Aeronautical Information Manual (AIM), page 85

Index   84 -- Page 85 -- 86

the following restrictions. When using WAAS at
an alternate airport, flight planning must be based
on flying the RNAV (GPS) LNAV or circling minima
line, or minima on a GPS approach procedure, or
conventional approach procedure with "or GPS" in
the title. Code of Federal Regulation (CFR) Part 91
non-precision weather requirements must be used for
planning. Upon arrival at an alternate, when the
WAAS navigation system indicates that LNAV/
VNAV or LPV service is available, then vertical
guidance may be used to complete the approach using
the displayed level of service. The FAA has begun
removing the NA (Alternate Minimums Not
Authorized) symbol from select RNAV (GPS) and
GPS approach procedures so they may be used by
approach approved WAAS receivers at alternate
airports. Some approach procedures will still require
the NA for other reasons, such as no weather
reporting, so it cannot be removed from all
procedures. Since every procedure must be individu-
ally evaluated, removal of the NA from RNAV
(GPS) and GPS procedures will take some time.

Properly trained and approved, as required, TSO-C145()
and TSO-C146() equipped users (WAAS users) with and
using approved baro-VNAV equipment may plan for
LNAV/VNAV DA at an alternate airport. Specifically
authorized WAAS users with and using approved
baro-VNAV equipment may also plan for RNP 0.3 DA at the
alternate airport as long as the pilot has verified RNP
availability through an approved prediction program.
d. Flying Procedures with WAAS

1. WAAS receivers support all basic GPS

approach functions and provide additional capabilit-

ies. 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 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.
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

Navigation Aids 1-1-31

Page 85 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   84 -- Page 85 -- 86