In rec.aviation.student Jay Honeck wrote:
Here we are, 13 years later, and apparently little has changed. We've
got a navigation system (GPS) that is accurate to within a meter, and
yet the entire system is still built around VORs, which is accurate to
within...a lot. (Anyone know how accurate it is to be flying a VOR
radial say, 30 miles from the VOR station? Is it a mile? A half
mile? 1000 feet? I have no idea...)

If your VOR is accurate to within one degree, then the inaccuracy is 30 *
pi/180 or about half a mile. Increase the inaccuracy proportional with the
VOR inaccuracy, so two degrees gets you to within a mile, etc.

However, there's the question of how accurate you need to be, and how
accurate you *want* to be. When you're 30 miles out, being within a mile
of where you want to be is probably fine. And having navigational systems
that are too accurate can be dangerous. There is some concern now that GPS
is leading to increased mid-air collisions due to pilots following the GPS
exactly, causing them to run into other aircraft who were also following
their GPS exactly along the same track. The obvious fix is to not follow
it so exactly, but I think a lot of people get trained to fly as precisely
as possible and then carry that over to following their GPS.

There's also the question of reliability. For IFR flight where you *need*
some kind of navigation system, having only one is dangerous. Maybe VOR
isn't the best backup system but there ought to be *something* in
operation other than GPS.

But I do know this: In the real world of (relatively unregulated) VFR
flying, GPS rules. The fact that the IFR system hasn't completed the
change-over in a decade is just another example of how glacial
progress can be in aviation.

Backups are more important for IFR, though. If I'm flying and suddenly GPS
goes out, I'll just shrug and keep looking out the window. If someone is
flying IFR in the clouds and the One True Nav System goes down, he's
pretty screwed.

--
Michael Ash
Rogue Amoeba Software