Sattelite Navigation
T o determine any location on Earth satellite navigation systems like GPS use signals from satellites which provide extremely accurate times. By finding their delay a computer can calculate how far away the satellites are. If you know the distance from at least three different satellites and the position of these satellites you can find the unique and exact position of the receiver on Earth. A metric space is the mathematical generalisation of measuring distances. Because of general relativity GPS receivers have to use the Kerr metric rather than the usual Euclidean metric.