A radio telescope at the Kokee Park Geophysical Observatory, NASA's VLBI station in Hawaii, one of the most active sites in the global geodetic/astrometric VLBI network (Image: US Navy/PMRF)
Global Positioning System (GPS) devices have permeated society to the point where millions of us rely on them daily for directions, locations and traffic avoidance (if only they could tell me where I left my car keys). GPS satellites send signals to a receiver in your handheld or car-based GPS navigator, which calculates your position on the planet based on the location of the satellites and your distance from them. The distance is determined by how long it took the signals from various satellites to reach your receiver. But have you ever thought what tells the GPS satellites where they are in the first place?