Hi guys,
I am embarresed to ask this one, but I am hoping that it is as simple as it sounds, and its not too much of a waste of time to ask! 
I have footage of one of our ships flying against a blue screen, I have keyed it, and dropped a nice moving lake background behind it. The camera in the background footage was placed on a boat, and shot off the back. This gives nice movement, and so the bobbing ship flies nicely towards us. I want to duplicate this layer, make it slightly transparent and us it as a reversed shadow. the expression I am hoping to get, is the one that will allow the shadow to move up and down, in the oposite direction to the original layer - so when they ship flies a little higher, the shadow drops a little lower - (I hope that makes sense - it did in my head! 
)
Thank you to all.
Pierre Devereux