Do you have any evidence or math to support your claim?
How do you propose that we measure 6 hundreths of a millimeter convergance of an object traveling around 180 km/hr?
...
First, you question is flawed in several ways:
1) The convergence is of two objects.
2) Your question assumes a given height and initial separation.
3) Your concern about speed is irrelevant. We need only determine the location that each object passed the "ground line".
4) Your concern about measure is irrelevant. We need only determine that the objects converge, not by how much, to conclude that the Earth is round.
Second, yes, we can, and have, measured to such accuracy. Ask Jack about measuring the tidal forces of GWs.