabilities.
One of the major improvements is the ability to
generate glide path guidance, independent of ground
equipment or barometric aiding. This eliminates
several problems such as hot and cold temperature
effects, incorrect altimeter setting or lack of a local
altimeter source. It also allows approach procedures
to be built without the cost of installing ground
stations at each airport or runway. Some approach
certified receivers may only generate a glide path
with performance similar to Baro−VNAV and are
only approved to fly the LNAV/VNAV line of minima
on the RNAV (GPS) approach charts. Receivers with
additional capability (including faster update rates
and 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−
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
7/31/08 AIM
AIM 2/14/08
1−1−40 Navigation Aids
+/−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
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