gps altitude accuracy

Mark James Boyd wrote:

It strikes me that with GPS altitude and weather soundings and
measurements, with just the GPS WAAS altitude it should
be possible to calculate pressure altitude.


When restricted to the conditions of usual soaring, no weather sounding
is necessary, we need only a reference point with known pressure and true
altitude. As we soar in constantly rising or sinking air and out of
clouds, i.e. no saturation, we can easily predict the result of the sounding
maybe with a constant offset that the reference point would drop : the
vertical temperature gradient in this conditions is the dry adiabatic lapse
(1°C per 100m) and everything can be computed from this.

All this talk about computing pressure altitude from GPS reading is
irrelevant to the task of determining flight level or altitude (defined as
height above MSL).

It seems pretty relevant if I can save money and weight and
not have to calibrate an altimeter/transponder encoder. If I can get a
certified GPS with a transponder that is able to simulate exactly what an
altimeter would display, I have no need for the weight or
expense or certification for the altimeter, I wouldn't ever
need to update a Kohlsman window, I wouldn't have the power
consumption of a traditional transponder, yet I could calculate
final glides very well and have excellent and accurate information
about height above terrain even in places where the weather
forecasters had no pressure information.

The pressure altimeter is a fine device, but it's only advantage
(since June 10) in the United States over GPS is that it requires no
electricity. I have no doubt that if WAAS capable GPS had
preceded the invention of the pressure sensitive altimeter,
that WAAS GPS would be the altitude standard in the US for
ATC. WAAS GPS is cheaper, insensitive to temperature and
pressure gradients, passive (no reseting Kohlsman windows),
gives accurate altitude with respect to the ground, and
uses less power than a traditional transponder (since the air
doesn't need to be heated to 55 degrees C).

I think the only thing missing to make this system work
with the old standard is a pressure data signal,
perhaps added to the current GPS signals. This would allow
the GPS to simulate the altimeter, yet also provide final
glide and terrain separation information.

Does the current system do a great job of separating traffic?
As pointed out it is fine. Does it do a great job of
avoiding terrain? Ask the families of those killed when
airliners crash because the Kohlsman windows were set incorrectly.

UPSAT is banking on 250 ft 3/4 mile vis precision WAAS GPS
approaches being published over the next two years. It isn't
such a stretch to imagine U.S. ATC using GPS altitude for
IFR traffic separation at some point. The altimeter and encoder
may go the way of the 90 and 720 channel radio in the next
10 years...

Mark James Boyd wrote:

If so, and this were reliable, there would be no need for
a power-hungry mode C transponder, one could use a GPS based
transponder and the ATC computer could simply spit out
altitude. This isn't so farfetched since that same computer
already compensates for pressure differences.

etc.

You're completely missing the point. We're talking gliders here, and
glider flights usually are uncontrolled. A pressure alitmeter works
without controller and without electricity, is fairly robust and
provides the pilot with everything he needs for airspace issues. If the
pilot is too dumb to compensate for wheather changes and flies into
terrain because he believes his altimeter more than the outside view, so
be it.

Stefan