The National Aeronautics and Space Agency of USA, NASA, are working on formation flying capabilities for spacecrafts. The first mission to use this technology is the gravitometry mission GRACE. The two GRACE spacecrafts will fly in close formation in low earth orbit, with a separation varying from 200 to 500 km. The spacecraft distance must be monitored with an accuracy of better than 1 micrometer/sec. IAU is together with the GRACE project at JPL developing the inter spacecraft attitude link which is a prerequisite for the mission objectives to be met.