What is GPS?

This article is a brief introduction of how Global Positioning works. This article is intended for the general public and will be considered trivial for advanced users.

The current Global Positioning System is a constellation of 32 satellites that obit the earth in six orbital planes. As these satellites orbit the earth they transmit streams of encoded data intended for users on the ground.

It is important and fundamental to realize that GPS receivers, are just that, receivers, they are not transceivers. Your units CAN NOT communicate in a two-way fashion with the satellites.

So what kind of data are the satellites transmitting? For simplicity reasons assume the satellites are all transmitting a signal that contains the answer to the questions:

  • “Here is the Time”
  • “Here is where I am at”

Knowing the amount of time that a signal needs to reach our receiver we can compute the distance from the satellite to our receiver.

With that information from a minimum of four satellites your receiver my compute it’s position utilizing a process called trilateralization.

Update: January 22, 2011

I’ve posted a 2D trilateralization example . I hope to have a 3D example up soon. Dave.

Important Concepts

Trilateralization is a process similar to the Side-Side-Side theorem of a triangle, except it works on several triangles at once.

Trilateralization, in general, can yield no solution, one solution or more than one solution and is often solved utilizing Least Squares Adjustments.

Typically three measurements from three points can yield a 3d intersection, however, GPS requires a fourth satellite to provide an independent reference to the time factor.