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flown. Outside the SBAS coverage or in the event of
a WAAS failure, GPS/WAAS equipment reverts to
GPS−only operation and satisfies the requirements
for basic GPS equipment.
7. Unlike TSO−C129 avionics, which were
certified as a supplement to other means of
navigation, WAAS avionics are evaluated without
reliance on other navigation systems. As such,
installation of WAAS avionics does not require the
aircraft to have other equipment appropriate to the
route to be flown.
(a) Due to initial system limitation, there are
certain restrictions on WAAS operations. Pilots may
plan to use any instrument approach authorized for
use with WAAS avionics at a required alternate.
However, when using WAAS at an alternate airport,
flight planning must be based on flying the RNAV
(GPS) LNAV 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 nonprecision weather
requirements must be used for planning. Upon arrival
at an alternate, when the WAAS navigation system
AIM 8/5/04
1−1−42 Air Navigation Aids
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 individually evaluated, removal of the
NA from RNAV (GPS) and GPS procedures will
take some time.
d. Flying Procedures with WAAS
1. WAAS receivers support all basic GPS
approach functions and will provide additional
capabilities. One of the major improvements is the
ability to generate an electronic glide path,
independent of ground equipment or barometric
aiding. This eliminates several problems such as cold
temperature effects, incorrect altimeter setting or lack
of a local altimeter source and allows approach
procedures to be built without the cost of installing
ground stations at each airport. Some approach
certified receivers will only support a glide path with
performance similar to Baro−VNAV, and are
authorized to fly the LNAV/VNAV line of minima on
the RNAV (GPS) approach charts. Receivers with
additional capability which support the performance
requirements for precision approaches (including
update rates and integrity limits) will be authorized 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 adds vertical
integrity monitoring, which for LNAV/VNAV and
LPV approaches bounds the vertical error to
50 meters.
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 “LPV
not available − use LNAV/VNAV minima,” even
though the WAAS signal would support LPV. If
flying an existing LNAV/VNAV procedure, the
receiver will notify the pilot “LNAV/VNAV
available” even if the receiver is certified for LPV and
the WAAS signal supports LPV. If the WAAS signal
does not support published minima lines which the
receiver is certified to fly, the receiver will notify the
pilot with a message such as “LPV not available − use
LNAV/VNAV minima” or “LPV not available − use
LNAV minima.” Once this notification has been
given, the receiver will operate in this mode for the
duration of that approach procedure. The receiver
cannot change back to a more accurate level of
service until the next time an approach is activated.
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
　
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