A question about GRACE
"Now in orbit, the two spacecraft will chase each other around the Earth 16 times a day at an altitude of 311 miles ( 500 kilometers). Separated by 137 miles (220 kilometers), a precise microwave ranging system will constantly measure the distance between the two satellites to within the equivalent of one-tenth the width of a human hair."
The quote above is from a Space.com article on the GRACE mission. I was wondering if the positioning system mentioned could be used on a constellation of optical interferometer spacecraft?