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6/17/21 

AIM 

(1) 

RAIM outages may occur due to an 

insufficient number of satellites or due to unsuitable 
satellite geometry which causes the error in the 
position solution to become too large. Loss of satellite 
reception and RAIM warnings may occur due to 
aircraft dynamics (changes in pitch or bank angle). 
Antenna location on the aircraft, satellite position 
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, 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. 

(3) 

If a RAIM failure/status annunciation 

o c c u rs  p r i or  to  t he  f i n al  a p pr o a ch  w a y p o i n t  
(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. 

(4) 

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 or 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. 

(5) 

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. 

(i)  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

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 naviga-
tion 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

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 un-
named DME fixes, beginning and ending points of 
DME arcs, and sensor (ground-based signal i.e., 

Navigation Aids 

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