How Does a GPS Work?

How Does a GPS Work?

A GPS works by using satellites to send signals to devices here on the planet’s surface to indicate where we are. A compute with a database of driving directions usually provides directions based on the information that’s being received from the satellites.

The GPS system in my car is my lifeline. It gets me to lunch meetings, important dates, and around traffic when I’m traveling. I’ve never stopped to think how it works before, and the process by which GPS guides people around the world is fascinating.

What is GPS?

GPS, or Global Positioning System, uses satellites to send signals to specific pieces of equipment here on Earth. All GPS receivers work passively. meaning they receive signals from those GPS satellites, but they don’t transmit information back to the satellites.

GPS receivers, such as the one in your car’s navigation system, require unblocked access to the sky. That’s why these receivers are used exclusively outdoors. In fact, I’ve found that my GPS system doesn’t perform up to snuff when I’m near a big forest or in a big city. The trees and buildings block the access to GPS satellites.

How Does a GPS Work?

How Does a GPS Work?

Because GPS systems depend on having an accurate understanding of the time, these satellites are all tied to the atomic clocks at the U.S. Naval Observatory. Atomic clocks are the most accurate clocks we have; they use atomic degradation and frequencies to keep time rather than relying on mechanical means like a wristwatch. Every GPS satellite has at least one atomic clock on board.

GPS Transmission

GPS satellites constantly transmit information that tells both the satellite’s location and the current time where it is. This allows pinpointing in both time and space.

GPS satellites are all synchronized so that their operations and repeating signals are transmitted at the exact same instant. This keeps GPS positions standard worldwide. The signals they transmit, which move at the speed of light, arrive at GPS receivers at slightly different times because of the fact that some satellites are farther away than others. This distance (from the satellite to the receiver) is what allows the exact location of GPS satellites to be determined. The time it takes for a satellite’s signals to reach a receiver is estimated, and when a receiver makes this estimation to four different GPS satellites, that receiver can figure out its position in three dimensions.

As of January 2010, there are at least 24 GPS satellites operating at all times. These satellites, put in place and kept running by the U.S. Air Force, orbit the Earth in a 12 hour period. Air Force ground stations track each satellite’s orbit within a few inches.

Determining Position

A GPS receiver “knows” the location of the satellites because a satellite’s position is transmitted along with whatever other data it is transmitting. Taking in this data from four sources, a GPS receiver determines the size of several spherical orbits, one for each satellite that it is pinging for location. The receiver is located where these spheres intersect–quadrangular positioning. This is what makes our modern GPS systems so accurate.

GPS Accuracy

The precision of accuracy of a position given by a GPS receiver depends on the type of GPS receiver being used.

Most cheaper hand-held GPS units now have about a twenty meter accuracy (about sixty feet). This is plenty good for most basic operations, including navigational.

Other types of GPS receivers use a method called Differential GPS (DGPS) to obtain a level of accuracy that is much greater than twenty meters. DGPS requires an additional receiver fixed at a known location nearby for this increased accuracy. DGPS can pinpoint the receiver’s location within two feet. This kind of GPS accuracy is reserved for military or research purposes and is not available to the public.

Concerns about GPS accuracy led to the Air Force’s decision to initially limit the accuracy available to civilian GPS operators. When the system was created, timing errors were intentionally inserted into GPS transmissions to make sure that non-military GPS receivers were only accurate within a few hundred feet. In the year 2000, this feature was removed.

GPS makes our lives easier. It helps us find a pharmacy or a drive-through restaurant when we really need them and military and scientific applications are even more important. Now that you understand how GPS works, you can appreciate the effort put into powering your Garmin on a daily basis.

As of January 2010, this Garmin GPS is the topselling GPS on Amazon. If you don’t already have a GPS, you could do a lot worse than to buy one using our link. (We make a tiny commission too.)

This is part of a series of blog posts we’re publishing about How Stuff Works. The other posts in this series include: