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What's RAIM?
RAIM stands for Receiver
Autonomous Integrity Monitoring.
It is a form of integrity monitoring
performed within the GPS receivers
themselves. It ensures available satellite
signals meet the integrity requirements for
a given phase of flight. By comparing the
pseudorange measurements of a number of
satellites, the RAIM function can identify a
satellite failure and issue an alert to the
pilot.
Many VFR GPS receivers
and most hand-held receivers do not have
RAIM capability. Without RAIM capability,
the pilot has no assurance of the accuracy
of the GPS position (and in a non-WAAS
environment, may not file IFR). A minimum of
five satellites is required to detect a bad
satellite. (Note: At least
six satellites are required to detect and
exclude a bad satellite from the navigation
solution if the receiver has a fault
detection and exclusion (FDE) RAIM
algorithm.)
The GPS receiver must
report when its RAIM function is
unavailable, at both present time/position
and at any selected future time/position.
Pilots can get information on satellite
outages through the NOTAM system. The
effect, however, of an outage on the
intended operation cannot be determined
unless the pilot has a RAIM availability
prediction program which allows exclusion of
a satellite which is predicted to be out of
service.
If you
are using GPS to fly an instrument approach
and receive a RAIM annunciation prior to the
final approach waypoint, you may not have
sufficient accuracy to complete the
approach.
RAIM function is required for instrument
flight (except when flying under WAAS).
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