How Accurate is GPS?

Get to know how GPS works.

It’s pretty mind-boggling when you consider a satellite orbiting 12,551 miles above the earth’s surface can transmit a signal to find your location with a GPS receiver. What’s even more impressive is the GPS signal in space provides a “worst case” pseudo-range accuracy of 7.8 meters, 95 percent of the time. But to truly understand what determines GPS accuracy, it helps to know how GPS works.

The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation and timing services. It was created by the U.S. Department of Defense, and was originally based on and run with 24 satellites. Today, nearly 30 active satellites are in space transmitting signals to GPS receivers on the earth’s surface. Here’s a little known fact: the GPS signal can be used without a fee by anyone who has a GPS receiver.

Of course, there are several factors outside of the government’s control that influence GPS accuracy. Some of the more common factors include:

  • Atmospheric effect: Any changes in atmospheric condition can affect GPS radio signals as signals travel through the thick layer of the atmosphere. Both ionosphere and troposphere can leave little or more impact on the speed of the radio signals.
  • Multipath errors: This occurs when signals are reflected or bounced by coming in contact with surrounding hills, lakes, buildings or any kind of hard core object before it reaches the receiver. This delay in signal travel time is what causes the error. Fortunately, most new GPS receivers have specialized antennas that are very effective at minimizing these errors.
  • Clock limitations: The internal clocks of both the satellite and receiver have limited accuracy, and they’re not precisely synchronized. Since position calculations are highly dependent on accurate timing information, small clock errors can cause significant errors in position computations.
  • Satellite geometry: This is the exact position of the satellite at any given point in time. When satellites are located in wide angles from each other, it becomes easy to measure distance. Conversely, poor satellite geometry occurs when they are in a line or in a tight grouping.

Higher accuracy is also available by using GPS in combination with an augmentation system. Augmentation systems help GPS accuracy by providing real-time positioning to within a few centimeters. Some of the more widely used augmentation systems rely on a network of ground stations to control and collect GPS signals to aid in positioning and navigation. In fact, there’s a good chance your in-vehicle navigation system is utilizing this technology today.

So, the next time you go out for a drive using your GPS, you’ll feel better knowing there’s an entire network in space and on the ground working to locate your position. Of course, you can also cause your own inaccuracies by driving with outdated maps in your device. That’s why it’s important to get map updates for your GPS device on a regular basis. It’s perhaps the easiest way to help ensure the accuracy of your GPS device.

Sources:
http://www.gps.gov/