AIM

4/20/23

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Navigation Aids

(4)

Receiver manufacturers and/or database suppliers may supply “NOTAM” type information

concerning database errors. Pilots should check these sources when available, to ensure that they have the most
current information concerning their electronic database.

(5)

If RAIM is not available, use another type of navigation and approach system; select another route

or destination; or delay the trip 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 an early
indication that an unscheduled satellite outage has occurred since takeoff.

(6)

If a RAIM failure/status annunciation occurs prior to the final approach waypoint (FAWP), the

approach should not be completed since GPS no longer provides the required integrity. The receiver performs
a RAIM prediction by 2 NM prior to the FAWP to ensure that RAIM is available as a condition for entering the
approach mode. The pilot should ensure 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 flying the approach.

(7)

If the receiver does not sequence into the approach mode or a RAIM failure/status annunciation

occurs prior to the FAWP, the pilot must not initiate the approach nor descend, but instead, proceed to the missed
approach waypoint (MAWP) via the FAWP, perform a missed approach, and contact ATC as soon as practical.
The GPS receiver may continue to operate after a RAIM flag/status annunciation appears, but the navigation
information should be considered advisory only. Refer to the receiver operating manual for specific indications
and instructions associated with loss of RAIM prior to the FAF.

(8)

If the RAIM flag/status annunciation appears after the FAWP, the pilot should initiate a climb and

execute the missed approach. The GPS receiver may continue to operate after a RAIM flag/status annunciation
appears, but the navigation information should be considered advisory only. Refer to the receiver operating
manual for operating mode information during a RAIM annunciation.

(h) Waypoints

(1)

GPS receivers navigate from one defined point to another retrieved from the aircraft’s onboard

navigational database. These points are waypoints (5-letter pronounceable name), existing VHF intersections,
DME fixes with 5

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letter pronounceable names and 3-letter NAVAID IDs. Each waypoint is a geographical

location defined by a latitude/longitude geographic coordinate. These 5

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letter waypoints, VHF intersections,

5

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letter  pronounceable DME fixes and 3

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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 Performance

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Based Navigation (PBN) operations. The GPS receiver uses CNFs in

conjunction with waypoints to navigate from point to point. However, CNFs are not recognized by 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 procedures 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

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over and fly

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by waypoints to join route segments on an approach. Fly

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by

waypoints connect the two segments by allowing the aircraft to turn prior to the current waypoint in order to roll