I am interested in knowing. AFAIK AM modulation would have more range than FM, but maybe will not transmit much data. Try to know what they used for the moon :) and that should work!
I came across a page yesterday for hobbyist who transmit video data from their RC airplanes (pretty cool thing actually!), but it probably won't go as far as 20KM. It might be of interest to you, though, so I fetched it just for you:
http://rcforces.com/
(check out FPV > Video Transmitter and Receiver)