Originally Posted by Alphactif
ultrasonic reflexion can perfectly work if the reflector (in this case the ground) is mobile ?
If it works imobile (with ground), it is going to work the same with mobile ground, because the speed of the plane is much lower than the speed of sound.
The Sparkfun sensor is nice, you would use the analog ports and create a variable frequency audio tone for the FPV transmitter. But again, only if it works, I think grass absorbs (ultra)sound..