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relative to the horizon, and aircraft attitude may affect
reception of one or more satellites. Since the relative
positions of the satellites are constantly changing,
prior experience with the airport does not guarantee
reception at all times, and RAIM availability should
always be checked.
2. If RAIM is not available, another type of
navigation and approach system must be used,
another destination selected, or the trip delayed until
RAIM is predicted to be available on arrival. On
longer flights, pilots should consider rechecking the
RAIM prediction for the destination during the flight.
This may provide early indications that an
unscheduled satellite outage has occurred since
takeoff.
3. If a RAIM failure/status annunciation
occurs prior to the final approach waypoint
(FAWP), the approach should not be completed
since GPS may no longer provide the required
accuracy. The receiver performs a RAIM prediction
by 2 NM prior to the FAWP to ensure that RAIM is
available at the FAWP as a condition for entering the
approach mode. The pilot should ensure that the
receiver has sequenced from “Armed” to
“Approach” prior to the FAWP (normally occurs
2NM prior). Failure to sequence may be an
indication of the detection of a satellite anomaly,
failure to arm the receiver (if required), or other
problems which preclude completing the approach.
4. If the receiver does not sequence into the
approach mode or a RAIM failure/status annunciation
occurs prior to the FAWP, the pilot should not
descend to Minimum Descent Altitude (MDA), but
should proceed to the missed approach waypoint
(MAWP) via the FAWP, perform a missed
approach, and contact ATC as soon as practical. Refer
to the receiver operating manual for specific
indications and instructions associated with loss of
RAIM prior to the FAF.
5. If a RAIM failure occurs after the FAWP, the
receiver is allowed to continue operating without an
annunciation for up to 5 minutes to allow completion
of the approach (see receiver operating manual). If
the RAIM flag/status annunciation appears after
the FAWP, the missed approach should be
executed immediately.
j. Waypoints
1. GPS approaches make use of both fly-over
and fly-by waypoints. Fly-by waypoints are used
when an aircraft should begin a turn to the next course
prior to reaching the waypoint separating the two
route segments. This is known as turn anticipation
and is compensated for in the airspace and terrain
clearances. Approach waypoints, except for the
MAWP and the missed approach holding waypoint
(MAHWP), are normally fly-by waypoints. Flyover
waypoints are used when the aircraft must fly
over the point prior to starting a turn. New approach
charts depict fly-over waypoints as a circled
waypoint symbol. Overlay approach charts and some
early stand alone GPS approach charts may not
reflect this convention.
2. Since GPS receivers are basically “To-To”
navigators, they must always be navigating to a
defined point. On overlay approaches, if no
pronounceable five-character name is published for
2/14/08 AIM
Navigation Aids 1-1-33
an approach waypoint or fix, it was given a database
identifier consisting of letters and numbers. These
points will appear in the list of waypoints in the
approach procedure database, but may not appear on
the approach chart. A point used for the purpose of
defining the navigation track for an airborne
computer system (i.e., GPS or FMS) is called a
Computer Navigation Fix (CNF). CNFs include
unnamed DME fixes, beginning and ending points of
DME arcs and sensor final approach fixes (FAFs) on
some GPS overlay approaches. To aid in the approach
chart/database correlation process, the FAA has
begun a program to assign five-letter names to CNFs
and to chart CNFs on various National Oceanic
Service aeronautical products. These CNFs are not to
be used for any air traffic control (ATC) application,
such as holding for which the fix has not already been
assessed. CNFs will be charted to distinguish them
from conventional reporting points, fixes, intersections,
and waypoints. The CNF name will be enclosed
in parenthesis, e.g., (MABEE), and the name will be
placed next to the CNF it defines. If the CNF is not at
an existing point defined by means such as crossing
radials or radial/DME, the point will be indicated by
an “X.” The CNF name will not be used in filing a
flight plan or in aircraft/ATC communications. Use
current phraseology, e.g., facility name, radial,
　
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