Setting altimeters with no radio

mike regish schrieb:
My point is that they both are, basically, the same frame of
reference-height above sea level.

Which is wrong.

Stefan

Well, what exactly are their reference datum(s?).

mike

"Stefan" wrote in message
...
mike regish schrieb:
My point is that they both are, basically, the same frame of
reference-height above sea level.

Which is wrong.

Stefan

mike regish writes:

My point is that they both are, basically, the same frame of
reference-height above sea level.

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

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Do those amounts vary with location? How large a difference is there between
the 2? I remember reading about the 2 standards, but forget how the mean
geoid is determined.

But you're right. If that's true, and I don't doubt it is, GPS would be
better suited to terrain avoidance and less so to aircraft separation.

mike

"Mxsmanic" wrote in message
...
mike regish writes:

My point is that they both are, basically, the same frame of
reference-height above sea level.

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

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Mxsmanic wrote:
mike regish writes:

My point is that they both are, basically, the same frame of
reference-height above sea level.

Nope. GPS is height above the mean surface of the geoid, altimeter is
height above mean sea level. They can be hundreds of feet apart.

You're thinking of height above the ellipsoid, which can be hundreds of
feet different from height above the geoid. But the geoid does
represent the mean sea level height - including in places that are far
from the sea. Internally GPS receivers generally initially calculate
height relative to the ellipsoid model of the earth's shape (using the
WGS-84 model parameters). However, recent models with which I'm
familiar then apply a correction term based on an internal lookup table
to convert the ellipsoid height to the geoid height (equivalent to
height above MSL) at that particular location. See:
http://www.esri.com/news/arcuser/0703/geoid1of3.html
The altitudes that end up being displayed by the GPS after its internal
correction are therefore based on elevation above MSL with some
measurement uncertainty that's dependent on the current satellite
geometry.

Stefan writes:

The point is not which one is more accurate. The point is that everybody
uses the same frame of reference.

Both are valid points. GPS is too inaccurate to use for measuring
altitude in aviation, _and_ it uses a different frame of reference,
which leads to increasingly large disparities between GPS and pressure
altitude at higher altitudes.

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mike regish writes:

Well, kind of. Most, if not all altimeters now, compensate for pressure
variations to show geometric (if that's the right term for it) altitude.

Not true. Altimeters show substantial differences with geometric
altitude as altitude increases. Air traffic control is based on
pressure altitudes, not geometric altitudes.

With SA disables in the GPS system, altitude is much more accurate than it
was. How accurate, I don't know.

GPS altitude can be from 200 to 500 feet off easily.

When I compare my altimeter with my GPS's,
they're usually about 100' apart. I'm not sure which is more accurate.

The altimeter is much more accurate.

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Above 18K feet, everybody is on pressure altitude. Down in the sewers, where
I fly, we're all on local pressure.

mike

"Mxsmanic" wrote in message
...

Not true. Altimeters show substantial differences with geometric
altitude as altitude increases. Air traffic control is based on
pressure altitudes, not geometric altitudes.

The altimeter is much more accurate.

That's what I would assume with my particular GPS.

"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. GPS was designed to measure lateral
positions accurately, but it performs poorly for altitude. It is
typically much less accurate than an altimeter for altitude, and the
computed altitude constantly changes (significantly) as the satellites
move, in a way that is, for aviation purposes, practically random.

... just set the altimeter to read the same.

This is a good way to fly into a mountain, or another aircraft, or the
runway.

... then you know adjusted
pressure. considering the legal requirements, it is
perfectly adequate.

No, it is not. It's especially dangerous for RVSM flight, but it's so
inaccurate that it should never be used for anything, except as a last
resort (if the altimeters disintegrate, or whatever).

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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.