I think the whole WAAS thing is in flux. The AIM gets wordier and more
difficult to interpret with each change to its discussion of GPS. IMHO,
until there are a whole lot of WAAS capable boxes flying in the system we
will be flailing about in the dark.

Did you see the change to the AIM with regard to a reversed W on approach
plates? Doesn't create much confidence in the system.

Bob Gardner

"Tarver Engineering" wrote in message
...

"Bob Gardner" wrote in message
news:R0hcc.78998$K91.171081@attbi_s02...
You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
altitude.
When WAAS is in common use this will probably change.

The altitude error is part of the WGS-84 model and is not a WAAS issue any
longer. The CNX-80 provides VNAV capability through the use of pressure
altitude from a transponder source and the pilot's baro correction input;
married to a partial TAWS data base. GPS/WAAS is not likely to become an
acceptable substitute for pressure altitude. Larger airplanes use air
data
derived pressure altitude in conjunction with GPS for improved RNP
capability.