How-GPS-Works

You're at the right place for quick & easy answers about GPS. We answer basic questions about how GPS works and point you to other helpful resources on the net.

How does GPS work?
(the really simple answer)

• GPS satellites broadcast the time and data about their location

• a GPS receiver compares signals from at least three or four GPS satellites to determine it's own location

How does a GPS receiver calculate its location?
(the simple answer)

• A GPS receiver figures out how far away it is from each satellite based on how much time it takes a broadcast signal to travel from the satellite to the receiver. (A signal takes more time to reach you if you're farther away.)

• Since the location of each GPS satellite is known, the receiver's location can be determined by "triangulating" the distances from several satellites

For this exercise, you'll need to dust off some simple skills you learned in geometry. Start by imagining a "distance sphere" (in three dimensions) surrounding a GPS satellite. Points on the surface of the sphere are all the same distance from the satellite that's located exactly at the center.

• knowing distance from one satellite places you somewhere on a spherical surface that's centered around the satellite

• knowing distances from two satellites places you somewhere along a circle that's between the two satellites (defined by the intersection of their "distance spheres")

• distances from three satellites usually intersect at two points, and if you're not flying around, one of these points will be on Earth's surface

• distances from four or more GPS satellites will intersect at just one point

This process works by finding the intersection of your distances from three or more satellites. Thus, describing it as "trilateration" is actually better than "triangulation", but neither term seems precisely correct from a technical standpoint

How does a GPS receiver figure its distance from a GPS satellite?

Yeah, the home page did kind of gloss over a few details. Let's back up and take a closer look, starting with some basic physics:

• [distance] = [speed] * [travel time]

We know [speed] because GPS signals are electromagnetic waves, which travel at light speed (approximately 300,000,000 meters per second).
We can figure [travel time] by comparing [time sent] and [time received].

• Each GPS satellite has an on-board clock and includes [time sent] in the the signal it broadcasts.

• A GPS receiver also keeps track of time, which gives [time received]

• [Travel time] is the difference between these times.

Piece of cake! Right?