IQue 3600 Elevation Problem?

Possible issue is multipath of the GPS signal.  Inside a commercial
airplane has got to be one of the worst GPS environments.

- John

The reception was very poor and I only had 3 weak signals for most of the
trip.  Thank you, Peter, for explaining this to me.  --Rip

Just a point of order about altitude and GPS. In all the GPS for beginers
books it states that the altitude is not accurate. If you want accurate
altitude you have to use a calibrated altimeter.

Yes, this is often stated, but that doesn't make it true.  Back when SA was
active the altitude number jumped around by hundreds of feet leading many
to conclude that GPS was almost worthless for altitude.  Of course the
horizontal positions also varied by almost as much but we weren't as upset
by that since we rarely know our horizontal position to such accuracy but
can readily see that we are 20' above sea level rather than the indicated
300' below.

Now that SA is gone I've found the elevation to be accurate within 40'
about 95% of the time using a non-WAAS receiver provided I have decent
satellite reception (>4 sats with reasonable geometry).

Pressure-based altimeters have their own set of measurement issues.  If
recently calibrated they are very good at measuring small changes in
elevation from the calibration point, but large altitude changes can give
incorrect results if the atmospheric conditions don't match the "standard
model" that's used to relate pressure to altitude.  Variations in the
temperature and lapse rate from the standard model can result in errors of
well over 100' of even a recently calibrated altimeter when it's taken up
thousands of feet from the calibration point.  I would put much more faith
in the absolute accuracy of a GPS altitude in that circumstance provided it
was getting good, unobstructed satellite reception unless I had detailed
information of the temperature variations of the atmospheric column of air
at my location so I could correct the pressure-based altitude.

[Of course if flying and needing to maintain vertical separation from other
planes with pressure altimeters I would also use a pressure altimeter (set
to the nominal 29.92" of Hg at sea level). That way my error would match
that of other pilots in that area and we'd all stay safely separated.  But
my GPS would be likely to give me a more accurate measure of my actual
altitude.]

Sounds like it was operating in "2D" mode for awhile resulting in the
constant 33000' reading and then switched to "3D" mode and corrected the
altitude reading.
In order to calculate time, lat., long., and altitude, the GPS receiver
needs to be receiving signals from at least 4 satellites.  If it only gets
signals from 3 (or sometimes 4 or more but with a bad geometry - e.g. all
in a line), then it can't solve for all 4 variables and it assumes the
altitude isn't changing so it can solve for the remaining 3 (time, lat.,
long.).  When it does this it'll report that it has a "2D GPS Location".
Usually the fixed altitude is a pretty good assumption since we don't
frequently have rapid altitude changes.  But in a plane that's descending
the assumption isn't a good one and will also lead to horizontal position
errors in addition to reporting an obviously wrong altitude.  Once  the
unit gets better satellite reception it'll switch to "3D GPS Location" and
correct the altitude and also provide a much more accurate horizontal position.

The cure is to have better satellite reception.  Holding the antenna right
up against the window will help and an external antenna that can be held
against the window would be even better.  Check the satellite page to see
how many signals are being received and what the geometry is like.