What is GPS and how does it work ?

GPS is the modern way for finding out where you are, something which people have been doing forever. They also wanted to be able to tell others so they use the info to make maps as seen here.

Africa 1812

To make old maps accurately and locate their position, navigators and map makers (cartographers) such as Christopher Columbus or Ptolemy made great maps for their time. However accuracy was always a problem - just look at the number of shipwrecks in those days !

To view a collection of old maps made the old fashio way - visit http://www.davidrumsey.com/view.html

Today maps are made with electronic technologies such as GPS and GIS.  GPS also tells you where you are for many other uses besides map making - like finding your way in a new city or that favorite fishing spot you loved last year !

GPS is really just about math - and really basic geometry that you learn in school. So you remember those triangles with right angles !!!

Above the earth are 24 satellites whose job is to send out distance signals converted from time.

Here's how they do the math:

2-D Trilateration
(- courtesy of How Stuff works at http://electronics.howstuffworks.com/gps2.htm)


Imagine you are somewhere in the United States on vacation and you are TOTALLY lost -- for whatever reason, you have absolutely no clue where you are. You find a friendly local and ask, "Where am I?" He says, "You are 625 miles from Boise, Idaho."



This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles, like this:


You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota." Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis.



If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are -- Denver, Colorado.



This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles.

This is called 3-D trilateration.

3-D trilateration

Here's how it works in 3 Dimensions (round earth) vs 2 Dimensions (flat earth).

First we get the signals from the satellites and measure the time which gets converted to distance.

Now lets see it really working to do the math - http://www.trimble.com/gps/triangulating1.html