That's RAIM *prediction*. RAIM is very capable of failing during an
approach if
you don't check the prediction ahead of time, at least on our Garmin
430.

Yes...and if you read his document, he doesn't distinguish between RAIM
(which many
non-IFR units do just fine) and predictive rain.

Ron,

I am open to argument and corrections of any misunderstandings that I might
have. It was my impression that RAIM is strictly a integrity monitoring
scheme rather than a predictive function. RAIM uses redundant satellite
signals as a cross check to monitor whether any satellite signals are bad.

Since a GPS knows the satellite orbits from the almanac, a program can be
designed to predict if satellite positioning will be adequate to support
RAIM at a given time and place. Receivers such as the Garmin 430 have this
RAIM prediction feature. Knowing the satellite orbit information, it is
also possible to predict and issue a notam for areas and times when RAIM
will be unavailable.

Using the same techniques for predicting RAIM coverage, it would be possible
to warn of impending satellite geometry and coverage problems. I do not
know whether or not any GPS receivers do this automatically beyond the RAIM
prediction program such as in the AUX menu of the Garmins.

I guess I understand RAIM as a monitoring the integrity of the satellite
signal rather than a predictive function. However, I will agree that there
are ways to predict whether or not RAIM will be available.

I am open to corrections if you will explain your reasoning and possibly
site sources.

Thanks,

John Bell