Re: A question about GRACE
Odds are that sort of navigation system (or an enhanced version of it) will be used on an optical interferometry satellites. Optical interferometry requires a great deal of precision. GRACE is simply measuring the orbital disruptions between two satellites to map gravity fields. If memory serves, most optical interferometry satellite proposals are not located in Earth's orbit because there are too many perturbation effects. A mission like that might be better located at a LaGrange point.