Hey guys,
I'm in the process of building my second Arduino RC airplane (NOT a quadcopter) and would like to incorporate an altitude sensor for a Terrain Following Autopilot feature. Here are the constraints:
1.) If it's a proximity sensor, it has to be able to reliably detect the distance between the ground and the plane from 6 inches to at least 10 feet.
2.) It should cost less than $45.
I've looked at barometric pressure sensors, but they are very finicky (especially when introduced to vibration).
If anyone knows of a sensor I might be able to use, please let me know. I've been looking for one for months and would really appreciate it 
.
-PB
Barometric sensors will not have a clue about the ground contour. You need something that generates a reflection from the ground (either ultrasonic or radar type) to determine altitude above the ground. Not sure what you will find in your price range - ultrasonic is probably cheaper, but I would need a bunch of testing to see just how it responds to tall grass for example (which may absorb most of the signal) etc. The bottom line though is you either need something that gives a ground reflection or (much more expensive) knows absolute position and has a built in terrain map so it can fly based on the map information).
The Lidar Lite from PulsedLight3D should work. It has at least 40 meters range.