Where to post questions?

WhenTurtlesFly writes:

'Cause I have one- Filed to cruise at 8500. Two GPS's (old simple one
in the plane and my new portable) say I'm at 8500, but pressure
altimeter says I'm at 8000. I of course set my altimeter to departing
airport elevation, which should calibrate it to current pressure and
nonstandard temperature, right? Do I...

* fly what seems to be accurate and correct readouts on both of my
GPS's? Then risk my altimeter being off and my Mode C reporting
incorrectly...

No. The altimeter is more accurate than the GPS units. The fact that the two
GPS units agree simply means that they are both subject to the same error when
operating in the same place at the same time.

* fly GPS's and adjust altimeter to read the GPS altitude, even though
the altimeter setting is not what is reported?

No. The altimeter is more accurate than the GPS units.

* fly the altimeter set at reported conditions, have my Mode C report
accurately, but run the risk of being off VFR altitude?

Yes. Fly the altimeter. The risk of being 500 feet off is virtually nil if
you've set the altimeter correctly to begin with.

GPS provides very poor vertical accuracy compared to a barometric altimeter.
GPS is designed for lateral accuracy rather than vertical accuracy.

Mxsmanic wrote:
WhenTurtlesFly writes:

'Cause I have one- Filed to cruise at 8500. Two GPS's (old simple
one in the plane and my new portable) say I'm at 8500, but pressure
altimeter says I'm at 8000. I of course set my altimeter to
departing airport elevation, which should calibrate it to current
pressure and nonstandard temperature, right? Do I...

* fly what seems to be accurate and correct readouts on both of my
GPS's? Then risk my altimeter being off and my Mode C reporting
incorrectly...

No. The altimeter is more accurate than the GPS units. The fact that
the two GPS units agree simply means that they are both subject to the
same error when operating in the same place at the same time.

His new portable would almost certainly use WAAS when available and would
not be off by 500 ft.

* fly GPS's and adjust altimeter to read the GPS altitude, even
though the altimeter setting is not what is reported?

No. The altimeter is more accurate than the GPS units.

WAAS is accurate to 25 ft vertically 95% of the time. Standard aneroid
altimeter have no adjustment for changes from non-standard temperature. A
temp difference of ~4 degrees Celsius from standard atmosphere would show a
500 ft difference.

* fly the altimeter set at reported conditions, have my Mode C report
accurately, but run the risk of being off VFR altitude?

Yes. Fly the altimeter. The risk of being 500 feet off is virtually
nil if you've set the altimeter correctly to begin with.

GPS provides very poor vertical accuracy compared to a barometric
altimeter. GPS is designed for lateral accuracy rather than vertical
accuracy.

Which is why WAAS was developed.

Jim Logajan writes:

His new portable would almost certainly use WAAS when available and would
not be off by 500 ft.

WAAS is independent of GPS. It's one of many GPS augmentation schemes.

WAAS is accurate to 25 ft vertically 95% of the time. Standard aneroid
altimeter have no adjustment for changes from non-standard temperature. A
temp difference of ~4 degrees Celsius from standard atmosphere would show a
500 ft difference.

So if you depend on your WAAS GPS for altitude, you might collide with traffic
that is depending on barometric altitude.

Which is why WAAS was developed.

WAAS was developed to improve GPS accuracy overall, not to allow GPS to
replace traditional barometric altimeters.

Mxsmanic wrote:
Jim Logajan writes:

His new portable would almost certainly use WAAS when available and
would not be off by 500 ft.

WAAS is independent of GPS. It's one of many GPS augmentation schemes.

WAAS cannot simultaneously be independent of GPS and an augmentation of it.
Only the second of your two quoted sentences is accurate in any useful way.

While your conclusion that he should use the altimeter set to the nearest
station setting was correct, the rationale leading up to it was based on an
invalid premise.

Jim Logajan wrote:
Mxsmanic wrote:
WhenTurtlesFly writes:

'Cause I have one- Filed to cruise at 8500. Two GPS's (old simple
one in the plane and my new portable) say I'm at 8500, but pressure
altimeter says I'm at 8000. I of course set my altimeter to
departing airport elevation, which should calibrate it to current
pressure and nonstandard temperature, right? Do I...

* fly what seems to be accurate and correct readouts on both of my
GPS's? Then risk my altimeter being off and my Mode C reporting
incorrectly...

No. The altimeter is more accurate than the GPS units. The fact that
the two GPS units agree simply means that they are both subject to the
same error when operating in the same place at the same time.

His new portable would almost certainly use WAAS when available and would
not be off by 500 ft.

* fly GPS's and adjust altimeter to read the GPS altitude, even
though the altimeter setting is not what is reported?

No. The altimeter is more accurate than the GPS units.

WAAS is accurate to 25 ft vertically 95% of the time. Standard aneroid
altimeter have no adjustment for changes from non-standard temperature. A
temp difference of ~4 degrees Celsius from standard atmosphere would show a
500 ft difference.

The temperature correction is affected by height. The rot is 4 feet per
thousand feet for each degree C of difference from standard. A 500 ft
difference at 4 degrees would only be true at 30,000 feet. So at my 250
foot high airport, it would have to be -35C to get my altimeter to be
50 feet high. I don't think I've ever seen it more than 20 feet off when
set to the broadcast setting on the ground.

* fly the altimeter set at reported conditions, have my Mode C report
accurately, but run the risk of being off VFR altitude?

Yes. Fly the altimeter. The risk of being 500 feet off is virtually
nil if you've set the altimeter correctly to begin with.

GPS provides very poor vertical accuracy compared to a barometric
altimeter. GPS is designed for lateral accuracy rather than vertical
accuracy.

Which is why WAAS was developed.

--
Don Poitras

"Stephen!" wrote:
Jim Logajan wrote in
:

His new portable would almost certainly use WAAS when available and
would not be off by 500 ft.

It could very well be 'off' by that much quite easily. Keep in mind
that the earth is neither smooth nor a sphere. Depending on where you
are on the theoretical sphere that the GPS uses for its calculations,
its altitude calculation could be off from actual AGL by quite a bit.
I saw that quite frequently on Guam where I would be in a boat and the
GPS would swear we were a couple hundred feet below sea level.

I don't believe WAAS is yet available in Guam.

Mxsmanic wrote:

GPS provides very poor vertical accuracy compared to a barometric altimeter.
GPS is designed for lateral accuracy rather than vertical accuracy.

Babbling nonsense when talking about aviation GPS.



--
Jim Pennino

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