We must have some incredible minds at NASA to take on a project this complex. Over 3 million miles the spacecraft would have to account for so many variables it's mind boggling, gravitational lensing from the sun should bend the laser light, open space isn't a perfect vacuum so the the light should slow down by a very small margin, redshift between 3 million miles would be incredibly miniscule, but at that accuracy very significant.... off the top of my head.
More is explained here, thanks and credit go to www.popsci.com.... again.