II Morrow Inc. NMC 2001 Operating Manual page 325

RAIM Operations
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 NMS is providing valid navigation
data, then it is still capable of providing good IFR navigation as long as
the satellite data is good.
During Approach operations, if you lose RAIM availability, your
Apollo NMS will continue to provide navigation data; however, your
navigation display will be flagged as invalid. You should discontinue
the approach and revert to other means of navigation. It should be
noted that even if this were to occur, canceling Approach-Active, by
pressing the OBS/HLD button, will result in an immediate transition to
Terminal RAIM from Approach RAIM. Terminal RAIM integrity
may still be available even when Approach RAIM integrity is not. If
so, your navigation display will again become valid, and you may
continue to use the Apollo GPS for missed approach navigation.
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 NMS 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. Satellite clock failures, one of the
more probable failures, is more likely to occur on the older Block-I
satellites, but this does not mean that it can't happen on the newer ones.
When a satellite failure does happen, it can result in undetected error of
miles in the position if a receiver is using the failed satellite in its
navigation solution.
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