What does it mean when I get a “RAIM not
available” message?
If RAIM is not available, your Apollo GX55 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
navigation as long as you verify the accuracy of
your position by other means at least every 15
minutes until RAIM becomes available again. This
can be accomplished by cross-checking your GPS
position
against
that
of
other
navigation
instruments. Even during times of no RAIM, if your
Apollo GPS determines that it can not provide an
accurate position due to poor satellite availability or
geometry, it will flag the navigation display as
invalid. If the Apollo GX55 is providing valid
navigation data, then it is still capable of providing
good IFR navigation as long as the satellite data is
good.
What Does A RAIM Alarm Mean?
A RAIM alarm means that the navigation data can
not be trusted to be within the alarm limit. A RAIM
alarm means that the Apollo GX55 has determined
that there is something wrong with the data it is
receiving from one or more satellites. The data may
be corrupted by a satellite failure, such as a clock
failure, or it may have been corrupted by some form
of signal interference. A RAIM alarm will also be
accompanied by flagging all navigational displays
as invalid. If you should ever see a RAIM alarm, do
not trust the GPS position, and revert to other means
of navigation as appropriate.
How Often Do Satellite Failures Occur?
There is not enough data to provide a good answer;
however, it is known that they can and do happen.
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