TSO C129 to mean within 30 nm of your departure
or destination.
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 GX55 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 required by TSO C129 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. This is the
reason that TSO C129 GPS installations are
required to have pressure altitude inputs.
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