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.