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En Route RAIM, or a 2.0 nm alarm limit is provided
at all other times.
Note that the three different equipment operation
states: En route, Approach-Transition, and
Approach-Active are somewhat but not directly
related to the three RAIM alarm limits of En route,
Terminal, and Approach. The equipment states are
defined by the CDI sensitivity and operational
requirements. When you depart an airport, the
equipment is operating En route, but it is providing
Terminal RAIM integrity.
How Often Will RAIM Be Unavailable?
RAIM availability depends on many factors, including
the number of operational satellites in the sky, the
time of day, the location, the receiver design, the
antenna mask angle and location, and on the method
of baro-aiding. The Apollo GX has been designed
such that with an optimal 21 satellites in operation,
Approach RAIM availability should exceed 95%.
There are currently, at the time this manual was
published, at least 24 satellites in operation; however,
the Federal Navigation Radio Plan only guarantees
that there is at least a 95% probability that 21 will be
operational at any given time.
What Is Baro-aiding?
Baro-aiding is used to enhance RAIM availability. It is
a method where pressure altitude data can be used to
provide RAIM availability during some times when
RAIM would not be available using the satellites
alone.
What does it mean when I get a “RAIM not
available” message?
If RAIM is not available, your Apollo GX may be
capable of continuing to provide good accurate IFR
navigation. What you have lost is the ability to tell if a
satellite is sending bad data. For all operations except
Approach, the FAA has determined that it is
acceptable for you to continue to use GPS for IFR
9-31
Approach Basics (GX50/60)