Setting altimeters with no radio

peter writes:

This indicates a basic lack of understanding of GPS technology.

It reveals exactly the opposite. That's how GPS determines position.

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.

Surprising though it may be, those "precise times of arrival" are the
sides of a triangle.

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.

Actually it does. The arrival times define spheres in 3D space around
the satellites (the geoid can also be used as a reference sphere).
The intersections of these spheres effectively isolate the position of
the receiver. It's just a fancy version of good old triangulation,
and it works very well.

Unfortunately, however, it is optimized for lateral positioning, not
vertical positioning. To achieve the same vertical accuracy as
lateral accuracy, a much higher measurement precision is required.
For this reason, vertical measurement accuracy is very poor.

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.

More than "somewhat" less accurate: they are usually unusable,
certainly for aviation.

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.

How were you able to accurately survey your altitude in the air?

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.

It's hard to do that in the air.

However, there are good reasons why barometric measurements are used
instead for aviation to ensure consistency and uniform procedures.

The main reason is that it's more accurate.

GPS altitude data is so poor and so variable that I've given up using
it even on the ground. It's almost never anywhere near surveyed
altitudes, and it drifts all over the place. Indeed, you can watch it
change as you stand still on the ground, and that's with SA turned
off. I definitely would not want to depend on that in the air.

--
Transpose mxsmanic and gmail to reach me by e-mail.