[ekianjo]

The problem is that every instrument involved in the chain of measurement has its own inaccuracies, and at the end of the day you would need to make sure that once you add each of their inaccuracies it does not compound up to more than what you are actually measuring. This is a very complex subject and I'm not sure it's as "settled" as you seem to portray it.

[teraflop]

Oh, sure, I don't mean to minimize the engineering challenges involved. I'm far from an expert in the details of how these particular satellites work; I'm just trying to describe the general principles, to make the point that this level of measurement accuracy shouldn't be viewed as intrinsically unattainable.