Tilman,
We have used the Maxbotix before on a fixed-wing and had an issue with
it depending on the surface we were flying above. We were using it for
altitude (just as you are) when flying over grass and the diffraction
pattern of the ground caused the sonar module to have massive error in
its reading. When we tried it on a nice, relatively smooth surface
such as cement or concrete it worked great. I'm not sure if anyone
else has seen something like this before either, but just wanted to
let you know about an issue we had with it.