Measuring Plate Motion
from Space
You are likely familiar with the Global Positioning Sys- tem (GPS), which is part of the navigation system used to locate one’s position in order to provide directions to some
other location. The GPS employs satellites that send radio signals that are intercepted by GPS receivers located at Earth’s surface. The exact position of a site is determined by simultaneously establishing the distance from the receiver to four or more satellites. Researchers use specifically designed equipment to locate the position of a pointon Earth to within a few millimeters (about the diameter of a small pea). To establish plate motions, GPS data are col- lected at numerous sites repeatedly over a number of years.