GPS

If you have used a GPS navigation system in a car, you know how valuable it can be. How does that little electronic device know exactly where you are? It relies on a system of satellites orbiting the earth about 12,600 miles up. At any location, the GPS receiver should be able to see at least 6 of these satellites, each of which transmits information about the locations of all the orbiting satellites along with precise time stamps. The GPS receiver is not capable of communicating back to the satellites, so it listens to these signals and, with lots of mathematics, determines its location.

Specifically, the GPS receiver can determine the synchronized time by comparing messages from different satellites. It can then determine precisely how long each signal took to travel from the satellite to the receiver. The signals travel at the speed of light, so a quick computation tells the device how far away it is from each satellite. On the program this week we showed how having only this information--the distances from several satellites--the GPS receiver can determine exactly where it is on the planet using a process called trilateration. How many satellites are needed to make this happen? The more you have the better, but a minimum of 4 is required (3 if you use information about your elevation on earth).

For more information, check out this very detailed Wikipedia entry.