Fabrice we are on our way toward making the quadcopter airframe. To keep my end up I need to make either a timer based or software extended PWM on the AVR Butterfly. May be it will be worth getting another more modern microcotroller board like the Teensy which has more PWM channels.
In one of the Cape Town IGARSS a presenter making a remote sensing UAV wanted ideas for mounting a mini radar on it. Andreas Reigber suggested a FMCW (Frequency Modulated Continuous Waveform) Radar scanner. I have seen Laser range finders mounted on quadcopters but not a radar yet. It is time to put one on and find the needles in the haystack, because with radar you can if the haystack is dry enough and the needle is not made of bone.