Over my 57 career years, I have been privileged to help in the development of GPS in the area of precise timing which is necessary for telecommunications, the internet, and the precise synchronization of cell phone towers. It has been an exciting ride. These three technologies have made an enormous impact on society. Technically, they are well described in the world wide encyclopedia, Wikipedia. (see links below)
Learn How GPS Works in Five Minutes!
Get Ready to Paint a Picture in Your Mind
Think in your minds eye that you can look up into the sky and see three GPS satellites in very different parts of the heavens. Think further that you have three looooooooooooooong measuring sticks that are able to reach out from you to each of the three satellites, so that you have a way of measuring the distance to each of them.
The picture you have is an upside down tripod with you at the focal point, and with each of the three GPS satellites at the end of each stick. This, of course, is totally absurd in practice, but there is a fascinating way to do the equivalent that is practical using the speed of light as your measuring stick. If you are traveling in your car at 60 miles per hour (~90 kph), and you travel for an hour, you know the distance you have traveled is 60 miles.