GPS altitude vs altimeter altitude

On Sun, 16 Apr 2006 19:58:19 -0500, Chris W wrote:

It seems as though those who actually read what I was really asking
didn't think it was important to find or know the answer so let me get a
little more specific. I am going to launch a remote control airplane
that has an autopilot. The autopilot has an altitude hold function that
is based on barometric pressure sensor. I will also have a GPS used for
guidance. The data from that GPS will be transmitted using APRS on
144.39 mhz to any amateur station listening. Once the autopilot is
turned on it will hold the pressure altitude it is at, so as it flies
along it's route (maybe as many as a few hundred miles) and the
barometric pressure changes the plane will climb and descend to maintain
the same pressure altitude. However the only data I will be getting
back is the GPS altitude. I need a way to do a reality check so if I
see the plane is descending or climbing I will know it is because of
changes in the barometric pressure and not the something that has gone
wrong. The plan is to get the latest METAR data from the closest
observation point to the current position of the plane and then do the
math compared to what it was where and when it launched so I will know
about what the GPS altitude should be reading as that is all I will be
able to see. For those who want to know why I don't just have it
transmit the pressure altitude back, I have four good reasons; cost,
weight, size, complexity. My first flights will be only 20 miles or
so. For safety I will be sure it steers clear of any class B, C, and D
air space. I'm not sure what pressure altitude I will have it fly at
.... probably somewhere between 1500' and 6000' AGL depending on the
distance for it to cover.

I do not believe there is any formulaic method to convert from GPS altitude
to pressure altitude.

GPS altitude may be more akin to true altitude, with variations based on
the precise geographic location that could be placed into a table.

But if altitude is varying with pressure altitude, bearing in mind that the
pressure altitude sensor is also sensitive to temperature, I think you have
your work cut out for yourself.

Perhaps you could graph the METAR derived data and compare it with the GPS
derived altitude, and if the trend (direction of change) is the same, be
satisfied that the aircraft is performing as designed.

If you had the lookup table to derive geographic position vs GPS altitude
error; and also something like a SKEW-T plot to look at temperatures aloft;
and also the METAR data, perhaps you could develop something to convert
true altitude (from your corrected GPS output) to assumed pressure altitude
reading on your altimeter) and draw some conclusions that way.

It sounds like an interesting problem.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)