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AIM
4/3/14
1−1−27
Navigation Aids
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 “Ap-
proach” 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.
4.
If the receiver does not sequence into the
approach mode or a RAIM failure/status annunci-
ation occurs prior to the FAWP, the pilot should not
descend to Minimum Descent Altitude (MDA), but
should proceed to the missed approach waypo-
int (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 receivers navigate from one defined
point to another retrieved from the aircraft’s on board
navigational database. These points are waypoints
(5-letter pronounceable name), existing VHF inter-
sections, DME fixes with 5-letter pronounceable
names and 3-letter NAVAID IDs. Each waypoint is a
geographical location defined by a latitude/longitude
geographic coordinate. These 5-letter waypoints,
VHF intersections, 5-letter pronounceable DME
fixes, and 3-letter NAVAID IDs are published on
various FAA aeronautical navigation products (IFR
Enroute Charts, VFR Charts, Terminal Procedures
Publications, etc.).
2.
A Computer Navigation Fix (CNF) is also a
point defined by a latitude/longitude coordinate and
is required to support area navigation (RNAV)
system operations. The GPS receiver uses CNFs in
conjunction with waypoints to navigate from point to
point. However, CNFs are not recognized by Air
Traffic Control (ATC). ATC does not maintain CNFs
in their database and they do not use CNFs for any air
traffic control purpose. CNFs may or may not be
charted on FAA aeronautical navigation products, are
listed in the chart legends, and are for advisory
purposes only. Pilots are not to use CNFs for point to
point navigation (proceed direct), filing a flight plan,
or in aircraft/ATC communications. CNFs that do
appear on aeronautical charts allow pilots increased
situational awareness by identifying points in the
aircraft database route of flight with points on the
aeronautical chart. CNFs are random five−letter
identifiers, not pronounceable like waypoints, and
placed in parenthesis. Eventually, all CNFs will begin
with the letters “CF” followed by three consonants
(for example, CFWBG). This five−letter identifier
will be found next to an “x” on enroute charts and
possibly on an approach chart. On instrument
approach procedures(charts) in the terminal proced-
ures publication, CNFs may represent unnamed
DME fixes, beginning and ending points of DME
arcs, and sensor (ground−based signal i.e., VOR,
NDB ILS) final approach fixes on GPS overlay
approaches. These CNFs provide the GPS with points
on the procedure that allow the overlay approach to
mirror the ground−based sensor approach. These
points should only be used by the GPS system for
navigation and should not be used by pilots for any
other purpose on the approach. The CNF concept has
not been adopted or recognized by the International
Civil Aviation Organization (ICAO).
3.
GPS approaches use fly−over and fly−by
waypoints to join route segments on an approach.
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