Jay Masino writes:

That said, I've had RAIM alarms on my IFR GPS, where my handheld GPSMAP
195 thought everything was OK, and I could see by looking out the window
that we were exactly where both GPSs thought we were.

Even a handheld GPS is more than sufficient for en route navigation.
Only takeoffs and landings require greater accuracy. Remember, typical
accuracies for GPS under an open sky without nearby obstacles (e.g., in
an aircraft) are in the range of a few metres, even with cheap,
handheld, uncertified units. Unless the sky is so crowded that aircraft
are only ten metres apart, this is more accuracy than anyone ever needs
en route. In fact, your commercial airline flights could navigate using
the GPS you're holding next to your window if they needed to, and things
would be just fine (at least outside of takeoff and landing).

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