Setting altimeters with no radio

"Stefan" wrote in message
...
mike regish schrieb:
The point is that the pressure altimeter measures, well, a pressure, not
an altitude. It displays a value in feet, but actually, this is wrong.

Certainly true.

... the whole aviatic system (airspace boundaries, ATC clearances, traffic
separation) is based on pressure altitude

I agree with that statement too.

.. If you are given an ATC clearance for a certain pressure altitude but
fly GPS altitude instead, then you act exactly like that bozo who drives
on the wrong side of the road.

The only ATC clearances for a pressure altitude would be in the flight
levels. Since the question was about setting a pressure altimeter, I would
say that the flight levels are irrelevant. Below the flight levels, ATC
clearances are for pressure compensated altitude above MSL, so yes it is
based on pressure and not true altitude but close enough. My GPS gives me a
calculated altitude above a theoretical sea level that's also close enough.

-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte writes:

The only ATC clearances for a pressure altitude would be in the flight
levels. Since the question was about setting a pressure altimeter, I would
say that the flight levels are irrelevant. Below the flight levels, ATC
clearances are for pressure compensated altitude above MSL, so yes it is
based on pressure and not true altitude but close enough. My GPS gives me a
calculated altitude above a theoretical sea level that's also close enough.

"Close enough": famous last words.

--
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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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mike regish writes:

Do those amounts vary with location?

Yes, but unfortunately that is only one of the variables. They also
vary dramatically with the changing positions of the satellites above
(and the satellites move significantly from one minute to the next).
Atmospheric conditions and other factors also come into play.

Overall, though, the problem is that the satellites are positioned to
optimize lateral navigation--but that also positions them in a way
that is unfavorable to vertical navigation. The vertical accuracy can
never be as high as the lateral accuracy; it can't even come close.
That's the way the system is designed.

Perhaps one day GPS will provide altitudes accurate to within a few
feet; but if it does, then by that time the lateral positions will be
accurate to within millimeters.

How large a difference is there between the 2?

It varies by location and conditions. I'm not sure of the exact
differences. One problem is that the geoid altitude is fixed, whereas
pressure altitudes depend on atmospheric conditions.

If everyone used GPS for altitude, it would probably work out okay in
some cases, particularly at high altitudes. But as long as anyone is
using a conventional altimeter, the differences are large enough to be
dangerous.

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.

I prefer to reserve GPS for lateral navigation only. For that it
works quite well, on a par with other navaids (depending on various
factors). For altitude it's a waste of time. Better to have a
standard altimeter and a radar altimeter for terrain.

--
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My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.

No. It isn't!

Karl
"Doug" wrote in message
oups.com...
My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.

In rec.aviation.piloting karl gruber wrote:
No. It isn't!

Don't know about the 90B, but my 89B has an input from the transponder/encoder.
You can use it for advisory VNAV. Since the input is pressure altitude, you
need to keep the pressure setting current. Mode C is only accurate to 100
feet though, so you will see it jump up and down by that amount. There is
also a page that shows the GPS calculated altitude. I've never seen it closer
than a couple of hundred feet to the true altitude. (on the ground; it could
be more acurate in the air, but I'd only be comparing it to the transponder
or the regular altimeter and maybe they go crazy in the air, but they always
agree within 100 feet of each other.) Sometimes I fly 50 feet higher or lower
to make the GPS presure-supplied altitude read my assigned altitude so that
my flight-aware track (and ATC) believe that I can maintain an altitude as
well as an auto-pilot.


Karl
"Doug" wrote in message
oups.com...
My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.


--
Don Poitras

Doug writes:

My IFR GPS, a King KLN90B is connected to the altimeter in my
transponder. It is also adjustable to the barometric setting.

It's the altimeter that provides the accuracy for measurement of
altitude in that case, not the GPS.

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

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.