"Bob Gardner" wrote in message
news:%Glcc.191812$_w.1879080@attbi_s53...
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.

I believe that VDL will be more of a factor moving forward than WAAS will
be. FAA has spent a lot of money on space based WAAS while being in
posession of a lot of underused VHF bandwidth.

"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.