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:

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

GPS uses a theoretical model of the earth's surface--a geoid. It may
or may not coincide with sea level.

However, that's not the main source of inaccuracy in GPS altitudes.
The main source is the difficulty of measuring altitude when the
position to be measured involves so many very oblique angles from the
reference satellites. It's easier to measure lateral position looking
straight down than it is to measure vertical position looking at a
shallow angle from 1000 miles away.

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Mxsmanic schrieb:

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

GPS uses a theoretical model of the earth's surface--a geoid. It may
or may not coincide with sea level.

Which is as theoretical a value.

Stefan

mike regish schrieb:
Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

GPS altitude gives you a geometrically derived value above some
reference surface. Mostly it's the WGS84 geoid. Of course this can be
converted to MSL altitude if you wish. But this isn't the point.

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.
It may be true in very specific conditions, but the real atmosphere very
seldom matches the theoretical ISA atmosphere model on which the
altimeter is based. And, most important, there is no way to calculate
the real (geometric) altitude from the displayed pressure altitude. (Of
course it could, if you knew the complete atmosphere profile, but you
usually don't.)

So the point is: GPS gives you the true altitude in reference to the
earth, but this doesn't help you, because the whole aviatic system
(airspace boundaries, ATC clearances, traffic separation) is based on
pressure altitude, and there is no practical way to convert one into the
other. 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 use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

But I'm sure you knew all this before, because after all, you've passed
a written.

Stefan

Gee, that was actually a well thought out, well explained, and in my case
anyway, very informative post.

It's been 9 years since my written, but I don't remember a whole lot about
geoid reference in it. I do remember the WSG84 reference in my handheld
(which I never use for altitude, BTW) though, so it must have been there.

mike

P.S. I got a 96


"Stefan" wrote in message
...
mike regish schrieb:
Well, what exactly are their reference datum(s?).

GPS altitude and pressure altitude are completely different and, most
important, *incompatible* systems.

very knowledgeable post snipped

But I'm sure you knew all this before, because after all, you've passed a
written.

Stefan

Stefan wrote:

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

Most of what you said is correct except this one. My belief is that
GPS altitude is fine for terrain separation. But not for aircraft
separation which you correctly state is based upon pressure altitude.

Ron Lee

"Ron Lee" wrote in message
...
Stefan wrote:

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

Most of what you said is correct except this one. My belief is that
GPS altitude is fine for terrain separation. But not for aircraft
separation which you correctly state is based upon pressure altitude.

Ron Lee

Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he can
give you an education on this subject.

Karl

"karl gruber" wrote:


"Ron Lee" wrote in message
...
Stefan wrote:

The only use for GPS altitude in aviation is to calculate the final
glide of a glider. (Or an IFR approach, but I don't know enough about
IFR flight to comment this.)

Most of what you said is correct except this one. My belief is that
GPS altitude is fine for terrain separation. But not for aircraft
separation which you correctly state is based upon pressure altitude.

Ron Lee

Your belief is wrong. Could be DEAD wrong. Read MX's posts...........he can
give you an education on this subject.

Not sure what he can help on since I understand GPS quite well. I
also fly in mountainous terrain which I doubt that he does.

Ron Lee

Ron Lee writes:

Not sure what he can help on since I understand GPS quite well.

If you think you can use GPS safely for altitude in aviation, you need
to understand it better.

I also fly in mountainous terrain which I doubt that he does.

In that case, you need to understand GPS better very soon.

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