ted wrote:


Ground based navaids can fail, can experience jamming (both intentional and
unintentional) and are much more vulnerable to terrain obscuration than
navaids in the sky. I'm not aware of any significant continuity issue for
GPS that are a bigger problem than what is experienced with ground based
navaids.


Those involved in planning see it differently. That is why the new RNAV
(RNP) criteria presumes a failure of GPS during a terrain-critical phase
of flight.

A long history of FAA ground-based nav aids has shown very, very little
problem with the ground components of the system. And, where it occurs
there is usually an alternate nav aid or aids available.

Terrain obscuration is irrelavant for IFR procedures because of design
and flight inspection confirmation.