How do I cast a shadow on a platform?

I am trying to make simple circular shadows for a platformer which has a bit of perspective, so not entirely flat.

So essentially the shadow would follow the X of the character but each time the character is above (either jumping/falling/landing) a new platform, the shadow would need to follow beneath it.

What would the best way of doing this be? I was looking everywhere but could not really find anythign concrete.

Cheers

Here is my version:

dropbox.com/s/3kjdpsn1nizae56/PlatformerWithShadow.capx

Note that the shadow has Source Atop blend mode and should be on the same layer with platforms.

Thanks, I have found a similar solution (although more complicated), I will try to implement yours and see if I can make it work.

Originally I Was trying to have the shadow constantly be cast beneath the character, even when he is jumping, but perhaps this won't be necessary.