Baro/GPS disagreement

I'm a little baffled at the huge difference between my GPS altitude
and the pressure altitude when I get up high. Yesterday on the
ground, I showed around 10 feet difference between the GPS altitude
and the pressure altitude. Up around 17000 feet, the GPS indicates
over 1000' higher. I also noted that the indicated pressure altitude
from the FR closely matches the altimeter (and the FR was recently
calibrated and no such huge errors noted). Is it possible that (at
least where I fly) the air just cools more rapidly than the accepted
model, lowering the indicated pressure altitude?

Jim

On Jul 6, 5:23 pm, wrote:
I'm a little baffled at the huge difference between my GPS altitude
and the pressure altitude when I get up high. Yesterday on the
ground, I showed around 10 feet difference between the GPS altitude
and the pressure altitude. Up around 17000 feet, the GPS indicates
over 1000' higher. I also noted that the indicated pressure altitude
from the FR closely matches the altimeter (and the FR was recently
calibrated and no such huge errors noted). Is it possible that (at
least where I fly) the air just cools more rapidly than the accepted
model, lowering the indicated pressure altitude?

Jim

How do you think that convective lift is generated? It is by the
atmosphere not being adiabatic (which is what altimeters assume).

Tom

I'm not an expert, so this response is pretty crude. You may find
more info by searching the archives...

GPS altitude is the least precise of its measurements, primarily
because position is best determined by satellites on the horizon, and
altitude is best measured by satellites above (and if possible) below
your position. So it will vary more than position.

Pressure altitude is just that, a measurement of air pressure. In a
perfect world, you'd set the altimeter to 29.92, climb to 20,000' AGL,
then drop a line that's 20,000' long and it would just touch the
ground. But air isn't perfect so you have to adjust for temperature,
humidity, and whatever else to find out *precisely* how high you are.

Lucky for all of us flying types, we really don't need to do this as
long as we all use a decently calibrated altimeter and get a nearby
altimeter setting from a trusted source, such as AWOS. Now, all
aircraft in your vicinity are flying at the same, most likely,
incorrect physically, altitude, but it's all relative and as long as
all our altimeters indicate a different altitude, we don't bump into
each other.

This is the reason that using pure GPS altitude is a bad idea for
vertical navigation at the current time. Because not everyone has a
GPS altimeter, and because each receiver may come up with a different
solution for altitude, it's quite likely that two aircraft at
different indicated altitudes, could very likely be at the same
altitude.

I know this is very simplistic, but I think you get the point.

You *could* use GPS to measure an altitude gain, but you could NOT use
it to verify, for example, compliance with Class A airspace in the
USA.

-Tom

On Jul 6, 5:23 pm, wrote:
I'm a little baffled at the huge difference between my GPS altitude
and the pressure altitude when I get up high.

I had the same question a few years back and finally figured out that
on a warmer than standard day -- which is what we see in the summer --
pressure altitude up high will read lower than true altitude due to
the air mass expanding as it heats.

As a simple example, suppose you are using the altimeter setting for
an airport essentially at sea level. For a standard atmosphere (which
includes a sea level temp of 59 degrees F), the half atmosphere point
is roughly at 18,000 feet, so when your altimeter reaches half an
atmosphere, it will read 18,000' and be right.

But on a warmer than normal day, the whole atmosphere expands and that
half atmosphere point will be higher up, maybe 19,000'. But your
altimeter knows nothing about temperature, so it will read 18,000'
when you are really at 19,000'.

Your GPS will say 19,000' when your altimeter says 18,000'. As far as
ATC is concerned, you're at 18,000', but in reality you are at 19,000'
MSL (modulo the error of the GPS, but that's only a couple of hundred
feet usually, even though as another poster pointed out, altitude is
less accurate than latitude and longitude).

If you want to check this out, take your GPS on a car trip and note
the altitude it reads at various passes or other places where the
highway department has a sign noting altitude. A friend of mine who
did this on a transcontinental trip told me that the GPS was always
within 50' of the marked altitude.

I've heard, but never confirmed, that on warm days ATC will not put
any IFR traffic in the flight levels lower than about 20,000' to avoid
potential errors of this nature.

This is also the reason that higher altitude airports tend to have
higher altimeter settings. The altimeter setting is obtained by
getting the altimeter to read the airport's true altitude and that
will require a higher altimeter setting for higher altitude airports
on warm days.

The opposite problem occurs on colder than standard days so your
altimeter (and soaring computer, if you have one) will be overly
optimistic about making airports. That's a potential danger, but most
of our soaring is on warmer than standard days.

Hope this helps.

Martin

I've heard, but never confirmed, that on warm days ATC will not put
any IFR traffic in the flight levels lower than about 20,000' to avoid
potential errors of this nature.

FAR 91.121(b), Lowest Usable Flight Levels are predicated on Altimeter
Settings, not temperature, but yes, high temperatures are associated with
abnormally high pressure.


This is also the reason that higher altitude airports tend to have
higher altimeter settings. The altimeter setting is obtained by
getting the altimeter to read the airport's true altitude and that
will require a higher altimeter setting for higher altitude airports
on warm days.


Altimeter settings are "adjusted to Sea Level", the altimeter is referenced
to sea level. True.
Altimeter settings to read the airports true altitude for warm days will be
higher than for colder days. Warm days are associated with High Pressure
weather patterns.

Yes, a set volume (column) of air, at standard sea level pressure will have
equal pressure readings at sea level and at a set "physical true" altitude
or elevation above sea level. Chill the column significantly and the
physical true altitude for the same pressure reading will be lower, raise
the temperature, and the column will expand and read a higher true altitude
for the same pressure altitude.

BT