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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.
18.9.2If 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.
ENR 4.1−30 AIP
15 MAR 07 United States of America
Nineteenth Edition Federal Aviation Administration
18.9.3If 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 2
NM 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.
18.9.4If 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 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.
18.9.5If 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.
18.10Waypoints
18.10.1GPS 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
tworoute 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.
Fly−over 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.
18.10.2Since 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
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 FAA 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, distance, to
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