Not a solved problem, GPS is too inaccurate for interferometry. The problem is you want to do correlations between radio signals; consider a 1 GHz signal, which has a 30 centimeter wavelength. Error compounds as you integrate longer and longer signals. To do anything decent, you need to know absolute position within millimeters, and it can't be drifting too much.

Also, the positioning error isn't the whole story; there's also timing drift. Rubidium GPS-disciplined oscillators actually drift too much instantaneously to be useful...