II Morrow Inc. NMC 2001 Operating Manual page 324

Operations RAIM
the Apollo NMS will automatically switch to Terminal RAIM
integrity, but you will be prompted to enable your approach and thus
transition to the Approach-Transition CDI scale only if you have an
approach loaded into your active flight plan. If you have an approach
loaded and enabled, the Apollo NMS will provide Approach RAIM
integrity starting 2 nm from the FAF. Approach-Active operation,
which requires both that Approach RAIM and Approach CDI
sensitivity be provided, begins at the FAF and is indicated by the
Approach Active annunciator on solid.
This may seem very confusing, but operationally, the Apollo NMS
makes it very simple. If the Approach Active annunciator comes on
solid at the FAF, then it is OK to continue the approach. If it does not
come on solid at the FAF, discontinue the approach. You will get a
message telling you what operational or integrity requirement was not
OK.
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 NMS 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 C129a 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 C129a
GPS installations are required to have pressure altitude inputs.
What does it mean when I get a "RAIM not available"
message?
If RAIM is not available, your Apollo NMS 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
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