Stratus 2s Altitude Accuracy

hish747

hish747

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Hish747
I noticed this weekend that Stratus 2s altitude was off by about 200 feet. Has anyone else using the stratus had this problem? If so, is there a solution?
 
hish747 said:
I noticed this weekend that Stratus 2s altitude was off by about 200 feet. Has anyone else using the stratus had this problem? If so, is there a solution?
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Does it pressure or GPS altitude ?
 
My Foreflight always agrees with the 430w and the Aera 560. It never agrees with the altimeter and the transponder.
 
I'd put money on the stratus being accurate. My altimeter is always a little off in comparison. I've noticed when checking on flightaware that my recorded altitude is usually a little lower than what my altimeter read.
 
GPS altitude is basd on a standard datum plane which is NOT sea level or the ground. I trust the altimeter, if its been checked & cert.
 
AIM 1-1-17 tells pilots "Do not use GPS-derived altitude....", so the "error" is meaningless.

Bob Gardner
 
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murphey said:
GPS altitude is basd on a standard datum plane which is NOT sea level or the ground. I trust the altimeter, if its been checked & cert.
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You don't just trust, it you *go by it* because ATC is expecting you to, because all the other buzzards flying around are using it, too.

This, of course, goes out the window if your altimeter also goes out the window.
 
GPS altitude is true altitude and regardless of what modeling it uses is most likely accurate within feet. Altimeters are ancient technology and only estimate your height using a standard pressure gradient. This is why you need greater terrain separation in mountainous areas, the altimeter setting could be from an airport in the valley and therefore be off by quite a bit near the peaks. Obviously use the altimeter to comply with ATC, but don't trust it for close terrain separation.
 
Sgu2-22 said:
GPS altitude is true altitude ...
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... above a mathematical model ellipsoid that approximates the shape of the earth. IIRC most GPSs use WGS 84. The operative words here are "model" and "approximates."
 
Clark1961 said:
And there you have it folks. The pointlessly argumentative nature of POA captured in one line (again).
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There really is a distinction. Barometric altimeters do make errors, even when properly adjusted, at nonstandard temperatures. They are systematics and are highly reproducible. The point is that everyone makes the same errors, so it doesn't matter, except for terrain avoidance. And terrain avoidance is done either visually or with large margins (though there are correction tables, and it can get significant when really, really cold).

The distinction is much more obvious in the flight levels, where the pressure is always presumed to be standard. You can make enough of a difference there using the GPS altitude to put you in an opposite-direction RVSM airway. Not good at all.
 
rbridges said:
I'd put money on the stratus being accurate. My altimeter is always a little off in comparison. I've noticed when checking on flightaware that my recorded altitude is usually a little lower than what my altimeter read.
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Flightaware is likely using your Mode C altitude, which is pressure altitude.
 
MAKG1 said:
There really is a distinction. Barometric altimeters do make errors, even when properly adjusted, at nonstandard temperatures. They are systematics and are highly reproducible. The point is that everyone makes the same errors, so it doesn't matter, except for terrain avoidance. And terrain avoidance is done either visually or with large margins (though there are correction tables, and it can get significant when really, really cold).

The distinction is much more obvious in the flight levels, where the pressure is always presumed to be standard. You can make enough of a difference there using the GPS altitude to put you in an opposite-direction RVSM airway. Not good at all.
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This. A WAAS GPS like the Stratus will have excellent and very repeatable altitudes above the datum plane. This locates you very accurately with regards to the Earth. That's why LPV approaches work so well. But...it doesn't do as well in regards to other aircraft. The pressure altimeter is the standard there. It won't work as well in locating you above the Earth, but it will do a better job of separating you from other aircraft also using an altimeter, since the errors you see will be consistent with the errors they see. That's why the Stratus 2S has the pressure altitude, too. (Which is not very accurate! It's depending on cabin pressure, which is usually a tenth or two less than outside air due to venturi effects of the moving cabin. Don't fly altitude by a Stratus, either GPS or pressure.)
 
airdale said:
... above a mathematical model ellipsoid that approximates the shape of the earth. IIRC most GPSs use WGS 84. The operative words here are "model" and "approximates."
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An oblate spheroid, to be exact. Slightly flatter at the poles, bulging a bit at the equator.
 
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