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How Does GPS Work?

One way to understand how GPS works is to imagine yourself floating in a room of zero gravity. To find your exact position, you use a tape measure to determine your distance to the closest wall. You then measure the distance to another wall and finally the distance to the floor. Because the positions of the walls, ceiling and floor are known, you position is also known because you know your distance to them. GPS works much the same way, using a system of twenty-four satellites that orbit 20,200 km above the earth and beam radio signals downward. When a GPS receiver is first turned on, it detects exactly where each satellite is located even though it is moving. It then monitors four of the satellites that are currently overhead. It uses the codes transmitted from the satellites like a tape measure to determine its distance from three satellites (the signals from the fourth satellite are still used to keep accurate time). Much like the zero gravity room, once the receiver knows its distance from three known objects it knows where it is. Although the system is very complex, it is very simple to use.

- Adapted from GPS Made Easy: Using Global Positioning Systems in the Outdoors, 4th Edition by Lawrence Letham, published by The Mountaineers Books, $15.95.

What Is WAAS and How Accurate Does It Make GPS?

WAAS stands for Wide Area Augmentation System. It is a collection of ground stations that use differential GPS techniques to calculate the amount of error in the GPS signals. Information to correct for the error in the GPS signals is then transmitted to one of two WAAS satellites stationed over the equator, which in turn transmit the correction information to GPS receivers. When using WAAS, a GPS receiver calculates its position using the GPS satellites’ signals and then uses information from one of the WAAS satellites to corrects its calculated position. WAAS does not help the receiver determine its location, but it provides data to help the receiver refine its position calculation once it is established. WAAS was developed to increase GPS accuracy for airport landing positions.

WAAS makes a GPS receiver accurate to 3 meters (9.8 feet) 95 percent of the time. Without WAAS, the range commonly quoted is 5-15 meters.

WAAS coverage is not global. It is designed to cover the Continental U.S. and Alaska. Hawaii will also be covered in the future. Agreements between the U.S. and Canadian governments will also provide coverage to Canada in the future. If you travel where there is WAAS coverage, be sure to use it because it increases accuracy significantly. If you travel in an area not covered by WAAS, be sure to turn this feature off on your receiver. If the receiver picks up WAAS signals in an area that WAAS does not cover, the receiver’s accuracy can be adversely affected and result in accuracy worse than 15 meters (49.2 feet).

- Adapted from GPS Made Easy: Using Global Positioning Systems in the Outdoors, 4th Edition by Lawrence Letham, published by The Mountaineers Books, $15.95.

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