GPS Altitude with WAAS

"Fred E. Pate" writes:

And this is for a decision altitude of only 294 ft AGL. Seems like
the FAA is moving towards taking into account temperature errors in
barometric alitmetry.

Thanks -- that's an interesting note.

And, by implication, this supports the premise that WAAS altitude
figures are more accurate than the trusty old "sensitive altimeter."
(In the legend they specifically state that WAAS-based VNAV can be
used when BARO-VNAV is not approved due to temperature.)

The altimeter becomes increasingly accurate near the ground (assuming
you have the correct altimeter setting) and increasingly inaccurate
away from the ground; WAAS, I'll guess, has about the same accuracy
all the way down (or up). I don't know at what point they typically
cross over, but it would vary depending on the temperature gradiant.


All the best,


David