Setting altimeters with no radio

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

"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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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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"Jim Macklin" writes:

If I take-off in a airplane with no radio and fly through a
cold front in VFR, the altimeter setting is likely to change
enough to have a 500 foot or greater error with a 100 mile
flight. A GPS can be used to get closer than that.

Unfortunately, no, it cannot.

Probably, most NORDO flights are at or below 3,000 AGL under
VFR. Terrain clearance is more critical to such a flight
based on the altimeter than is the mid-air collision problem
since the "rule" only applies above 3,000 AGL.

GPS altitudes are especially dangerous to use near the ground, becuase
they are so inaccurate.

Someday, the rules may change, but within today's rules,
using GPS "as the best available" or only source is better
than no source at all.

True, if you are far from the ground and you cannot see the ground.
Don't trust it to keep you clear of terrain at low altitudes.

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"Jim Macklin" writes:

The point about GPS, it is accurate in relation to terrain.

It is dangerously inaccurate with respect to terrain.

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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:

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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Neil Gould writes:

I'm not sure why you think that "GPS is far less accurate than an
altimiter..." ...

Because I studied GPS for a long time and was using it before anyone
else knew what it was, and I know how badly it measures altitude. GPS
was designed to accurately measure longitude and latitude; at best, it
gives only a rough estimate of altitude (ships and troops, the
original users of GPS, don't need to know their altitudes).

... as an altimeter only need be accurate to 75' to be legal.

GPS is routinely off by hundreds of feet.

GPS can do much better than that ...

No, it cannot ... not vertically. It's actually much worse.

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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