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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

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.