Richard Kaplan wrote:

The recent thread regarding GPS as the sole navaid sometime in the future
has focused on terrorism as the "what if" scenario.

I am not focusing specifically on terrorism in my concern the need for
GPS redundancy; I am simply observing that there is no system or device I am
aware of anywhere in aviation which works 100% of the time, so certainly it
is always worh having a backup. Even if GPS has 12 satellites, it seems
clear by common sense that the best backup system would use a technology
different from GPS.

With that as background, look at the following NOTAM, which is currently
available on DUAT. We can theorize all we want about whether this NOTAM
exists due to military testing or some atmospheric irregularity or whatever,
but the point is that the NOTAM really is out there and will be effective in
a few days. If my plane had nothing but GPS-based avionics, what would my
options be?

GPS 07/015 ZDC GPS UNRELIABLE WITHIN A 100 NM RADIUS OF PATUXENT
VORTAC (PXT) AT 10,000 MSL THROUGH FL400, AND DECREASING IN AREA
WITH DECREASE IN ALTITUDE TO 80 NM RADIUS AT 4000 FT AGL
1200-2000 DLY WEF 0307211200-0307252000


Again, there are toy airplanes then there are real airplanes. All high-end jets
have triple IRUs feeding position to the FMS and LNAV. So, if the primary nav
sensor shoots craps around Podunk, Merrykanasa, you just coast along using
triple-mized inertial position until the GPS kicks back in.