Mxsmanic writes:

RAIM isn't about additional accuracy, that's WAAS. A receiver with
RAIM can't give you a better position fix, but it can tell you when the
the possible error in the position fix has gotten unacceptably large.

How large is unacceptable?

Most of the time my handheld GPS receiver is fine, but sometimes
(extremely rarely) it loses signal or has an inadequate coverage.
RAIM is essentially a requirement for the GPS to let you know when
something has gone wrong. If you're in VMC, presumably, you can tell
just by looking out the window, but in IMC, if the GPS is your sole
means (rather than just a cross-check on VOR and ADF), you have to
know. You could be talking about an error of dozens or even hundreds
of miles.

It's a lot like electricity. For your home, the electricity works
99.[multiple 9's] percent of the time, and on the very rare occasions
when it goes out, you just pull out the flashlights and battery
radio. In a hospital, the electricity *cannot* stop working, so there
are monitoring systems and backup generators.

The VOR and LOC/GS also have an error-monitoring system in the form of
the flags on the NAV head, for precisely the same reason -- if the
flags drop, you cannot trust the instrument. Ditto for some newer
AI's and TC's. The ADF is grandparented without any such error
indicator, which makes it a bit more dangerous: some snap to 90 deg
when they lose signal (which is hard to miss), and many people just
leave the ident volume on low. I have to admit that I occasionally
change course 10 degrees for 10 or 20 seconds to make sure that the
ADF is still working, since I frequently fly Romeo (LF/MF) airways
between Ottawa and Kingston.

For en route navigation you don't have to be that accurate. Older
forms of navigation are considerably less accurate, and people still
use those.

I don't think people are usually worried about being a mile or two off
course enroute.


All the best,


David