How do I retrieve a signal from a modulated return?

I have a laser beam which I am modulating the intensity of with a psuedorandom maximum length sequence code. It propagates through the atmosphere, and some of the light is tered back- this is the atmospheric signal that depends on distance from the transmitter. I want to get the atmospheric signal (with all its valuable atmospheric information), but it is convoluted (?) with the psuedorandom code. I read that I need to cross-correlate the signal with the psuedorandom code- but what does that mean? Do you know what I should do to get my atmospheric signal?