Image credit: Scott Ehardt, Wikimedia Commons; image source; larger image
How does GPS work?
The photo shows a Global Positioning System satellite. To find out how the system works, visit How does GPS work?. Be sure to see the video to understand how three or four different GPS satellites specify your position on Earth.
Einstein's Relativity and Everyday Life
If you rely on a GPS to navigate, then you need Einstein's theories of relativity--special relativity because the GPS satellites are moving fast relative to you, and general relativity because the GPS satellites are in a different gravitational field than you are. Without correction for relativistic effects, errors in GPS positions would increase by 10 km for every day the system operates! To learn more visit Einstein's Relativity and Everyday Life, and also this Ohio State astronomy page.