The Global Positioning System is based on a "constellation" of satellites that emit timed signals. Developed as a navigation (and weapons guidance) aid, GPS is now used as a very precise surveying tool. Under good conditions (and repeated fixes over several days), sites can be located within 2 cm. So repeat surveys over several years can determine accumulated plate motion. On this site there are examples of rotation of a single plate (Antarctica) and displacement fields within the deforming Anatolian/Aegean area.