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AIM

10/12/17

1

−1−25

Navigation Aids

(2)

The terms UNRELIABLE and MAY

NOT BE AVAILABLE are used in conjunction with
GPS NOTAMs. Both UNRELIABLE and MAY NOT
BE AVAILABLE are advisories to pilots indicating
the expected level of service may not be available.
UNRELIABLE does not mean there is a problem
with GPS signal integrity. If GPS service is available,
pilots may continue operations. If the LNAV or
LNAV/VNAV service is available, pilots may use the
displayed level of service to fly the approach. GPS
operation may be NOTAMed UNRELIABLE or
MAY NOT BE AVAILABLE due to testing or
anomalies. (Pilots are encouraged to report GPS
anomalies, including degraded operation and/or loss
of service, as soon as possible, reference paragraph
1

−1−13.) When GPS testing NOTAMS are published

and testing is actually occurring, Air Traffic Control
will advise pilots requesting or cleared for a GPS or
RNAV (GPS) approach that GPS may not be
available and request intentions. If pilots have
reported GPS anomalies, Air Traffic Control will
request the pilot’s intentions and/or clear the pilot for
an alternate approach, if available and operational.

EXAMPLE

The following is an example of a GPS testing NOTAM: 
!GPS 06/001 ZAB NAV GPS (INCLUDING WAAS, GBAS,
AND ADS-B) MAY NOT BE AVAILABLE WITHIN A
468NM RADIUS CENTERED AT 330702N1062540W
(TCS 093044) FL400-UNL DECREASING IN AREA
WITH A DECREASE IN ALTITUDE DEFINED AS:
425NM RADIUS AT FL250, 360NM RADIUS AT
10000FT, 354NM RADIUS AT 4000FT AGL, 327NM
RADIUS AT 50FT AGL.  1406070300-1406071200.

(3)

Civilian pilots may obtain GPS RAIM

availability information for non

−precision approach

procedures by using a manufacturer-supplied RAIM
prediction tool, or using the Service Availability
Prediction Tool (SAPT) on the FAA en route and
terminal RAIM prediction website. Pilots can also
request GPS RAIM aeronautical information from a
flight service station during preflight briefings. GPS
RAIM aeronautical information can be obtained for
a period of 3 hours (for example, if you are scheduled
to arrive at 1215 hours, then the GPS RAIM
information is available from 1100 to 1400 hours) or
a 24

−hour timeframe at a particular airport. FAA

briefers will provide RAIM information for a period
of 1 hour before to 1 hour after the ETA hour, unless
a specific timeframe is requested by the pilot. If flying
a published GPS departure, a RAIM prediction
should also be requested for the departure airport.

(4)

The military provides airfield specific

GPS RAIM NOTAMs for non

−precision approach

procedures at military airfields. The RAIM outages
are issued as M

−series NOTAMs and may be obtained

for up to 24 hours from the time of request.

(5)

Receiver manufacturers and/or data-

base 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.

(h) Receiver Autonomous Integrity Moni-

toring (RAIM)

(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.