Mxsmanic wrote:
"Jim Macklin" writes:

they give altitude, accurate to within a few feet.

Unfortunately, no, they do not. GPS is accurate for lateral
navigation, not vertical navigation. GPS altitudes can easily be off
by as much as 200 feet at ground level in comparison to a correctly
set altimeter, and at altitude the disparity can reach 500 feet.

The reason for this is that the angles used for triangulation of
lateral positions are large and permit a high level of precision, but
the angles for triangulation of altitude are very small and it's very
easy to be off by a wide margin.

This indicates a basic lack of understanding of GPS technology. The
GPS receiver never deals with measurement of any angles nor with
triangulation. What is measured are the precise times of arrival of
the signals from the satellites. Since the satellites encode the
signals with timing information from their sychronized atomic clocks
and also send detailed orbital data to define their own positions, the
receiver is able to determine the relative distances to the various
satellites based on the speed of light/radio and the observed relative
signal delays. Using this distance information together with the known
positions of the satellites then allows for a determination of the
position of the receiver. Note that this never involves a measurement
of any angles.

It is true that altitude measurements are generally somewhat less
accurate than horizontal position measurements due to the basic
geometry of receiving satellite signals from only the satellites that
are above you. Ideal measurement of altitude would also involve some
satellites below you but of course their signals are blocked by the
earth. Similarly, east-west positions are a bit better accuracy than
north-south since the satellites are equally likely to be east and west
of you but there's a greater likelihood of them being to the south
rather than the north (at least from the northern hemisphere).

My long-term evaluation of GPS altitude accuracy has shown that I get
values within 35' of accurately surveyed altitudes at least 95% of the
time ever since Selective Availability was turned off. Using the WAAS
correction data improves this to get the accuracy down to 20' with 95%
confidence. Both of these are based on having a reasonably
unobstructed view of the sky (which generally isn't hard in an aircraft
unless the antenna is poorly positioned).

So from a technical standpoint GPS altitudes these days are pretty good
although some care should be taken to check the actual satellite
geometry and reception at the time of any critical measurements.
However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.