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The GPS System is composed of:

• Satellites
• Earth Monitoring Stations
• GPS Receivers

What Is a GPS Satellite?

There are currently 24 satellites orbiting the earth in 6 distinct trajectories. Each satellite completes two orbits around the earth each day, traveling at a speed of about 7,000 MPH. At the highest point in their orbit they are more than 12,000 miles above the earth. From this height, the satellite signals can be received over a large percentage of the surface of the earth. A GPS unit works by receiving multiple satellite signals and calculating the receiver’s position based on triangulation of the different signals. Accuracy is increased with each additional satellite signal that is received so the orbits of the satellites are set so that at most points on the earth there are at least 4 signals that can be received. Of course, this is in theory only since the landscape around you can block some or all of the GPS signals.

What Is a GPS Earth Monitoring Station?

There are four unmanned monitoring stations in the US and one manned master station which is located in Colorado Springs, Colorado. The four unmanned stations relay position data from the satellites to the master station. The master station uses the relayed information to correct the position of the sat tiles and sends the corrected information back to the satellites. This synchronizes the data that each satellite transmits to your GPS receiver and ensures that your receiver is always working with accurate information.

What Is a GPS Receiver?

GPS satellites transmit low power radio signals that can only be picked up if your GPS unit has a direct “line of sight” to the satellite. The signals can penetrate things like glass, plastic and clouds but not solid structures like buildings or rocks. A GPS unit works by triangulating your location based on the position of at least three satellites in the GPS system. It compares the time it takes to receive a signal from each of the satellites in range and calculates your location based on the data.

If you are only able to pick up three satellite signals, your GPS unit can report your location in latitude and longitude. However, when the GPS unit picks up signals from at least four satellites it also calculates your altitude. Now it can report your trip distance, distance to a destination, travel speed and even your altitude if your GPS receiver features an altimeter.

How accurate a GPS unit is depends on its ability to find and lock onto a satellite’s signal. As you might expect, you get what you pay for and the more expensive GPS units are generally more accurate. The best consumer units claim accuracy to within a few feet while the older or less expensive units can be off up to 30-40 feet or more. Where you plan to use your GPS receiver could determine if it’s worth shelling out the extra money for increased accuracy. If you’re using your GPS unit in flatter terrain with a good open view of the sky a less expensive unit could perform nearly as well as the top of the line receiver. However, if you plan to use your receiver in dense tree cover, while hiking in ravines or narrow valleys, or among tall buildings you’ll gain much better accuracy with a more higher end GPS unit.

Now you know the basics about how GPS works and can answer the question “What is GPS?”.

The best consumer GPS technology available as of 2009 achieves an accuracy level of 15 to 30 feet under normal conditions. Garmin, currently the most popular manufacturer of GPS receivers, claims their GPS units are accurate to within 50 feet 95% of the time. Even within the Garmin GPS receiver line, accuracy specs differ because of the different chips used in the design of different receivers.

GPS Receiver Design Affects GPS Accuracy

If you are looking for the most accurate consumer GPS unit, it’s important to pay attention to the unit’s chip type. The latest technology in GPS receiver chips is the SiRFStar III from SiRF. With this chip in your receiver and WAAS enabled you can expect to reach the best GPS accuracy levels available under current conditions.

Environmental Factors Affect GPS Accuracy

Many different environmental factors can also affect the accuracy of your GPS receiver. Heavy tree cover, tall buildings, deep canyons, and any location in which your view of the sky is limited can affect how accurate your GPS is. Atmospheric conditions can also degrade the accuracy of your GPS unit.

As you can see, there is no easy answer to the question “how accurate is GPS?”. However, you now know the main factors that can affect GPS accuracy.